libstdc++: atomic: Add missing clear_padding in __atomic_float constructor
Checks
Commit Message
Hi, I found __atomic_float constructor does not clear padding,
while __compare_exchange assumes it as zeroed padding. So it is easy to
reproducing a infinite loop in X86-64 with long double type like:
---
-O0 -std=c++23 -mlong-double-80
#include <stdio.h>
#include <atomic>
#define T long double
int main() {
std::atomic<T> t(0.5);
t.fetch_add(0.5);
float x = t;
printf("%f\n", x);
}
---
So we should add __builtin_clear_padding in __atomic_float constructor,
just like the generic atomic struct.
regtested on x86_64-linux. Is it OK for trunk?
---
libstdc++: atomic: Add missing clear_padding in __atomic_float constructor.
libstdc++-v3/ChangeLog:
* include/bits/atomic_base.h: add __builtin_clear_padding in __atomic_float
constructor.
---
libstdc++-v3/include/bits/atomic_base.h | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
Comments
Ping. Thanks.
xndcn <xndchn@gmail.com> 于2024年1月8日周一 09:01写道：
> Hi, I found __atomic_float constructor does not clear padding,
> while __compare_exchange assumes it as zeroed padding. So it is easy to
> reproducing a infinite loop in X86-64 with long double type like:
> ---
> -O0 -std=c++23 -mlong-double-80
> #include <stdio.h>
> #include <atomic>
>
> #define T long double
> int main() {
> std::atomic<T> t(0.5);
> t.fetch_add(0.5);
> float x = t;
> printf("%f\n", x);
> }
> ---
>
> So we should add __builtin_clear_padding in __atomic_float constructor,
> just like the generic atomic struct.
>
> regtested on x86_64-linux. Is it OK for trunk?
>
> ---
> libstdc++: atomic: Add missing clear_padding in __atomic_float constructor.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/atomic_base.h: add __builtin_clear_padding in
> __atomic_float constructor.
> ---
> libstdc++-v3/include/bits/atomic_base.h | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/bits/atomic_base.h
> b/libstdc++-v3/include/bits/atomic_base.h
> index f4ce0fa53..d59c2209e 100644
> --- a/libstdc++-v3/include/bits/atomic_base.h
> +++ b/libstdc++-v3/include/bits/atomic_base.h
> @@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> constexpr
> __atomic_float(_Fp __t) : _M_fp(__t)
> - { }
> + {
> +#if __has_builtin(__builtin_clear_padding)
> + if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
> + __builtin_clear_padding(std::__addressof(_M_fp));
> +#endif
> + }
>
> __atomic_float(const __atomic_float&) = delete;
> __atomic_float& operator=(const __atomic_float&) = delete;
> --
> 2.25.1
>
On Sun, Jan 7, 2024, 5:02 PM xndcn <xndchn@gmail.com> wrote:
> Hi, I found __atomic_float constructor does not clear padding,
> while __compare_exchange assumes it as zeroed padding. So it is easy to
> reproducing a infinite loop in X86-64 with long double type like:
> ---
> -O0 -std=c++23 -mlong-double-80
> #include <stdio.h>
> #include <atomic>
>
> #define T long double
> int main() {
> std::atomic<T> t(0.5);
> t.fetch_add(0.5);
> float x = t;
> printf("%f\n", x);
> }
> ---
>
> So we should add __builtin_clear_padding in __atomic_float constructor,
> just like the generic atomic struct.
>
> regtested on x86_64-linux. Is it OK for trunk?
>
> ---
> libstdc++: atomic: Add missing clear_padding in __atomic_float constructor.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/atomic_base.h: add __builtin_clear_padding in
> __atomic_float constructor.
> ---
> libstdc++-v3/include/bits/atomic_base.h | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/bits/atomic_base.h
> b/libstdc++-v3/include/bits/atomic_base.h
> index f4ce0fa53..d59c2209e 100644
> --- a/libstdc++-v3/include/bits/atomic_base.h
> +++ b/libstdc++-v3/include/bits/atomic_base.h
> @@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> constexpr
> __atomic_float(_Fp __t) : _M_fp(__t)
> - { }
> + {
> +#if __has_builtin(__builtin_clear_padding)
> + if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
> + __builtin_clear_padding(std::__addressof(_M_fp));
> +#endif
> + }
>
> __atomic_float(const __atomic_float&) = delete;
> __atomic_float& operator=(const __atomic_float&) = delete;
> --
> 2.25.1
>
Can you add a testcase?
Thanks.
H.J.
>
Thanks, so I add a test: atomic_float/compare_exchange_padding.cc, which
will fail due to timeout without the patch.
---
libstdc++-v3/ChangeLog:
* include/bits/atomic_base.h: add __builtin_clear_padding in
__atomic_float constructor.
* testsuite/lib/dg-options.exp: enable libatomic for IA32 and X86-64.
* testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
---
libstdc++-v3/include/bits/atomic_base.h | 7 ++-
.../atomic_float/compare_exchange_padding.cc | 50 +++++++++++++++++++
libstdc++-v3/testsuite/lib/dg-options.exp | 1 +
3 files changed, 57 insertions(+), 1 deletion(-)
create mode 100644
libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
diff --git a/libstdc++-v3/include/bits/atomic_base.h
b/libstdc++-v3/include/bits/atomic_base.h
index d3a2c4f3805..cbe3749e17f 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
constexpr
__atomic_float(_Fp __t) : _M_fp(__t)
- { }
+ {
+#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
+ if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
+ __builtin_clear_padding(std::__addressof(_M_fp));
+#endif
+ }
__atomic_float(const __atomic_float&) = delete;
__atomic_float& operator=(const __atomic_float&) = delete;
diff --git
a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
new file mode 100644
index 00000000000..9376ab22850
--- /dev/null
+++
b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
@@ -0,0 +1,50 @@
+// { dg-do run { target c++20 } }
+// { dg-options "-O0" }
+// { dg-timeout 10 }
+// { dg-additional-options "-mlong-double-80" { target x86_64-*-* } }
+// { dg-do run { target { ia32 || x86_64-*-* } } }
+// { dg-add-options libatomic }
+
+#include <atomic>
+#include <testsuite_hooks.h>
+
+template<typename T>
+void __attribute__((noinline,noipa))
+fill_padding(T& f)
+{
+ T mask;
+ __builtin_memset(&mask, 0xff, sizeof(T));
+ __builtin_clear_padding(&mask);
+ unsigned char* ptr_f = (unsigned char*)&f;
+ unsigned char* ptr_mask = (unsigned char*)&mask;
+ for (int i = 0; i < sizeof(T); i++)
+ {
+ if (ptr_mask[i] == 0x00)
+ {
+ ptr_f[i] = 0xff;
+ }
+ }
+}
+
+void
+test01()
+{
+ long double f = 0.5f; // long double may contains padding on X86
+ fill_padding(f);
+ std::atomic<long double> as{ f }; // padding cleared on constructor
+ long double t = 1.5;
+
+ as.fetch_add(t);
+ long double s = f + t;
+ t = as.load();
+ VERIFY(s == t); // padding ignored on float comparing
+ fill_padding(s);
+ VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
+ fill_padding(f);
+ VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
+}
+
+int main()
+{
+ test01();
+}
diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp
b/libstdc++-v3/testsuite/lib/dg-options.exp
index bc387d17ed7..d9a19dadd7f 100644
--- a/libstdc++-v3/testsuite/lib/dg-options.exp
+++ b/libstdc++-v3/testsuite/lib/dg-options.exp
@@ -337,6 +337,7 @@ proc add_options_for_libatomic { flags } {
|| ([istarget powerpc*-*-*] && [check_effective_target_ilp32])
|| [istarget riscv*-*-*]
|| ([istarget sparc*-*-linux-gnu] && [check_effective_target_ilp32])
+ || ([istarget i?86-*-*] || [istarget x86_64-*-*])
} {
global TOOL_OPTIONS
On Tue, 2024-01-16 at 17:53 +0800, xndcn wrote:
> Thanks, so I add a test: atomic_float/compare_exchange_padding.cc,
> which will fail due to timeout without the patch.
Please resend in plain text instead of HTML. Sending in HTML causes the
patch mangled.
And libstdc++ patches should CC libstdc++@gcc.gnu.org, in addition to
gcc-patches@gcc.gnu.org.
> ---
> libstdc++-v3/ChangeLog:
>
> * include/bits/atomic_base.h: add __builtin_clear_padding in __atomic_float constructor.
> * testsuite/lib/dg-options.exp: enable libatomic for IA32 and X86-64.
> * testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
> ---
> libstdc++-v3/include/bits/atomic_base.h | 7 ++-
> .../atomic_float/compare_exchange_padding.cc | 50 +++++++++++++++++++
> libstdc++-v3/testsuite/lib/dg-options.exp | 1 +
> 3 files changed, 57 insertions(+), 1 deletion(-)
> create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
>
> diff --git a/libstdc++-v3/include/bits/atomic_base.h b/libstdc++-v3/include/bits/atomic_base.h
> index d3a2c4f3805..cbe3749e17f 100644
> --- a/libstdc++-v3/include/bits/atomic_base.h
> +++ b/libstdc++-v3/include/bits/atomic_base.h
> @@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> constexpr
> __atomic_float(_Fp __t) : _M_fp(__t)
> - { }
> + {
> +#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
> + if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
> + __builtin_clear_padding(std::__addressof(_M_fp));
> +#endif
> + }
>
> __atomic_float(const __atomic_float&) = delete;
> __atomic_float& operator=(const __atomic_float&) = delete;
> diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> new file mode 100644
> index 00000000000..9376ab22850
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> @@ -0,0 +1,50 @@
> +// { dg-do run { target c++20 } }
> +// { dg-options "-O0" }
> +// { dg-timeout 10 }
> +// { dg-additional-options "-mlong-double-80" { target x86_64-*-* } }
> +// { dg-do run { target { ia32 || x86_64-*-* } } }
> +// { dg-add-options libatomic }
> +
> +#include <atomic>
> +#include <testsuite_hooks.h>
> +
> +template<typename T>
> +void __attribute__((noinline,noipa))
> +fill_padding(T& f)
> +{
> + T mask;
> + __builtin_memset(&mask, 0xff, sizeof(T));
> + __builtin_clear_padding(&mask);
> + unsigned char* ptr_f = (unsigned char*)&f;
> + unsigned char* ptr_mask = (unsigned char*)&mask;
> + for (int i = 0; i < sizeof(T); i++)
> + {
> + if (ptr_mask[i] == 0x00)
> + {
> + ptr_f[i] = 0xff;
> + }
> + }
> +}
> +
> +void
> +test01()
> +{
> + long double f = 0.5f; // long double may contains padding on X86
> + fill_padding(f);
> + std::atomic<long double> as{ f }; // padding cleared on constructor
> + long double t = 1.5;
> +
> + as.fetch_add(t);
> + long double s = f + t;
> + t = as.load();
> + VERIFY(s == t); // padding ignored on float comparing
> + fill_padding(s);
> + VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
> + fill_padding(f);
> + VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
> +}
> +
> +int main()
> +{
> + test01();
> +}
> diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp b/libstdc++-v3/testsuite/lib/dg-options.exp
> index bc387d17ed7..d9a19dadd7f 100644
> --- a/libstdc++-v3/testsuite/lib/dg-options.exp
> +++ b/libstdc++-v3/testsuite/lib/dg-options.exp
> @@ -337,6 +337,7 @@ proc add_options_for_libatomic { flags } {
> || ([istarget powerpc*-*-*] && [check_effective_target_ilp32])
> || [istarget riscv*-*-*]
> || ([istarget sparc*-*-linux-gnu] && [check_effective_target_ilp32])
> + || ([istarget i?86-*-*] || [istarget x86_64-*-*])
This seems too overkill as "dg-add-options libatomic" is not intended to
handle 16-byte atomics. Maybe we can fork this to a new dg-add-options
like "add_options_for_libatomic_16b"?
