diff mbox series

[committed] libstdc++: Fix undefined shift when _Atomic_word is 64-bit

Message ID CACb0b4kFPDWmG-u-QMznjWwbCn53etpq16X2F0=Lr3ftQzX-6w@mail.gmail.com
State Committed
Commit c15aa46cca0649b68613d3292cf71c7cc57ef78f
Headers
Series [committed] libstdc++: Fix undefined shift when _Atomic_word is 64-bit |

Commit Message

Jonathan Wakely Dec. 8, 2021, 11:47 p.m. UTC 
  On Wed, 8 Dec 2021 at 19:27, Jonathan Wakely wrote:
>
> On Wed, 8 Dec 2021 at 19:21, Jonathan Wakely wrote:
> >
> > On Wed, 8 Dec 2021 at 19:17, Rainer Orth wrote:
> > >
> > > Hi Jonathan,
> > >
> > > > I've pushed this change to trunk now (it was posted and reviewed in
> > > > stage 1, I just didn't get around to pushing it until now).
> > > >
> > > > The final version of the patch is attached to this mail.
> > >
> > > unfortunately, it breaks Solaris/SPARC bootstrap:
> > >
> > > In file included from /var/gcc/regression/master/11.4-gcc-gas/build/sparc-sun-solaris2.11/sparcv9/libstdc++-v3/include/bits/shared_ptr.h:53,
> > >                  from /var/gcc/regression/master/11.4-gcc-gas/build/sparc-sun-solaris2.11/sparcv9/libstdc++-v3/include/memory:77,
> > >                  from /vol/gcc/src/hg/master/local/libstdc++-v3/include/precompiled/stdc++.h:82:
> > > /var/gcc/regression/master/11.4-gcc-gas/build/sparc-sun-solaris2.11/sparcv9/libstdc++-v3/include/bits/shared_ptr_base.h: In member function 'void std::_Sp_counted_base<_Lp>::_M_release() [with __gnu_cxx::_Lock_policy _Lp = __gnu_cxx::_S_atomic]':
> > > /var/gcc/regression/master/11.4-gcc-gas/build/sparc-sun-solaris2.11/sparcv9/libstdc++-v3/include/bits/shared_ptr_base.h:329:26: error: right operand of shift expression '(1 << 64)' is greater than or equal to the precision 64 of the left operand [-fpermissive]
> > >   329 |             = 1LL + (1LL << (__CHAR_BIT__ * sizeof(_Atomic_word)));
> > >       |                     ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > make[9]: *** [Makefile:1875: sparc-sun-solaris2.11/bits/stdc++.h.gch/O2ggnu++0x.gch] Error 1
> > >
> > > For 64-bit SPARC, _Atomic_word is long.
> >
> > Ah yes, so we need to disable this optimization. Patch coming up ...
>
> Gah, I remembered to check that:
>
>       constexpr bool __double_word
>         = sizeof(long long) == 2 * sizeof(_Atomic_word);
>       // The ref-count members follow the vptr, so are aligned to
>       // alignof(void*).
>       constexpr bool __aligned = __alignof(long long) <= alignof(void*);
>       if _GLIBCXX17_CONSTEXPR (__lock_free && __double_word && __aligned)
>
>
> But for C++11 and C++14 that is a normal runtime condition not
> if-constexpr, so the undefined shift still gets compiled, even though
> it can never be reached at runtime.

Fixed like so. Tested sparc-sun-solaris2.11, pushed to trunk.
commit c15aa46cca0649b68613d3292cf71c7cc57ef78f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Dec 8 19:36:24 2021

    libstdc++: Fix undefined shift when _Atomic_word is 64-bit
    
    The check for _Atomic_word being 32-bit is just a normal runtime
    condition for C++11 and C++14, because it doesn't use if-constexpr. That
    means the 1LL << (CHAR_BIT * sizeof(_Atomic_word)) expression expands to
    1LL << 64 on Solaris, which is ill-formed.
    
    This adds another indirection so that the shift width is zero if the
    code is unreachable.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/shared_ptr_base.h (_Sp_counted_base::_M_release()):
            Make shift width conditional on __double_word condition.

Comments

Rainer Orth Dec. 9, 2021, 12:28 p.m. UTC | #1 
Hi Jonathan,

>> > Ah yes, so we need to disable this optimization. Patch coming up ...
>>
>> Gah, I remembered to check that:
>>
>>       constexpr bool __double_word
>>         = sizeof(long long) == 2 * sizeof(_Atomic_word);
>>       // The ref-count members follow the vptr, so are aligned to
>>       // alignof(void*).
>>       constexpr bool __aligned = __alignof(long long) <= alignof(void*);
>>       if _GLIBCXX17_CONSTEXPR (__lock_free && __double_word && __aligned)
>>
>>
>> But for C++11 and C++14 that is a normal runtime condition not
>> if-constexpr, so the undefined shift still gets compiled, even though
>> it can never be reached at runtime.
>
> Fixed like so. Tested sparc-sun-solaris2.11, pushed to trunk.

great, thanks a lot.

	Rainer
diff mbox series

Patch

diff --git a/libstdc++-v3/include/bits/shared_ptr_base.h b/libstdc++-v3/include/bits/shared_ptr_base.h
index 90ad30947b0..f315d8f354f 100644
--- a/libstdc++-v3/include/bits/shared_ptr_base.h
+++ b/libstdc++-v3/include/bits/shared_ptr_base.h
@@ -325,8 +325,9 @@  _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr bool __aligned = __alignof(long long) <= alignof(void*);
       if _GLIBCXX17_CONSTEXPR (__lock_free && __double_word && __aligned)
 	{
-	  constexpr long long __unique_ref
-	    = 1LL + (1LL << (__CHAR_BIT__ * sizeof(_Atomic_word)));
+	  constexpr int __wordbits = __CHAR_BIT__ * sizeof(_Atomic_word);
+	  constexpr int __shiftbits = __double_word ? __wordbits : 0;
+	  constexpr long long __unique_ref = 1LL + (1LL << __shiftbits);
 	  auto __both_counts = reinterpret_cast<long long*>(&_M_use_count);
 
 	  _GLIBCXX_SYNCHRONIZATION_HAPPENS_BEFORE(&_M_weak_count);