> } {
> global TOOL_OPTIONS
>
> --
> 2.25.1
Sorry about the mangled content...
So I add a new add-options for libatomic_16b:
---
libstdc++-v3/ChangeLog:
* include/bits/atomic_base.h: add __builtin_clear_padding in
__atomic_float constructor.
* testsuite/lib/dg-options.exp: add new add-options for libatomic_16b.
* testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
---
libstdc++-v3/include/bits/atomic_base.h | 7 ++-
.../atomic_float/compare_exchange_padding.cc | 50 +++++++++++++++++++
libstdc++-v3/testsuite/lib/dg-options.exp | 22 ++++++++
3 files changed, 78 insertions(+), 1 deletion(-)
create mode 100644
libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
diff --git a/libstdc++-v3/include/bits/atomic_base.h
b/libstdc++-v3/include/bits/atomic_base.h
index f4ce0fa53..104ddfdbe 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
constexpr
__atomic_float(_Fp __t) : _M_fp(__t)
- { }
+ {
+#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
+ if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
+ __builtin_clear_padding(std::__addressof(_M_fp));
+#endif
+ }
__atomic_float(const __atomic_float&) = delete;
__atomic_float& operator=(const __atomic_float&) = delete;
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
new file mode 100644
index 000000000..d538b3d55
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
@@ -0,0 +1,50 @@
+// { dg-do run { target c++20 } }
+// { dg-options "-O0" }
+// { dg-timeout 10 }
+// { dg-additional-options "-mlong-double-80" { target x86_64-*-* } }
+// { dg-do run { target { ia32 || x86_64-*-* } } }
+// { dg-add-options libatomic_16b }
+
+#include <atomic>
+#include <testsuite_hooks.h>
+
+template<typename T>
+void __attribute__((noinline,noipa))
+fill_padding(T& f)
+{
+ T mask;
+ __builtin_memset(&mask, 0xff, sizeof(T));
+ __builtin_clear_padding(&mask);
+ unsigned char* ptr_f = (unsigned char*)&f;
+ unsigned char* ptr_mask = (unsigned char*)&mask;
+ for (int i = 0; i < sizeof(T); i++)
+ {
+ if (ptr_mask[i] == 0x00)
+ {
+ ptr_f[i] = 0xff;
+ }
+ }
+}
+
+void
+test01()
+{
+ long double f = 0.5f; // long double may contains padding on X86
+ fill_padding(f);
+ std::atomic<long double> as{ f }; // padding cleared on constructor
+ long double t = 1.5;
+
+ as.fetch_add(t);
+ long double s = f + t;
+ t = as.load();
+ VERIFY(s == t); // padding ignored on float comparing
+ fill_padding(s);
+ VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
+ fill_padding(f);
+ VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
+}
+
+int main()
+{
+ test01();
+}
diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp
b/libstdc++-v3/testsuite/lib/dg-options.exp
index 850442b6b..25da20d58 100644
--- a/libstdc++-v3/testsuite/lib/dg-options.exp
+++ b/libstdc++-v3/testsuite/lib/dg-options.exp
@@ -356,6 +356,28 @@ proc add_options_for_libatomic { flags } {
return $flags
}
+# Add option to link to libatomic, if required for atomics on 16-byte (128-bit)
+proc add_options_for_libatomic_16b { flags } {
+ if { ([istarget i?86-*-*] || [istarget x86_64-*-*])
+ } {
+ global TOOL_OPTIONS
+
+ set link_flags ""
+ if ![is_remote host] {
+ if [info exists TOOL_OPTIONS] {
+ set link_flags "[atomic_link_flags [get_multilibs ${TOOL_OPTIONS}]]"
+ } else {
+ set link_flags "[atomic_link_flags [get_multilibs]]"
+ }
+ }
+
+ append link_flags " -latomic "
+
+ return "$flags $link_flags"
+ }
+ return $flags
+}
+
# Add options to enable use of deprecated features.
proc add_options_for_using-deprecated { flags } {
return "$flags -U_GLIBCXX_USE_DEPRECATED -D_GLIBCXX_USE_DEPRECATED=1"
Hi, is it OK for trunk? I do not have access to the repo, can you
please help me submit the patch? Thanks.
xndcn <xndchn@gmail.com> 于2024年1月17日周三 00:16写道：
>
> Sorry about the mangled content...
> So I add a new add-options for libatomic_16b:
>
> ---
> libstdc++-v3/ChangeLog:
>
> * include/bits/atomic_base.h: add __builtin_clear_padding in
> __atomic_float constructor.
> * testsuite/lib/dg-options.exp: add new add-options for libatomic_16b.
> * testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
> ---
> libstdc++-v3/include/bits/atomic_base.h | 7 ++-
> .../atomic_float/compare_exchange_padding.cc | 50 +++++++++++++++++++
> libstdc++-v3/testsuite/lib/dg-options.exp | 22 ++++++++
> 3 files changed, 78 insertions(+), 1 deletion(-)
> create mode 100644
> libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
>
> diff --git a/libstdc++-v3/include/bits/atomic_base.h
> b/libstdc++-v3/include/bits/atomic_base.h
> index f4ce0fa53..104ddfdbe 100644
> --- a/libstdc++-v3/include/bits/atomic_base.h
> +++ b/libstdc++-v3/include/bits/atomic_base.h
> @@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> constexpr
> __atomic_float(_Fp __t) : _M_fp(__t)
> - { }
> + {
> +#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
> + if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
> + __builtin_clear_padding(std::__addressof(_M_fp));
> +#endif
> + }
>
> __atomic_float(const __atomic_float&) = delete;
> __atomic_float& operator=(const __atomic_float&) = delete;
> diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> new file mode 100644
> index 000000000..d538b3d55
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> @@ -0,0 +1,50 @@
> +// { dg-do run { target c++20 } }
> +// { dg-options "-O0" }
> +// { dg-timeout 10 }
> +// { dg-additional-options "-mlong-double-80" { target x86_64-*-* } }
> +// { dg-do run { target { ia32 || x86_64-*-* } } }
> +// { dg-add-options libatomic_16b }
> +
> +#include <atomic>
> +#include <testsuite_hooks.h>
> +
> +template<typename T>
> +void __attribute__((noinline,noipa))
> +fill_padding(T& f)
> +{
> + T mask;
> + __builtin_memset(&mask, 0xff, sizeof(T));
> + __builtin_clear_padding(&mask);
> + unsigned char* ptr_f = (unsigned char*)&f;
> + unsigned char* ptr_mask = (unsigned char*)&mask;
> + for (int i = 0; i < sizeof(T); i++)
> + {
> + if (ptr_mask[i] == 0x00)
> + {
> + ptr_f[i] = 0xff;
> + }
> + }
> +}
> +
> +void
> +test01()
> +{
> + long double f = 0.5f; // long double may contains padding on X86
> + fill_padding(f);
> + std::atomic<long double> as{ f }; // padding cleared on constructor
> + long double t = 1.5;
> +
> + as.fetch_add(t);
> + long double s = f + t;
> + t = as.load();
> + VERIFY(s == t); // padding ignored on float comparing
> + fill_padding(s);
> + VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
> + fill_padding(f);
> + VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
> +}
> +
> +int main()
> +{
> + test01();
> +}
> diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp
> b/libstdc++-v3/testsuite/lib/dg-options.exp
> index 850442b6b..25da20d58 100644
> --- a/libstdc++-v3/testsuite/lib/dg-options.exp
> +++ b/libstdc++-v3/testsuite/lib/dg-options.exp
> @@ -356,6 +356,28 @@ proc add_options_for_libatomic { flags } {
> return $flags
> }
>
> +# Add option to link to libatomic, if required for atomics on 16-byte (128-bit)
> +proc add_options_for_libatomic_16b { flags } {
> + if { ([istarget i?86-*-*] || [istarget x86_64-*-*])
> + } {
> + global TOOL_OPTIONS
> +
> + set link_flags ""
> + if ![is_remote host] {
> + if [info exists TOOL_OPTIONS] {
> + set link_flags "[atomic_link_flags [get_multilibs ${TOOL_OPTIONS}]]"
> + } else {
> + set link_flags "[atomic_link_flags [get_multilibs]]"
> + }
> + }
> +
> + append link_flags " -latomic "
> +
> + return "$flags $link_flags"
> + }
> + return $flags
> +}
> +
> # Add options to enable use of deprecated features.
> proc add_options_for_using-deprecated { flags } {
> return "$flags -U_GLIBCXX_USE_DEPRECATED -D_GLIBCXX_USE_DEPRECATED=1"
> --
> 2.25.1
>
>
> Xi Ruoyao <xry111@xry111.site> 于2024年1月16日周二 18:12写道：
> >
> > On Tue, 2024-01-16 at 17:53 +0800, xndcn wrote:
> > > Thanks, so I add a test: atomic_float/compare_exchange_padding.cc,
> > > which will fail due to timeout without the patch.
> >
> > Please resend in plain text instead of HTML. Sending in HTML causes the
> > patch mangled.
> >
> > And libstdc++ patches should CC libstdc++@gcc.gnu.org, in addition to
> > gcc-patches@gcc.gnu.org.
> >
> > > ---
> > > libstdc++-v3/ChangeLog:
> > >
> > > * include/bits/atomic_base.h: add __builtin_clear_padding in __atomic_float constructor.
> > > * testsuite/lib/dg-options.exp: enable libatomic for IA32 and X86-64.
> > > * testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
> > > ---
> > > libstdc++-v3/include/bits/atomic_base.h | 7 ++-
> > > .../atomic_float/compare_exchange_padding.cc | 50 +++++++++++++++++++
> > > libstdc++-v3/testsuite/lib/dg-options.exp | 1 +
> > > 3 files changed, 57 insertions(+), 1 deletion(-)
> > > create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> > >
> > > diff --git a/libstdc++-v3/include/bits/atomic_base.h b/libstdc++-v3/include/bits/atomic_base.h
> > > index d3a2c4f3805..cbe3749e17f 100644
> > > --- a/libstdc++-v3/include/bits/atomic_base.h
> > > +++ b/libstdc++-v3/include/bits/atomic_base.h
> > > @@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >
> > > constexpr
> > > __atomic_float(_Fp __t) : _M_fp(__t)
> > > - { }
> > > + {
> > > +#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
> > > + if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
> > > + __builtin_clear_padding(std::__addressof(_M_fp));
> > > +#endif
> > > + }
> > >
> > > __atomic_float(const __atomic_float&) = delete;
> > > __atomic_float& operator=(const __atomic_float&) = delete;
> > > diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> > > new file mode 100644
> > > index 00000000000..9376ab22850
> > > --- /dev/null
> > > +++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> > > @@ -0,0 +1,50 @@
> > > +// { dg-do run { target c++20 } }
> > > +// { dg-options "-O0" }
> > > +// { dg-timeout 10 }
> > > +// { dg-additional-options "-mlong-double-80" { target x86_64-*-* } }
> > > +// { dg-do run { target { ia32 || x86_64-*-* } } }
> > > +// { dg-add-options libatomic }
> > > +
> > > +#include <atomic>
> > > +#include <testsuite_hooks.h>
> > > +
> > > +template<typename T>
> > > +void __attribute__((noinline,noipa))
> > > +fill_padding(T& f)
> > > +{
> > > + T mask;
> > > + __builtin_memset(&mask, 0xff, sizeof(T));
> > > + __builtin_clear_padding(&mask);
> > > + unsigned char* ptr_f = (unsigned char*)&f;
> > > + unsigned char* ptr_mask = (unsigned char*)&mask;
> > > + for (int i = 0; i < sizeof(T); i++)
> > > + {
> > > + if (ptr_mask[i] == 0x00)
> > > + {
> > > + ptr_f[i] = 0xff;
> > > + }
> > > + }
> > > +}
> > > +
> > > +void
> > > +test01()
> > > +{
> > > + long double f = 0.5f; // long double may contains padding on X86
> > > + fill_padding(f);
> > > + std::atomic<long double> as{ f }; // padding cleared on constructor
> > > + long double t = 1.5;
> > > +
> > > + as.fetch_add(t);
> > > + long double s = f + t;
> > > + t = as.load();
> > > + VERIFY(s == t); // padding ignored on float comparing
> > > + fill_padding(s);
> > > + VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
> > > + fill_padding(f);
> > > + VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
> > > +}
> > > +
> > > +int main()
> > > +{
> > > + test01();
> > > +}
> > > diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp b/libstdc++-v3/testsuite/lib/dg-options.exp
> > > index bc387d17ed7..d9a19dadd7f 100644
> > > --- a/libstdc++-v3/testsuite/lib/dg-options.exp
> > > +++ b/libstdc++-v3/testsuite/lib/dg-options.exp
> > > @@ -337,6 +337,7 @@ proc add_options_for_libatomic { flags } {
> > > || ([istarget powerpc*-*-*] && [check_effective_target_ilp32])
> > > || [istarget riscv*-*-*]
> > > || ([istarget sparc*-*-linux-gnu] && [check_effective_target_ilp32])
> > > + || ([istarget i?86-*-*] || [istarget x86_64-*-*])
> >
> > This seems too overkill as "dg-add-options libatomic" is not intended to
> > handle 16-byte atomics. Maybe we can fork this to a new dg-add-options
> > like "add_options_for_libatomic_16b"?
> >
> > > } {
> > > global TOOL_OPTIONS
> > >
> > > --
> > > 2.25.1
> >
> > --
> > Xi Ruoyao <xry111@xry111.site>
> > School of Aerospace Science and Technology, Xidian University
Ping, thanks.
I do not have access to the repo, anyone can please help me submit the
patch? Thanks.
xndcn <xndchn@gmail.com> 于2024年1月17日周三 00:16写道：
>
> Sorry about the mangled content...
> So I add a new add-options for libatomic_16b:
>
> ---
> libstdc++-v3/ChangeLog:
>
> * include/bits/atomic_base.h: add __builtin_clear_padding in
> __atomic_float constructor.
> * testsuite/lib/dg-options.exp: add new add-options for libatomic_16b.
> * testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
> ---
> libstdc++-v3/include/bits/atomic_base.h | 7 ++-
> .../atomic_float/compare_exchange_padding.cc | 50 +++++++++++++++++++
> libstdc++-v3/testsuite/lib/dg-options.exp | 22 ++++++++
> 3 files changed, 78 insertions(+), 1 deletion(-)
> create mode 100644
> libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
>
> diff --git a/libstdc++-v3/include/bits/atomic_base.h
> b/libstdc++-v3/include/bits/atomic_base.h
> index f4ce0fa53..104ddfdbe 100644
> --- a/libstdc++-v3/include/bits/atomic_base.h
> +++ b/libstdc++-v3/include/bits/atomic_base.h
> @@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> constexpr
> __atomic_float(_Fp __t) : _M_fp(__t)
> - { }
> + {
> +#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
> + if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
> + __builtin_clear_padding(std::__addressof(_M_fp));
> +#endif
> + }
>
> __atomic_float(const __atomic_float&) = delete;
> __atomic_float& operator=(const __atomic_float&) = delete;
> diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> new file mode 100644
> index 000000000..d538b3d55
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> @@ -0,0 +1,50 @@
> +// { dg-do run { target c++20 } }
> +// { dg-options "-O0" }
> +// { dg-timeout 10 }
> +// { dg-additional-options "-mlong-double-80" { target x86_64-*-* } }
> +// { dg-do run { target { ia32 || x86_64-*-* } } }
> +// { dg-add-options libatomic_16b }
> +
> +#include <atomic>
> +#include <testsuite_hooks.h>
> +
> +template<typename T>
> +void __attribute__((noinline,noipa))
> +fill_padding(T& f)
> +{
> + T mask;
> + __builtin_memset(&mask, 0xff, sizeof(T));
> + __builtin_clear_padding(&mask);
> + unsigned char* ptr_f = (unsigned char*)&f;
> + unsigned char* ptr_mask = (unsigned char*)&mask;
> + for (int i = 0; i < sizeof(T); i++)
> + {
> + if (ptr_mask[i] == 0x00)
> + {
> + ptr_f[i] = 0xff;
> + }
> + }
> +}
> +
> +void
> +test01()
> +{
> + long double f = 0.5f; // long double may contains padding on X86
> + fill_padding(f);
> + std::atomic<long double> as{ f }; // padding cleared on constructor
> + long double t = 1.5;
> +
> + as.fetch_add(t);
> + long double s = f + t;
> + t = as.load();
> + VERIFY(s == t); // padding ignored on float comparing
> + fill_padding(s);
> + VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
> + fill_padding(f);
> + VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
> +}
> +
> +int main()
> +{
> + test01();
> +}
> diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp
> b/libstdc++-v3/testsuite/lib/dg-options.exp
> index 850442b6b..25da20d58 100644
> --- a/libstdc++-v3/testsuite/lib/dg-options.exp
> +++ b/libstdc++-v3/testsuite/lib/dg-options.exp
> @@ -356,6 +356,28 @@ proc add_options_for_libatomic { flags } {
> return $flags
> }
>
> +# Add option to link to libatomic, if required for atomics on 16-byte (128-bit)
> +proc add_options_for_libatomic_16b { flags } {
> + if { ([istarget i?86-*-*] || [istarget x86_64-*-*])
> + } {
> + global TOOL_OPTIONS
> +
> + set link_flags ""
> + if ![is_remote host] {
> + if [info exists TOOL_OPTIONS] {
> + set link_flags "[atomic_link_flags [get_multilibs ${TOOL_OPTIONS}]]"
> + } else {
> + set link_flags "[atomic_link_flags [get_multilibs]]"
> + }
> + }
> +
> + append link_flags " -latomic "
> +
> + return "$flags $link_flags"
> + }
> + return $flags
> +}
> +
> # Add options to enable use of deprecated features.
> proc add_options_for_using-deprecated { flags } {
> return "$flags -U_GLIBCXX_USE_DEPRECATED -D_GLIBCXX_USE_DEPRECATED=1"
> --
> 2.25.1
>
>
> Xi Ruoyao <xry111@xry111.site> 于2024年1月16日周二 18:12写道：
> >
> > On Tue, 2024-01-16 at 17:53 +0800, xndcn wrote:
> > > Thanks, so I add a test: atomic_float/compare_exchange_padding.cc,
> > > which will fail due to timeout without the patch.
> >
> > Please resend in plain text instead of HTML. Sending in HTML causes the
> > patch mangled.
> >
> > And libstdc++ patches should CC libstdc++@gcc.gnu.org, in addition to
> > gcc-patches@gcc.gnu.org.
> >
> > > ---
> > > libstdc++-v3/ChangeLog:
> > >
> > > * include/bits/atomic_base.h: add __builtin_clear_padding in __atomic_float constructor.
> > > * testsuite/lib/dg-options.exp: enable libatomic for IA32 and X86-64.
> > > * testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
> > > ---
> > > libstdc++-v3/include/bits/atomic_base.h | 7 ++-
> > > .../atomic_float/compare_exchange_padding.cc | 50 +++++++++++++++++++
> > > libstdc++-v3/testsuite/lib/dg-options.exp | 1 +
> > > 3 files changed, 57 insertions(+), 1 deletion(-)
> > > create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> > >
> > > diff --git a/libstdc++-v3/include/bits/atomic_base.h b/libstdc++-v3/include/bits/atomic_base.h
> > > index d3a2c4f3805..cbe3749e17f 100644
> > > --- a/libstdc++-v3/include/bits/atomic_base.h
> > > +++ b/libstdc++-v3/include/bits/atomic_base.h
> > > @@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >
> > > constexpr
> > > __atomic_float(_Fp __t) : _M_fp(__t)
> > > - { }
> > > + {
> > > +#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
> > > + if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
> > > + __builtin_clear_padding(std::__addressof(_M_fp));
> > > +#endif
> > > + }
> > >
> > > __atomic_float(const __atomic_float&) = delete;
> > > __atomic_float& operator=(const __atomic_float&) = delete;
> > > diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> > > new file mode 100644
> > > index 00000000000..9376ab22850
> > > --- /dev/null
> > > +++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> > > @@ -0,0 +1,50 @@
> > > +// { dg-do run { target c++20 } }
> > > +// { dg-options "-O0" }
> > > +// { dg-timeout 10 }
> > > +// { dg-additional-options "-mlong-double-80" { target x86_64-*-* } }
> > > +// { dg-do run { target { ia32 || x86_64-*-* } } }
> > > +// { dg-add-options libatomic }
> > > +
> > > +#include <atomic>
> > > +#include <testsuite_hooks.h>
> > > +
> > > +template<typename T>
> > > +void __attribute__((noinline,noipa))
> > > +fill_padding(T& f)
> > > +{
> > > + T mask;
> > > + __builtin_memset(&mask, 0xff, sizeof(T));
> > > + __builtin_clear_padding(&mask);
> > > + unsigned char* ptr_f = (unsigned char*)&f;
> > > + unsigned char* ptr_mask = (unsigned char*)&mask;
> > > + for (int i = 0; i < sizeof(T); i++)
> > > + {
> > > + if (ptr_mask[i] == 0x00)
> > > + {
> > > + ptr_f[i] = 0xff;
> > > + }
> > > + }
> > > +}
> > > +
> > > +void
> > > +test01()
> > > +{
> > > + long double f = 0.5f; // long double may contains padding on X86
> > > + fill_padding(f);
> > > + std::atomic<long double> as{ f }; // padding cleared on constructor
> > > + long double t = 1.5;
> > > +
> > > + as.fetch_add(t);
> > > + long double s = f + t;
> > > + t = as.load();
> > > + VERIFY(s == t); // padding ignored on float comparing
> > > + fill_padding(s);
> > > + VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
> > > + fill_padding(f);
> > > + VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
> > > +}
> > > +
> > > +int main()
> > > +{
> > > + test01();
> > > +}
> > > diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp b/libstdc++-v3/testsuite/lib/dg-options.exp
> > > index bc387d17ed7..d9a19dadd7f 100644
> > > --- a/libstdc++-v3/testsuite/lib/dg-options.exp
> > > +++ b/libstdc++-v3/testsuite/lib/dg-options.exp
> > > @@ -337,6 +337,7 @@ proc add_options_for_libatomic { flags } {
> > > || ([istarget powerpc*-*-*] && [check_effective_target_ilp32])
> > > || [istarget riscv*-*-*]
> > > || ([istarget sparc*-*-linux-gnu] && [check_effective_target_ilp32])
> > > + || ([istarget i?86-*-*] || [istarget x86_64-*-*])
> >
> > This seems too overkill as "dg-add-options libatomic" is not intended to
> > handle 16-byte atomics. Maybe we can fork this to a new dg-add-options
> > like "add_options_for_libatomic_16b"?
> >
> > > } {
> > > global TOOL_OPTIONS
> > >
> > > --
> > > 2.25.1
> >
> > --
> > Xi Ruoyao <xry111@xry111.site>
> > School of Aerospace Science and Technology, Xidian University
On Tue, 16 Jan 2024 at 16:17, xndcn <xndchn@gmail.com> wrote:
>
> Sorry about the mangled content...
> So I add a new add-options for libatomic_16b:
>
> ---
> libstdc++-v3/ChangeLog:
>
> * include/bits/atomic_base.h: add __builtin_clear_padding in
> __atomic_float constructor.
> * testsuite/lib/dg-options.exp: add new add-options for libatomic_16b.
> * testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
> ---
> libstdc++-v3/include/bits/atomic_base.h | 7 ++-
> .../atomic_float/compare_exchange_padding.cc | 50 +++++++++++++++++++
> libstdc++-v3/testsuite/lib/dg-options.exp | 22 ++++++++
> 3 files changed, 78 insertions(+), 1 deletion(-)
> create mode 100644
> libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
>
> diff --git a/libstdc++-v3/include/bits/atomic_base.h
> b/libstdc++-v3/include/bits/atomic_base.h
> index f4ce0fa53..104ddfdbe 100644
> --- a/libstdc++-v3/include/bits/atomic_base.h
> +++ b/libstdc++-v3/include/bits/atomic_base.h
> @@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> constexpr
> __atomic_float(_Fp __t) : _M_fp(__t)
> - { }
> + {
> +#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
> + if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
This code is not compiled for C++11 and C++14, so this should just use
constexpr not _GLIBCXX17_CONSTEXPR.
Apart from that I think this patch looks OK.
> + __builtin_clear_padding(std::__addressof(_M_fp));
> +#endif
> + }
>
> __atomic_float(const __atomic_float&) = delete;
> __atomic_float& operator=(const __atomic_float&) = delete;
> diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> new file mode 100644
> index 000000000..d538b3d55
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> @@ -0,0 +1,50 @@
> +// { dg-do run { target c++20 } }
> +// { dg-options "-O0" }
> +// { dg-timeout 10 }
> +// { dg-additional-options "-mlong-double-80" { target x86_64-*-* } }
> +// { dg-do run { target { ia32 || x86_64-*-* } } }
> +// { dg-add-options libatomic_16b }
> +
> +#include <atomic>
> +#include <testsuite_hooks.h>
> +
> +template<typename T>
> +void __attribute__((noinline,noipa))
> +fill_padding(T& f)
> +{
> + T mask;
> + __builtin_memset(&mask, 0xff, sizeof(T));
> + __builtin_clear_padding(&mask);
> + unsigned char* ptr_f = (unsigned char*)&f;
> + unsigned char* ptr_mask = (unsigned char*)&mask;
> + for (int i = 0; i < sizeof(T); i++)
> + {
> + if (ptr_mask[i] == 0x00)
> + {
> + ptr_f[i] = 0xff;
> + }
> + }
> +}
> +
> +void
> +test01()
> +{
> + long double f = 0.5f; // long double may contains padding on X86
> + fill_padding(f);
> + std::atomic<long double> as{ f }; // padding cleared on constructor
> + long double t = 1.5;
> +
> + as.fetch_add(t);
> + long double s = f + t;
> + t = as.load();
> + VERIFY(s == t); // padding ignored on float comparing
> + fill_padding(s);
> + VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
> + fill_padding(f);
> + VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
> +}
> +
> +int main()
> +{
> + test01();
> +}
> diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp
> b/libstdc++-v3/testsuite/lib/dg-options.exp
> index 850442b6b..25da20d58 100644
> --- a/libstdc++-v3/testsuite/lib/dg-options.exp
> +++ b/libstdc++-v3/testsuite/lib/dg-options.exp
> @@ -356,6 +356,28 @@ proc add_options_for_libatomic { flags } {
> return $flags
> }
>
> +# Add option to link to libatomic, if required for atomics on 16-byte (128-bit)
> +proc add_options_for_libatomic_16b { flags } {
> + if { ([istarget i?86-*-*] || [istarget x86_64-*-*])
> + } {
> + global TOOL_OPTIONS
> +
> + set link_flags ""
> + if ![is_remote host] {
> + if [info exists TOOL_OPTIONS] {
> + set link_flags "[atomic_link_flags [get_multilibs ${TOOL_OPTIONS}]]"
> + } else {
> + set link_flags "[atomic_link_flags [get_multilibs]]"
> + }
> + }
> +
> + append link_flags " -latomic "
> +
> + return "$flags $link_flags"
> + }
> + return $flags
> +}
> +
> # Add options to enable use of deprecated features.
> proc add_options_for_using-deprecated { flags } {
> return "$flags -U_GLIBCXX_USE_DEPRECATED -D_GLIBCXX_USE_DEPRECATED=1"
> --
> 2.25.1
>
>
> Xi Ruoyao <xry111@xry111.site> 于2024年1月16日周二 18:12写道：
> >
> > On Tue, 2024-01-16 at 17:53 +0800, xndcn wrote:
> > > Thanks, so I add a test: atomic_float/compare_exchange_padding.cc,
> > > which will fail due to timeout without the patch.
> >
> > Please resend in plain text instead of HTML. Sending in HTML causes the
> > patch mangled.
> >
> > And libstdc++ patches should CC libstdc++@gcc.gnu.org, in addition to
> > gcc-patches@gcc.gnu.org.
> >
> > > ---
> > > libstdc++-v3/ChangeLog:
> > >
> > > * include/bits/atomic_base.h: add __builtin_clear_padding in __atomic_float constructor.
> > > * testsuite/lib/dg-options.exp: enable libatomic for IA32 and X86-64.
> > > * testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
> > > ---
> > > libstdc++-v3/include/bits/atomic_base.h | 7 ++-
> > > .../atomic_float/compare_exchange_padding.cc | 50 +++++++++++++++++++
> > > libstdc++-v3/testsuite/lib/dg-options.exp | 1 +
> > > 3 files changed, 57 insertions(+), 1 deletion(-)
> > > create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> > >
> > > diff --git a/libstdc++-v3/include/bits/atomic_base.h b/libstdc++-v3/include/bits/atomic_base.h
> > > index d3a2c4f3805..cbe3749e17f 100644
> > > --- a/libstdc++-v3/include/bits/atomic_base.h
> > > +++ b/libstdc++-v3/include/bits/atomic_base.h
> > > @@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >
> > > constexpr
> > > __atomic_float(_Fp __t) : _M_fp(__t)
> > > - { }
> > > + {
> > > +#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
> > > + if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
> > > + __builtin_clear_padding(std::__addressof(_M_fp));
> > > +#endif
> > > + }
> > >
> > > __atomic_float(const __atomic_float&) = delete;
> > > __atomic_float& operator=(const __atomic_float&) = delete;
> > > diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> > > new file mode 100644
> > > index 00000000000..9376ab22850
> > > --- /dev/null
> > > +++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> > > @@ -0,0 +1,50 @@
> > > +// { dg-do run { target c++20 } }
> > > +// { dg-options "-O0" }
> > > +// { dg-timeout 10 }
> > > +// { dg-additional-options "-mlong-double-80" { target x86_64-*-* } }
> > > +// { dg-do run { target { ia32 || x86_64-*-* } } }
> > > +// { dg-add-options libatomic }
> > > +
> > > +#include <atomic>
> > > +#include <testsuite_hooks.h>
> > > +
> > > +template<typename T>
> > > +void __attribute__((noinline,noipa))
> > > +fill_padding(T& f)
> > > +{
> > > + T mask;
> > > + __builtin_memset(&mask, 0xff, sizeof(T));
> > > + __builtin_clear_padding(&mask);
> > > + unsigned char* ptr_f = (unsigned char*)&f;
> > > + unsigned char* ptr_mask = (unsigned char*)&mask;
> > > + for (int i = 0; i < sizeof(T); i++)
> > > + {
> > > + if (ptr_mask[i] == 0x00)
> > > + {
> > > + ptr_f[i] = 0xff;
> > > + }
> > > + }
> > > +}
> > > +
> > > +void
> > > +test01()
> > > +{
> > > + long double f = 0.5f; // long double may contains padding on X86
> > > + fill_padding(f);
> > > + std::atomic<long double> as{ f }; // padding cleared on constructor
> > > + long double t = 1.5;
> > > +
> > > + as.fetch_add(t);
> > > + long double s = f + t;
> > > + t = as.load();
> > > + VERIFY(s == t); // padding ignored on float comparing
> > > + fill_padding(s);
> > > + VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
> > > + fill_padding(f);
> > > + VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
> > > +}
> > > +
> > > +int main()
> > > +{
> > > + test01();
> > > +}
> > > diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp b/libstdc++-v3/testsuite/lib/dg-options.exp
> > > index bc387d17ed7..d9a19dadd7f 100644
> > > --- a/libstdc++-v3/testsuite/lib/dg-options.exp
> > > +++ b/libstdc++-v3/testsuite/lib/dg-options.exp
> > > @@ -337,6 +337,7 @@ proc add_options_for_libatomic { flags } {
> > > || ([istarget powerpc*-*-*] && [check_effective_target_ilp32])
> > > || [istarget riscv*-*-*]
> > > || ([istarget sparc*-*-linux-gnu] && [check_effective_target_ilp32])
> > > + || ([istarget i?86-*-*] || [istarget x86_64-*-*])
> >
> > This seems too overkill as "dg-add-options libatomic" is not intended to
> > handle 16-byte atomics. Maybe we can fork this to a new dg-add-options
> > like "add_options_for_libatomic_16b"?
> >
> > > } {
> > > global TOOL_OPTIONS
> > >
> > > --
> > > 2.25.1
> >
> > --
> > Xi Ruoyao <xry111@xry111.site>
> > School of Aerospace Science and Technology, Xidian University
>
On Tue, 16 Jan 2024 at 16:17, xndcn <xndchn@gmail.com> wrote:
>
> Sorry about the mangled content...
> So I add a new add-options for libatomic_16b:
>
> ---
> libstdc++-v3/ChangeLog:
>
> * include/bits/atomic_base.h: add __builtin_clear_padding in
> __atomic_float constructor.
> * testsuite/lib/dg-options.exp: add new add-options for libatomic_16b.
> * testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
> ---
> libstdc++-v3/include/bits/atomic_base.h | 7 ++-
> .../atomic_float/compare_exchange_padding.cc | 50 +++++++++++++++++++
> libstdc++-v3/testsuite/lib/dg-options.exp | 22 ++++++++
> 3 files changed, 78 insertions(+), 1 deletion(-)
> create mode 100644
> libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
>
> diff --git a/libstdc++-v3/include/bits/atomic_base.h
> b/libstdc++-v3/include/bits/atomic_base.h
> index f4ce0fa53..104ddfdbe 100644
> --- a/libstdc++-v3/include/bits/atomic_base.h
> +++ b/libstdc++-v3/include/bits/atomic_base.h
> @@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> constexpr
> __atomic_float(_Fp __t) : _M_fp(__t)
> - { }
> + {
> +#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
> + if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
> + __builtin_clear_padding(std::__addressof(_M_fp));
> +#endif
> + }
>
> __atomic_float(const __atomic_float&) = delete;
> __atomic_float& operator=(const __atomic_float&) = delete;
> diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> new file mode 100644
> index 000000000..d538b3d55
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> @@ -0,0 +1,50 @@
> +// { dg-do run { target c++20 } }
> +// { dg-options "-O0" }
> +// { dg-timeout 10 }
> +// { dg-additional-options "-mlong-double-80" { target x86_64-*-* } }
> +// { dg-do run { target { ia32 || x86_64-*-* } } }
> +// { dg-add-options libatomic_16b }
Why not just use -latomic unconditionally here?
You're adding a new libatomic_16b option that expands to -latomic for
x86 and nothing otherwise, and then using it in a test that only runs
for x86. So it's just adding -latomic to all targets that run the
test, right?
Also, this patch needs either a copyright assignment or a DCO sign-off:
https://gcc.gnu.org/contribute.html#legal
> +
> +#include <atomic>
> +#include <testsuite_hooks.h>
> +
> +template<typename T>
> +void __attribute__((noinline,noipa))
> +fill_padding(T& f)
> +{
> + T mask;
> + __builtin_memset(&mask, 0xff, sizeof(T));
> + __builtin_clear_padding(&mask);
> + unsigned char* ptr_f = (unsigned char*)&f;
> + unsigned char* ptr_mask = (unsigned char*)&mask;
> + for (int i = 0; i < sizeof(T); i++)
> + {
> + if (ptr_mask[i] == 0x00)
> + {
> + ptr_f[i] = 0xff;
> + }
> + }
> +}
> +
> +void
> +test01()
> +{
> + long double f = 0.5f; // long double may contains padding on X86
> + fill_padding(f);
> + std::atomic<long double> as{ f }; // padding cleared on constructor
> + long double t = 1.5;
> +
> + as.fetch_add(t);
> + long double s = f + t;
> + t = as.load();
> + VERIFY(s == t); // padding ignored on float comparing
> + fill_padding(s);
> + VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
> + fill_padding(f);
> + VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
> +}
> +
> +int main()
> +{
> + test01();
> +}
> diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp
> b/libstdc++-v3/testsuite/lib/dg-options.exp
> index 850442b6b..25da20d58 100644
> --- a/libstdc++-v3/testsuite/lib/dg-options.exp
> +++ b/libstdc++-v3/testsuite/lib/dg-options.exp
> @@ -356,6 +356,28 @@ proc add_options_for_libatomic { flags } {
> return $flags
> }
>
> +# Add option to link to libatomic, if required for atomics on 16-byte (128-bit)
> +proc add_options_for_libatomic_16b { flags } {
> + if { ([istarget i?86-*-*] || [istarget x86_64-*-*])
> + } {
> + global TOOL_OPTIONS
> +
> + set link_flags ""
> + if ![is_remote host] {
> + if [info exists TOOL_OPTIONS] {
> + set link_flags "[atomic_link_flags [get_multilibs ${TOOL_OPTIONS}]]"
> + } else {
> + set link_flags "[atomic_link_flags [get_multilibs]]"
> + }
> + }
> +
> + append link_flags " -latomic "
> +
> + return "$flags $link_flags"
> + }
> + return $flags
> +}
> +
> # Add options to enable use of deprecated features.
> proc add_options_for_using-deprecated { flags } {
> return "$flags -U_GLIBCXX_USE_DEPRECATED -D_GLIBCXX_USE_DEPRECATED=1"
> --
> 2.25.1
>
>
> Xi Ruoyao <xry111@xry111.site> 于2024年1月16日周二 18:12写道：
> >
> > On Tue, 2024-01-16 at 17:53 +0800, xndcn wrote:
> > > Thanks, so I add a test: atomic_float/compare_exchange_padding.cc,
> > > which will fail due to timeout without the patch.
> >
> > Please resend in plain text instead of HTML. Sending in HTML causes the
> > patch mangled.
> >
> > And libstdc++ patches should CC libstdc++@gcc.gnu.org, in addition to
> > gcc-patches@gcc.gnu.org.
> >
> > > ---
> > > libstdc++-v3/ChangeLog:
> > >
> > > * include/bits/atomic_base.h: add __builtin_clear_padding in __atomic_float constructor.
> > > * testsuite/lib/dg-options.exp: enable libatomic for IA32 and X86-64.
> > > * testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
> > > ---
> > > libstdc++-v3/include/bits/atomic_base.h | 7 ++-
> > > .../atomic_float/compare_exchange_padding.cc | 50 +++++++++++++++++++
> > > libstdc++-v3/testsuite/lib/dg-options.exp | 1 +
> > > 3 files changed, 57 insertions(+), 1 deletion(-)
> > > create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> > >
> > > diff --git a/libstdc++-v3/include/bits/atomic_base.h b/libstdc++-v3/include/bits/atomic_base.h
> > > index d3a2c4f3805..cbe3749e17f 100644
> > > --- a/libstdc++-v3/include/bits/atomic_base.h
> > > +++ b/libstdc++-v3/include/bits/atomic_base.h
> > > @@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > >
> > > constexpr
> > > __atomic_float(_Fp __t) : _M_fp(__t)
> > > - { }
> > > + {
> > > +#if __cplusplus >= 201402L && __has_builtin(__builtin_clear_padding)
> > > + if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
> > > + __builtin_clear_padding(std::__addressof(_M_fp));
> > > +#endif
> > > + }
> > >
> > > __atomic_float(const __atomic_float&) = delete;
> > > __atomic_float& operator=(const __atomic_float&) = delete;
> > > diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> > > new file mode 100644
> > > index 00000000000..9376ab22850
> > > --- /dev/null
> > > +++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> > > @@ -0,0 +1,50 @@
> > > +// { dg-do run { target c++20 } }
> > > +// { dg-options "-O0" }
> > > +// { dg-timeout 10 }
> > > +// { dg-additional-options "-mlong-double-80" { target x86_64-*-* } }
> > > +// { dg-do run { target { ia32 || x86_64-*-* } } }
> > > +// { dg-add-options libatomic }
> > > +
> > > +#include <atomic>
> > > +#include <testsuite_hooks.h>
> > > +
> > > +template<typename T>
> > > +void __attribute__((noinline,noipa))
> > > +fill_padding(T& f)
> > > +{
> > > + T mask;
> > > + __builtin_memset(&mask, 0xff, sizeof(T));
> > > + __builtin_clear_padding(&mask);
> > > + unsigned char* ptr_f = (unsigned char*)&f;
> > > + unsigned char* ptr_mask = (unsigned char*)&mask;
> > > + for (int i = 0; i < sizeof(T); i++)
> > > + {
> > > + if (ptr_mask[i] == 0x00)
> > > + {
> > > + ptr_f[i] = 0xff;
> > > + }
> > > + }
> > > +}
> > > +
> > > +void
> > > +test01()
> > > +{
> > > + long double f = 0.5f; // long double may contains padding on X86
> > > + fill_padding(f);
> > > + std::atomic<long double> as{ f }; // padding cleared on constructor
> > > + long double t = 1.5;
> > > +
> > > + as.fetch_add(t);
> > > + long double s = f + t;
> > > + t = as.load();
> > > + VERIFY(s == t); // padding ignored on float comparing
> > > + fill_padding(s);
> > > + VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
> > > + fill_padding(f);
> > > + VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
> > > +}
> > > +
> > > +int main()
> > > +{
> > > + test01();
> > > +}
> > > diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp b/libstdc++-v3/testsuite/lib/dg-options.exp
> > > index bc387d17ed7..d9a19dadd7f 100644
> > > --- a/libstdc++-v3/testsuite/lib/dg-options.exp
> > > +++ b/libstdc++-v3/testsuite/lib/dg-options.exp
> > > @@ -337,6 +337,7 @@ proc add_options_for_libatomic { flags } {
> > > || ([istarget powerpc*-*-*] && [check_effective_target_ilp32])
> > > || [istarget riscv*-*-*]
> > > || ([istarget sparc*-*-linux-gnu] && [check_effective_target_ilp32])
> > > + || ([istarget i?86-*-*] || [istarget x86_64-*-*])
> >
> > This seems too overkill as "dg-add-options libatomic" is not intended to
> > handle 16-byte atomics. Maybe we can fork this to a new dg-add-options
> > like "add_options_for_libatomic_16b"?
> >
> > > } {
> > > global TOOL_OPTIONS
> > >
> > > --
> > > 2.25.1
> >
> > --
> > Xi Ruoyao <xry111@xry111.site>
> > School of Aerospace Science and Technology, Xidian University
>
On Tue, Jan 30, 2024 at 04:34:30PM +0000, Jonathan Wakely wrote:
> > --- /dev/null
> > +++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> > @@ -0,0 +1,50 @@
> > +// { dg-do run { target c++20 } }
> > +// { dg-options "-O0" }
> > +// { dg-timeout 10 }
Why dg-timeout?
I don't see it used in any of the libstdc++ tests and I don't see anything
expensive on this test. Just leave it out.
> > +// { dg-additional-options "-mlong-double-80" { target x86_64-*-* } }
> > +// { dg-do run { target { ia32 || x86_64-*-* } } }
Also, the target selectors above look wrong.
Generally, one should use { i?86-*-* x86_64-*-* } because e.g. on some
OSes like Solaris, one builds i?86 compiler which supports -m32 or -m64
options.
I wouldn't add the -mlong-double-80 at all, it is an ABI changing option,
if you rely in the test on some 80-bit long double properties (but on which
ones? Sure, if -mlong-double-64 or -mlong-double-128 is in effect, the
type won't have any padding, but __builtin_clear_padding should reflect
that), it is better to guard the part of the test with some checks, whether
std::numeric_limits<long double>::digits == 64, or
preprocessor __LDBL_MANT_DIG__ == 64.
> > +// { dg-add-options libatomic_16b }
>
> Why not just use -latomic unconditionally here?
>
> You're adding a new libatomic_16b option that expands to -latomic for
> x86 and nothing otherwise, and then using it in a test that only runs
> for x86. So it's just adding -latomic to all targets that run the
> test, right?
>
> Also, this patch needs either a copyright assignment or a DCO sign-off:
> https://gcc.gnu.org/contribute.html#legal
And if somebody would need to commit it for you, also patch in non-garbled
state (e.g. sent as an attachment, or even compressed attachment if using
really bad MUAs).
Jakub
Thanks!
> Why not just use -latomic unconditionally here?
testsuites of libstdc++ seems to have some tricks to find and link
libatomic.a by using "dg-add-options libatomic", which do nothing for
x86 target. In previous email, we decide not to pollute the current
option, so we add a new libatomic_16b option here.
> Why dg-timeout?
without the patch, "as.fetch_add(t);" will result in an infinite loop,
so I add timeout to help it escape. Should I keep it or not?
> Also, the target selectors above look wrong.
Thanks, fixed with checking "std::numeric_limits<long double>::digits == 64".
> This code is not compiled for C++11 and C++14, so this should just use
> constexpr not _GLIBCXX17_CONSTEXPR.
Thanks, fixed with "constexpr".
So here is the fixed patch, please review it again, thanks:
---
libstdc++-v3/ChangeLog:
* include/bits/atomic_base.h: add __builtin_clear_padding in
__atomic_float constructor.
* testsuite/lib/dg-options.exp: add new add-options for libatomic_16b.
* testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
Signed-off-by: xndcn <xndchn@gmail.com>
---
libstdc++-v3/include/bits/atomic_base.h | 7 ++-
.../atomic_float/compare_exchange_padding.cc | 54 +++++++++++++++++++
libstdc++-v3/testsuite/lib/dg-options.exp | 22 ++++++++
3 files changed, 82 insertions(+), 1 deletion(-)
create mode 100644
libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
diff --git a/libstdc++-v3/include/bits/atomic_base.h
b/libstdc++-v3/include/bits/atomic_base.h
index f4ce0fa53..90e3f0e4b 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
constexpr
__atomic_float(_Fp __t) : _M_fp(__t)
- { }
+ {
+#if __has_builtin(__builtin_clear_padding)
+ if constexpr (__atomic_impl::__maybe_has_padding<_Fp>())
+ __builtin_clear_padding(std::__addressof(_M_fp));
+#endif
+ }
__atomic_float(const __atomic_float&) = delete;
__atomic_float& operator=(const __atomic_float&) = delete;
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
new file mode 100644
index 000000000..e6e17e4b5
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
@@ -0,0 +1,54 @@
+// { dg-do run { target c++20 } }
+// { dg-options "-O0" }
+// { dg-timeout 10 }
+// { dg-do run { target { i?86-*-* x86_64-*-* } } }
+// { dg-add-options libatomic_16b }
+
+#include <atomic>
+#include <limits>
+#include <testsuite_hooks.h>
+
+template<typename T>
+void __attribute__((noinline,noipa))
+fill_padding(T& f)
+{
+ T mask;
+ __builtin_memset(&mask, 0xff, sizeof(T));
+ __builtin_clear_padding(&mask);
+ unsigned char* ptr_f = (unsigned char*)&f;
+ unsigned char* ptr_mask = (unsigned char*)&mask;
+ for (int i = 0; i < sizeof(T); i++)
+ {
+ if (ptr_mask[i] == 0x00)
+ {
+ ptr_f[i] = 0xff;
+ }
+ }
+}
+
+void
+test01()
+{
+ // test for long double with padding (float80)
+ if constexpr (std::numeric_limits<long double>::digits == 64)
+ {
+ long double f = 0.5f; // long double may contains padding on X86
+ fill_padding(f);
+ std::atomic<long double> as{ f }; // padding cleared on constructor
+ long double t = 1.5;
+
+ as.fetch_add(t);
+ long double s = f + t;
+ t = as.load();
+ VERIFY(s == t); // padding ignored on float comparing
+ fill_padding(s);
+ VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
+ fill_padding(f);
+ VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
+ }
+}
+
+int main()
+{
+ test01();
+}
diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp
b/libstdc++-v3/testsuite/lib/dg-options.exp
index 850442b6b..75b27a136 100644
--- a/libstdc++-v3/testsuite/lib/dg-options.exp
+++ b/libstdc++-v3/testsuite/lib/dg-options.exp
@@ -356,6 +356,28 @@ proc add_options_for_libatomic { flags } {
return $flags
}
+# Add option to link to libatomic, if required for atomics on 16-byte (128-bit)
+proc add_options_for_libatomic_16b { flags } {
+ if { ([istarget i?86-*-*] || [istarget x86_64-*-*])
+ } {
+ global TOOL_OPTIONS
+
+ set link_flags ""
+ if ![is_remote host] {
+ if [info exists TOOL_OPTIONS] {
+ set link_flags "[atomic_link_flags [get_multilibs ${TOOL_OPTIONS}]]"
+ } else {
+ set link_flags "[atomic_link_flags [get_multilibs]]"
+ }
+ }
+
+ append link_flags " -latomic "
+
+ return "$flags $link_flags"
+ }
+ return $flags
+}
+
# Add options to enable use of deprecated features.
proc add_options_for_using-deprecated { flags } {
return "$flags -U_GLIBCXX_USE_DEPRECATED -D_GLIBCXX_USE_DEPRECATED=1"
On Wed, 31 Jan 2024 at 17:20, xndcn <xndchn@gmail.com> wrote:
>
> Thanks!
>
> > Why not just use -latomic unconditionally here?
> testsuites of libstdc++ seems to have some tricks to find and link
> libatomic.a by using "dg-add-options libatomic", which do nothing for
> x86 target. In previous email, we decide not to pollute the current
> option, so we add a new libatomic_16b option here.
Right, we don't want to change the existing libatomic option. But we
don't need a new one if it's going to be used in exactly one test and
if the new option does the same thing for all targets that run the
test.
However, on that subject, does the test really need to be restricted
to x86? It shouldn't loop on any target, right?
>
> > Why dg-timeout?
> without the patch, "as.fetch_add(t);" will result in an infinite loop,
> so I add timeout to help it escape. Should I keep it or not?
No, because the patch is supposed to prevent the infinite loop, and so
there's no need to stop it looping after 10s. It won't loop at all.
>
> > Also, the target selectors above look wrong.
> Thanks, fixed with checking "std::numeric_limits<long double>::digits == 64".
>
> > This code is not compiled for C++11 and C++14, so this should just use
> > constexpr not _GLIBCXX17_CONSTEXPR.
> Thanks, fixed with "constexpr".
Actually, why are you using the built-in directly in the first place? See below.
>
> So here is the fixed patch, please review it again, thanks:
> ---
> libstdc++-v3/ChangeLog:
>
> * include/bits/atomic_base.h: add __builtin_clear_padding in
> __atomic_float constructor.
> * testsuite/lib/dg-options.exp: add new add-options for libatomic_16b.
> * testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
>
> Signed-off-by: xndcn <xndchn@gmail.com>
> ---
> libstdc++-v3/include/bits/atomic_base.h | 7 ++-
> .../atomic_float/compare_exchange_padding.cc | 54 +++++++++++++++++++
> libstdc++-v3/testsuite/lib/dg-options.exp | 22 ++++++++
> 3 files changed, 82 insertions(+), 1 deletion(-)
> create mode 100644
> libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
>
> diff --git a/libstdc++-v3/include/bits/atomic_base.h
> b/libstdc++-v3/include/bits/atomic_base.h
> index f4ce0fa53..90e3f0e4b 100644
> --- a/libstdc++-v3/include/bits/atomic_base.h
> +++ b/libstdc++-v3/include/bits/atomic_base.h
> @@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> constexpr
> __atomic_float(_Fp __t) : _M_fp(__t)
> - { }
> + {
> +#if __has_builtin(__builtin_clear_padding)
> + if constexpr (__atomic_impl::__maybe_has_padding<_Fp>())
> + __builtin_clear_padding(std::__addressof(_M_fp));
> +#endif
> + }
Why can't this be simply:
{ __atomic_impl::__clear_padding(_M_fp); }
?
We only need to clear padding for long double, not float and double, right?
>
> __atomic_float(const __atomic_float&) = delete;
> __atomic_float& operator=(const __atomic_float&) = delete;
> diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> new file mode 100644
> index 000000000..e6e17e4b5
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
> @@ -0,0 +1,54 @@
> +// { dg-do run { target c++20 } }
> +// { dg-options "-O0" }
> +// { dg-timeout 10 }
> +// { dg-do run { target { i?86-*-* x86_64-*-* } } }
Why can't we run this on all targets?
> +// { dg-add-options libatomic_16b }
Let's just add -latomic to the dg-options line for this one test. We
don't want that in general because we want to know that atomics work
without it in most cases. It should be fine to use it for one test.
> +
> +#include <atomic>
> +#include <limits>
> +#include <testsuite_hooks.h>
> +
> +template<typename T>
> +void __attribute__((noinline,noipa))
> +fill_padding(T& f)
> +{
> + T mask;
> + __builtin_memset(&mask, 0xff, sizeof(T));
There's no reason to use __builtin_memset here, just include <cstring>
and use std::memcpy.
> + __builtin_clear_padding(&mask);
> + unsigned char* ptr_f = (unsigned char*)&f;
> + unsigned char* ptr_mask = (unsigned char*)&mask;
> + for (int i = 0; i < sizeof(T); i++)
> + {
> + if (ptr_mask[i] == 0x00)
> + {
> + ptr_f[i] = 0xff;
> + }
> + }
> +}
> +
> +void
> +test01()
> +{
> + // test for long double with padding (float80)
> + if constexpr (std::numeric_limits<long double>::digits == 64)
> + {
> + long double f = 0.5f; // long double may contains padding on X86
It definitely does have padding, just say "long double has padding bits on x86"
> + fill_padding(f);
> + std::atomic<long double> as{ f }; // padding cleared on constructor
> + long double t = 1.5;
> +
> + as.fetch_add(t);
> + long double s = f + t;
> + t = as.load();
> + VERIFY(s == t); // padding ignored on float comparing
> + fill_padding(s);
> + VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
> + fill_padding(f);
> + VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
> + }
> +}
> +
> +int main()
> +{
> + test01();
> +}
> diff --git a/libstdc++-v3/testsuite/lib/dg-options.exp
> b/libstdc++-v3/testsuite/lib/dg-options.exp
> index 850442b6b..75b27a136 100644
> --- a/libstdc++-v3/testsuite/lib/dg-options.exp
> +++ b/libstdc++-v3/testsuite/lib/dg-options.exp
> @@ -356,6 +356,28 @@ proc add_options_for_libatomic { flags } {
> return $flags
> }
>
> +# Add option to link to libatomic, if required for atomics on 16-byte (128-bit)
> +proc add_options_for_libatomic_16b { flags } {
> + if { ([istarget i?86-*-*] || [istarget x86_64-*-*])
> + } {
> + global TOOL_OPTIONS
> +
> + set link_flags ""
> + if ![is_remote host] {
> + if [info exists TOOL_OPTIONS] {
> + set link_flags "[atomic_link_flags [get_multilibs ${TOOL_OPTIONS}]]"
> + } else {
> + set link_flags "[atomic_link_flags [get_multilibs]]"
> + }
> + }
> +
> + append link_flags " -latomic "
> +
> + return "$flags $link_flags"
> + }
> + return $flags
> +}
> +
> # Add options to enable use of deprecated features.
> proc add_options_for_using-deprecated { flags } {
> return "$flags -U_GLIBCXX_USE_DEPRECATED -D_GLIBCXX_USE_DEPRECATED=1"
> --
> 2.25.1
>
Thank you for your careful review!
> But we don't need a new one if it's going to be used in exactly one test and if the new option does the same thing for all targets that run the test.
Got it, thanks. Now add option "-latomic" directly, but it still rely
on the trick "[atomic_link_flags [get_multilibs]]"
> No, because the patch is supposed to prevent the infinite loop, and so there's no need to stop it looping after 10s. It won't loop at all.
Thanks, deleted.
> We only need to clear padding for long double, not float and double, right?
Yes, actually there is a check "if constexpr
(__atomic_impl::__maybe_has_padding<_Fp>())".
But "__atomic_impl::__clear_padding(_M_fp); " is indeed simply, so fixed here.
> Why can't we run this on all targets?
Got it, now target option deleted.
> There's no reason to use __builtin_memset here, just include <cstring> and use std::memcpy.
Thanks, fixed.
> It definitely does have padding, just say "long double has padding bits on x86"
Thanks, fixed.
So here comes the latest patch:
---
libstdc++-v3/ChangeLog:
* include/bits/atomic_base.h: add __builtin_clear_padding in
__atomic_float constructor.
* testsuite/29_atomics/atomic_float/compare_exchange_padding.cc: New test.
Signed-off-by: xndcn <xndchn@gmail.com>
---
libstdc++-v3/include/bits/atomic_base.h | 2 +-
.../atomic_float/compare_exchange_padding.cc | 53 +++++++++++++++++++
2 files changed, 54 insertions(+), 1 deletion(-)
create mode 100644
libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
diff --git a/libstdc++-v3/include/bits/atomic_base.h
b/libstdc++-v3/include/bits/atomic_base.h
index f4ce0fa53..cdd6f07da 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -1283,7 +1283,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
constexpr
__atomic_float(_Fp __t) : _M_fp(__t)
- { }
+ { __atomic_impl::__clear_padding(_M_fp); }
__atomic_float(const __atomic_float&) = delete;
__atomic_float& operator=(const __atomic_float&) = delete;
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
new file mode 100644
index 000000000..eeace251c
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
@@ -0,0 +1,53 @@
+// { dg-do run { target c++20 } }
+// { dg-options "-O0" }
+// { dg-additional-options "[atomic_link_flags [get_multilibs]] -latomic" }
+
+#include <atomic>
+#include <cstring>
+#include <limits>
+#include <testsuite_hooks.h>
+
+template<typename T>
+void __attribute__((noinline,noipa))
+fill_padding(T& f)
+{
+ T mask;
+ std::memset(&mask, 0xff, sizeof(T));
+ __builtin_clear_padding(&mask);
+ unsigned char* ptr_f = (unsigned char*)&f;
+ unsigned char* ptr_mask = (unsigned char*)&mask;
+ for (int i = 0; i < sizeof(T); i++)
+ {
+ if (ptr_mask[i] == 0x00)
+ {
+ ptr_f[i] = 0xff;
+ }
+ }
+}
+
+void
+test01()
+{
+ // test for long double with padding (float80)
+ if constexpr (std::numeric_limits<long double>::digits == 64)
+ {
+ long double f = 0.5f; // long double has padding bits on x86
+ fill_padding(f);
+ std::atomic<long double> as{ f }; // padding cleared on constructor
+ long double t = 1.5;
+
+ as.fetch_add(t);
+ long double s = f + t;
+ t = as.load();
+ VERIFY(s == t); // padding ignored on float comparing
+ fill_padding(s);
+ VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
+ fill_padding(f);
+ VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
+ }
+}
+
+int main()
+{
+ test01();
+}
On Fri, 2 Feb 2024 at 16:52, xndcn <xndchn@gmail.com> wrote:
>
> Thank you for your careful review!
>
> > But we don't need a new one if it's going to be used in exactly one test and if the new option does the same thing for all targets that run the test.
> Got it, thanks. Now add option "-latomic" directly, but it still rely
> on the trick "[atomic_link_flags [get_multilibs]]"
>
> > No, because the patch is supposed to prevent the infinite loop, and so there's no need to stop it looping after 10s. It won't loop at all.
> Thanks, deleted.
>
> > We only need to clear padding for long double, not float and double, right?
> Yes, actually there is a check "if constexpr
> (__atomic_impl::__maybe_has_padding<_Fp>())".
> But "__atomic_impl::__clear_padding(_M_fp); " is indeed simply, so fixed here.
>
> > Why can't we run this on all targets?
> Got it, now target option deleted.
>
> > There's no reason to use __builtin_memset here, just include <cstring> and use std::memcpy.
> Thanks, fixed.
>
> > It definitely does have padding, just say "long double has padding bits on x86"
> Thanks, fixed.
>
> So here comes the latest patch:
Thanks. I've applied the patch to my tree, but the new test fails
pretty reliably.
The infinite loop in std::atomic<long double>::fetch_add is fixed by
clearing padding in the constructor, but the test fails on the
compare_exchange_weak or compare_exchange_strong lines here:
> + as.fetch_add(t);
> + long double s = f + t;
> + t = as.load();
> + VERIFY(s == t); // padding ignored on float comparing
> + fill_padding(s);
> + VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
> + fill_padding(f);
> + VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
>
I think the problem is here in __atomic_impl::__compare_exchange:
if (__atomic_compare_exchange(__pval, __pexp, __pi,
__is_weak, int(__s), int(__f)))
return true;
Even though padding in *__pexp and *__pi has been cleared, the value
of *__pval after a successful __atomic_compare_exchange has non-zero
padding. That means that the next compare_exchange will fail, because
we assume that the stored value always has zeroed padding bits.
Here's a gdb session showing that __atomic_compare_exchange stores a
value with non-zero padding:
Breakpoint 2, test01 () at compare_exchange_padding.cc:43
43 long double s2 = s;
(gdb) n
44 fill_padding(s2);
(gdb)
45 while (!as.compare_exchange_weak(s2, f)) // padding cleared
on compexchg
(gdb) p/x as._M_fp
$11 = 0x40008000000000000000
(gdb) step
std::__atomic_float<long double>::compare_exchange_weak
(this=0x7fffffffd8c0, __expected=@0x7fffffffd8a0: 2, __desired=0.5,
__order=std::memory_order::seq_cst) at
/home/jwakely/gcc/14/include/c++/14.0.1/bits/atomic_base.h:1387
1387 return compare_exchange_weak(__expected, __desired, __order,
(gdb) step
std::__atomic_float<long double>::compare_exchange_weak
(this=0x7fffffffd8c0, __expected=@0x7fffffffd8a0: 2, __desired=0.5,
__success=std::memory_order::seq_cst,
__failure=std::memory_order::seq_cst) at
/home/jwakely/gcc/14/include/c++/14.0.1/bits/atomic_base.h:1347
1347 return __atomic_impl::compare_exchange_weak(&_M_fp,
(gdb) step
std::__atomic_impl::compare_exchange_weak<false, long double>
(__check_padding=false, __failure=std::memory_order::seq_cst,
__success=std::memory_order::seq_cst, __desired=0.5,
__expected=@0x7fffffffd8a0: 2, __ptr=0x7fffffffd8c0)
at /home/jwakely/gcc/14/include/c++/14.0.1/bits/atomic_base.h:1123
1123 return __atomic_impl::__compare_exchange<_AtomicRef>(
(gdb)
std::__atomic_impl::__compare_exchange<false, long double>
(__f=std::memory_order::seq_cst, __s=std::memory_order::seq_cst,
__is_weak=true,
__i=<optimized out>, __e=@0x7fffffffd8a0: 2,
__val=@0x7fffffffd8c0: 2) at
/home/jwakely/gcc/14/include/c++/14.0.1/bits/atomic_base.h:994
994 __glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
(gdb) n
997 _Tp* const __pval = std::__addressof(__val);
(gdb)
1008 _Vp* const __pi = __atomic_impl::__clear_padding(__i);
(gdb)
1010 _Vp __exp = __e;
(gdb)
1012 _Vp* const __pexp = __atomic_impl::__clear_padding(__exp);
(gdb)
1016 if (__atomic_compare_exchange(__pval, __pexp, __pi,
(gdb) p/x *__pval
$12 = 0x40008000000000000000
(gdb) p/x *__pexp
$13 = 0x40008000000000000000
(gdb) p/x *__pi
$14 = 0x3ffe8000000000000000
(gdb) n
1018 return true;
(gdb) p/x *__pval
$15 = 0x7ffff7bf3ffe8000000000000000
(gdb)
We stored *__pi which has zero padding, but the result in *__pval has
non-zero padding. This doesn't seem to be gdb being misleading by
loading *__pval into a FP register which doesn't preserve the zero
padding, because if I do this then it fails:
as.fetch_add(t);
VERIFY(as.load() == s);
__builtin_clear_padding(&s);
VERIFY( std::memcmp(&s, &as, sizeof(s)) == 0 );
So the value stored by fetch_add (which uses compare_exchange_weak in
a loop) really doesn't have zero padding bits.
I'm not sure what's going on here yet, maybe __atomic_compare_exchange
recognizes that the pointers are long double* and so only copies the
non-padding bits into the value?
I think I'll push the fix to the __atomic_float constructor, but with
a simplified test case that doesn't include the
compare_exchange_{weak,strong} calls, until I figure out how to fix
it.
On Fri, 16 Feb 2024 at 12:38, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On Fri, 2 Feb 2024 at 16:52, xndcn <xndchn@gmail.com> wrote:
> >
> > Thank you for your careful review!
> >
> > > But we don't need a new one if it's going to be used in exactly one test and if the new option does the same thing for all targets that run the test.
> > Got it, thanks. Now add option "-latomic" directly, but it still rely
> > on the trick "[atomic_link_flags [get_multilibs]]"
> >
> > > No, because the patch is supposed to prevent the infinite loop, and so there's no need to stop it looping after 10s. It won't loop at all.
> > Thanks, deleted.
> >
> > > We only need to clear padding for long double, not float and double, right?
> > Yes, actually there is a check "if constexpr
> > (__atomic_impl::__maybe_has_padding<_Fp>())".
> > But "__atomic_impl::__clear_padding(_M_fp); " is indeed simply, so fixed here.
> >
> > > Why can't we run this on all targets?
> > Got it, now target option deleted.
> >
> > > There's no reason to use __builtin_memset here, just include <cstring> and use std::memcpy.
> > Thanks, fixed.
> >
> > > It definitely does have padding, just say "long double has padding bits on x86"
> > Thanks, fixed.
> >
> > So here comes the latest patch:
>
>
> Thanks. I've applied the patch to my tree, but the new test fails
> pretty reliably.
>
> The infinite loop in std::atomic<long double>::fetch_add is fixed by
> clearing padding in the constructor, but the test fails on the
> compare_exchange_weak or compare_exchange_strong lines here:
>
>
> > + as.fetch_add(t);
> > + long double s = f + t;
> > + t = as.load();
> > + VERIFY(s == t); // padding ignored on float comparing
> > + fill_padding(s);
> > + VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
> > + fill_padding(f);
> > + VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
> >
>
>
> I think the problem is here in __atomic_impl::__compare_exchange:
>
> if (__atomic_compare_exchange(__pval, __pexp, __pi,
> __is_weak, int(__s), int(__f)))
> return true;
>
> Even though padding in *__pexp and *__pi has been cleared, the value
> of *__pval after a successful __atomic_compare_exchange has non-zero
> padding. That means that the next compare_exchange will fail, because
> we assume that the stored value always has zeroed padding bits.
>
> Here's a gdb session showing that __atomic_compare_exchange stores a
> value with non-zero padding:
>
> Breakpoint 2, test01 () at compare_exchange_padding.cc:43
> 43 long double s2 = s;
> (gdb) n
> 44 fill_padding(s2);
> (gdb)
> 45 while (!as.compare_exchange_weak(s2, f)) // padding cleared
> on compexchg
> (gdb) p/x as._M_fp
> $11 = 0x40008000000000000000
> (gdb) step
> std::__atomic_float<long double>::compare_exchange_weak
> (this=0x7fffffffd8c0, __expected=@0x7fffffffd8a0: 2, __desired=0.5,
> __order=std::memory_order::seq_cst) at
> /home/jwakely/gcc/14/include/c++/14.0.1/bits/atomic_base.h:1387
> 1387 return compare_exchange_weak(__expected, __desired, __order,
> (gdb) step
> std::__atomic_float<long double>::compare_exchange_weak
> (this=0x7fffffffd8c0, __expected=@0x7fffffffd8a0: 2, __desired=0.5,
> __success=std::memory_order::seq_cst,
> __failure=std::memory_order::seq_cst) at
> /home/jwakely/gcc/14/include/c++/14.0.1/bits/atomic_base.h:1347
> 1347 return __atomic_impl::compare_exchange_weak(&_M_fp,
> (gdb) step
> std::__atomic_impl::compare_exchange_weak<false, long double>
> (__check_padding=false, __failure=std::memory_order::seq_cst,
> __success=std::memory_order::seq_cst, __desired=0.5,
> __expected=@0x7fffffffd8a0: 2, __ptr=0x7fffffffd8c0)
> at /home/jwakely/gcc/14/include/c++/14.0.1/bits/atomic_base.h:1123
> 1123 return __atomic_impl::__compare_exchange<_AtomicRef>(
> (gdb)
> std::__atomic_impl::__compare_exchange<false, long double>
> (__f=std::memory_order::seq_cst, __s=std::memory_order::seq_cst,
> __is_weak=true,
> __i=<optimized out>, __e=@0x7fffffffd8a0: 2,
> __val=@0x7fffffffd8c0: 2) at
> /home/jwakely/gcc/14/include/c++/14.0.1/bits/atomic_base.h:994
> 994 __glibcxx_assert(__is_valid_cmpexch_failure_order(__f));
> (gdb) n
> 997 _Tp* const __pval = std::__addressof(__val);
> (gdb)
> 1008 _Vp* const __pi = __atomic_impl::__clear_padding(__i);
> (gdb)
> 1010 _Vp __exp = __e;
> (gdb)
> 1012 _Vp* const __pexp = __atomic_impl::__clear_padding(__exp);
> (gdb)
> 1016 if (__atomic_compare_exchange(__pval, __pexp, __pi,
> (gdb) p/x *__pval
> $12 = 0x40008000000000000000
> (gdb) p/x *__pexp
> $13 = 0x40008000000000000000
> (gdb) p/x *__pi
> $14 = 0x3ffe8000000000000000
> (gdb) n
> 1018 return true;
> (gdb) p/x *__pval
> $15 = 0x7ffff7bf3ffe8000000000000000
> (gdb)
>
> We stored *__pi which has zero padding, but the result in *__pval has
> non-zero padding. This doesn't seem to be gdb being misleading by
> loading *__pval into a FP register which doesn't preserve the zero
> padding, because if I do this then it fails:
>
> as.fetch_add(t);
> VERIFY(as.load() == s);
> __builtin_clear_padding(&s);
> VERIFY( std::memcmp(&s, &as, sizeof(s)) == 0 );
>
> So the value stored by fetch_add (which uses compare_exchange_weak in
> a loop) really doesn't have zero padding bits.
>
> I'm not sure what's going on here yet, maybe __atomic_compare_exchange
> recognizes that the pointers are long double* and so only copies the
> non-padding bits into the value?
Ah, although __atomic_compare_exchange only takes pointers, the
compiler replaces that with a call to __atomic_compare_exchange_n
which takes the newval by value, which presumably uses an 80-bit FP
register and so the padding bits become indeterminate again.
Maybe we can make std::atomic<long double> actually store a 16-byte
struct containing an opaque char buffer, which we store a long double
into. That should stop FP registers being used for it. We never need
an actual long double, because we only access the value using the
__atomic_xxx built-ins.
Maybe we need that for every std::atomic<T> where T has padding bits,
because even for struct S { int i; char c; } passing S by value could
be scalarized and only copy the value representation, not preserve the
padding that we've carefully cleared.
>
> I think I'll push the fix to the __atomic_float constructor, but with
> a simplified test case that doesn't include the
> compare_exchange_{weak,strong} calls, until I figure out how to fix
> it.
On Fri, Feb 16, 2024 at 01:51:54PM +0000, Jonathan Wakely wrote:
> Ah, although __atomic_compare_exchange only takes pointers, the
> compiler replaces that with a call to __atomic_compare_exchange_n
> which takes the newval by value, which presumably uses an 80-bit FP
> register and so the padding bits become indeterminate again.
__atomic_compare_exchange_n only works with integers, so I guess
it is doing VIEW_CONVERT_EXPR (aka union-style type punning) on the
argument.
Do you have preprocessed source for the testcase?
Jakub
On Fri, 16 Feb 2024 at 14:10, Jakub Jelinek wrote:
>
> On Fri, Feb 16, 2024 at 01:51:54PM +0000, Jonathan Wakely wrote:
> > Ah, although __atomic_compare_exchange only takes pointers, the
> > compiler replaces that with a call to __atomic_compare_exchange_n
> > which takes the newval by value, which presumably uses an 80-bit FP
> > register and so the padding bits become indeterminate again.
>
> __atomic_compare_exchange_n only works with integers, so I guess
> it is doing VIEW_CONVERT_EXPR (aka union-style type punning) on the
> argument.
>
> Do you have preprocessed source for the testcase?
Sent offlist.
On Fri, 16 Feb 2024 at 15:15, Jonathan Wakely wrote:
>
> On Fri, 16 Feb 2024 at 14:10, Jakub Jelinek wrote:
> >
> > On Fri, Feb 16, 2024 at 01:51:54PM +0000, Jonathan Wakely wrote:
> > > Ah, although __atomic_compare_exchange only takes pointers, the
> > > compiler replaces that with a call to __atomic_compare_exchange_n
> > > which takes the newval by value, which presumably uses an 80-bit FP
> > > register and so the padding bits become indeterminate again.
> >
> > __atomic_compare_exchange_n only works with integers, so I guess
> > it is doing VIEW_CONVERT_EXPR (aka union-style type punning) on the
> > argument.
> >
> > Do you have preprocessed source for the testcase?
>
> Sent offlist.
Jakub fixed the compiler, so I've pushed the attached patch now.
Tested x86_64-linux.
commit 0adc8c5f146b108f99c4df09e43276e3a2419262
Author: xndcn <xndchn@gmail.com>
Date: Fri Feb 16 11:00:13 2024
libstdc++: Add missing clear_padding in __atomic_float constructor
For 80-bit long double we need to clear the padding bits on
construction.
libstdc++-v3/ChangeLog:
* include/bits/atomic_base.h (__atomic_float::__atomic_float(Fp)):
Clear padding.
* testsuite/29_atomics/atomic_float/compare_exchange_padding.cc:
New test.
Signed-off-by: xndcn <xndchn@gmail.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
diff --git a/libstdc++-v3/include/bits/atomic_base.h b/libstdc++-v3/include/bits/atomic_base.h
index b857b441169..dd360302f80 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -1283,7 +1283,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
constexpr
__atomic_float(_Fp __t) : _M_fp(__t)
- { }
+ { __atomic_impl::__clear_padding(_M_fp); }
__atomic_float(const __atomic_float&) = delete;
__atomic_float& operator=(const __atomic_float&) = delete;
diff --git a/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
new file mode 100644
index 00000000000..49626ac6651
--- /dev/null
+++ b/libstdc++-v3/testsuite/29_atomics/atomic_float/compare_exchange_padding.cc
@@ -0,0 +1,53 @@
+// { dg-do run { target c++20 } }
+// { dg-options "-O0" }
+// { dg-additional-options "[atomic_link_flags [get_multilibs]] -latomic" }
+
+#include <atomic>
+#include <cstring>
+#include <limits>
+#include <testsuite_hooks.h>
+
+template<typename T>
+void __attribute__((noinline,noipa))
+fill_padding(T& f)
+{
+ T mask;
+ std::memset(&mask, 0xff, sizeof(T));
+ __builtin_clear_padding(&mask);
+ unsigned char* ptr_f = (unsigned char*)&f;
+ unsigned char* ptr_mask = (unsigned char*)&mask;
+ for (unsigned i = 0; i < sizeof(T); i++)
+ {
+ if (ptr_mask[i] == 0x00)
+ {
+ ptr_f[i] = 0xff;
+ }
+ }
+}
+
+void
+test01()
+{
+ // test for long double with padding (float80)
+ if constexpr (std::numeric_limits<long double>::digits == 64)
+ {
+ long double f = 0.5f; // long double has padding bits on x86
+ fill_padding(f);
+ std::atomic<long double> as{ f }; // padding cleared on constructor
+ long double t = 1.5;
+
+ as.fetch_add(t);
+ long double s = f + t;
+ t = as.load();
+ VERIFY(s == t); // padding ignored on comparison
+ fill_padding(s);
+ VERIFY(as.compare_exchange_weak(s, f)); // padding cleared on cmpexchg
+ fill_padding(f);
+ VERIFY(as.compare_exchange_strong(f, t)); // padding cleared on cmpexchg
+ }
+}
+
+int main()
+{
+ test01();
+}
Wow, thank you all, you guys!
在 2024年3月14日星期四，Jonathan Wakely <jwakely@redhat.com> 写道：
> On Fri, 16 Feb 2024 at 15:15, Jonathan Wakely wrote:
> >
> > On Fri, 16 Feb 2024 at 14:10, Jakub Jelinek wrote:
> > >
> > > On Fri, Feb 16, 2024 at 01:51:54PM +0000, Jonathan Wakely wrote:
> > > > Ah, although __atomic_compare_exchange only takes pointers, the
> > > > compiler replaces that with a call to __atomic_compare_exchange_n
> > > > which takes the newval by value, which presumably uses an 80-bit FP
> > > > register and so the padding bits become indeterminate again.
> > >
> > > __atomic_compare_exchange_n only works with integers, so I guess
> > > it is doing VIEW_CONVERT_EXPR (aka union-style type punning) on the
> > > argument.
> > >
> > > Do you have preprocessed source for the testcase?
> >
> > Sent offlist.
>
> Jakub fixed the compiler, so I've pushed the attached patch now.
>
> Tested x86_64-linux.
>
b/libstdc++-v3/include/bits/atomic_base.h
@@ -1283,7 +1283,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
constexpr
__atomic_float(_Fp __t) : _M_fp(__t)
- { }
+ {
+#if __has_builtin(__builtin_clear_padding)
+ if _GLIBCXX17_CONSTEXPR (__atomic_impl::__maybe_has_padding<_Fp>())
+ __builtin_clear_padding(std::__addressof(_M_fp));
+#endif
+ }
__atomic_float(const __atomic_float&) = delete;
__atomic_float& operator=(const __atomic_float&) = delete;