diff mbox series

config: Remove HAVE_BUILTIN_MEMSET macro

Message ID 20210726083433.385572-1-naohirot@fujitsu.com
State Rejected
Headers show
Series config: Remove HAVE_BUILTIN_MEMSET macro | expand

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Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent
dj/TryBot-32bit success Build for i686

Commit Message

Naohiro Tamura July 26, 2021, 8:34 a.m. UTC
s patch removed HAVE_BUILTIN_MEMSET macro because GCC 6.2 that is
minimum requirement to compile glibc already support
__builtin_memset()[1].

Interestingly, removed code had a critical bug that is
HAVE_BUILTIN_MEMSET macro never be defined, because yes/no assignment
to libc_cv_gcc_builtin_memset was reversed in configure.ac as below:

1519 if AC_TRY_COMMAND([${CC-cc} -O3 -S conftest.c -o - | grep -F "memset" > /dev/null]);
1520 then
1521   libc_cv_gcc_builtin_memset=no   # shold be yes
1522 else
1523   libc_cv_gcc_builtin_memset=yes  # should be no
1524 fi
1525 rm -f conftest* ])
1526 if test "$libc_cv_gcc_builtin_memset" = yes ; then
1527   AC_DEFINE(HAVE_BUILTIN_MEMSET)
1528 fi

Therefor __builtin_memset() in elf/rtld.c was never be compiled.

 534 # ifdef HAVE_BUILTIN_MEMSET
 535   __builtin_memset (bootstrap_map.l_info, '\0', sizeof (bootstrap_map.l_info));
 536 # else

[1] https://gcc.gnu.org/onlinedocs/gcc-6.2.0/gcc/Other-Builtins.html
---
 config.h.in  |  3 ---
 configure    | 31 -------------------------------
 configure.ac | 19 -------------------
 elf/rtld.c   | 15 ++++-----------
 4 files changed, 4 insertions(+), 64 deletions(-)

Comments

naohirot--- via Libc-alpha July 26, 2021, 8:48 a.m. UTC | #1
I'll fix the typo

> s patch removed HAVE_BUILTIN_MEMSET macro because GCC 6.2 that is

s/s/This/

> minimum requirement to compile glibc already support
>__builtin_memset()[1].

Thanks.
Naohiro
Andreas Schwab July 26, 2021, 8:49 a.m. UTC | #2
On Jul 26 2021, Naohiro Tamura via Libc-alpha wrote:

> Interestingly, removed code had a critical bug that is
> HAVE_BUILTIN_MEMSET macro never be defined, because yes/no assignment
> to libc_cv_gcc_builtin_memset was reversed in configure.ac as below:

No, the point of the check is that __buildin_memset does *not* expand to
a memset libcall, as explained in the comment you removed.

Andreas.
naohirot--- via Libc-alpha July 26, 2021, 9:42 a.m. UTC | #3
Hi Andreas,

Thank you for the comment.

> From: Andreas Schwab <schwab@linux-m68k.org>

> On Jul 26 2021, Naohiro Tamura via Libc-alpha wrote:
> 
> > Interestingly, removed code had a critical bug that is
> > HAVE_BUILTIN_MEMSET macro never be defined, because yes/no assignment
> > to libc_cv_gcc_builtin_memset was reversed in configure.ac as below:
> 
> No, the point of the check is that __buildin_memset does *not* expand to
> a memset libcall, as explained in the comment you removed.
> 

Is the comment you mentioned below?

   /* Partly clean the `bootstrap_map' structure up.  Don't use
      `memset' since it might not be built in or inlined and we cannot
-     make function calls at this point.  Use '__builtin_memset' if we
-     know it is available.  We do not have to clear the memory if we
-     do not have to use the temporary bootstrap_map.  Global variables
-     are initialized to zero by default.  */
+     make function calls at this point.  Use '__builtin_memset' instead.
+     We do not have to clear the memory if we do not have to use the
+     temporary bootstrap_map.  Global variables are initialized to zero
+     by default.  */

Do you mean that yes/no assignment to libc_cv_gcc_builtin_memset was NOT reversed?
If it was not reversed, config.h never had " #define HAVE_BUILTIN_MEMSET 1" 
even if gcc 8.3 is used.

Thanks.
Naohiro
Andreas Schwab July 26, 2021, 9:51 a.m. UTC | #4
On Jul 26 2021, naohirot@fujitsu.com wrote:

> If it was not reversed, config.h never had " #define HAVE_BUILTIN_MEMSET 1" 
> even if gcc 8.3 is used.

This is correct if the builtin just expands to a memset libcall.

Andreas.
naohirot--- via Libc-alpha July 26, 2021, 1:16 p.m. UTC | #5
Hi Andreas,

Thanks for the explanation.

> From: Andreas Schwab <schwab@linux-m68k.org>

> > If it was not reversed, config.h never had " #define HAVE_BUILTIN_MEMSET 1"
> > even if gcc 8.3 is used.
>
> This is correct if the builtin just expands to a memset libcall.

Now I understood that by looking at the grep argument again in the line below:

1519 if AC_TRY_COMMAND([${CC-cc} -O3 -S conftest.c -o - | grep -F "memset" > /dev/null]);

So the macro name "HAVE_BUILTIN_MEMSET" is very confusing,
since GCC 6.2 manual mentions that __builtin_memset is supported.
Can we agree to change it to "HAVE_NON_LIB_EXPAND_BUILTIN_MEMSET"  or "HAVE_NON_LIBCALL_BUILTIN_MEMSET"?

Thanks.
Naohiro
diff mbox series

Patch

diff --git a/config.h.in b/config.h.in
index 8b45a3a61d77..4647632f2632 100644
--- a/config.h.in
+++ b/config.h.in
@@ -40,9 +40,6 @@ 
    shared between GNU libc and GNU gettext projects.  */
 #define HAVE_BUILTIN_EXPECT 1
 
-/* Define if the compiler supports __builtin_memset.  */
-#undef	HAVE_BUILTIN_MEMSET
-
 /* Define if compiler accepts -ftree-loop-distribute-patterns.  */
 #undef  HAVE_CC_INHIBIT_LOOP_TO_LIBCALL
 
diff --git a/configure b/configure
index 9619c10991d0..6f85d28ea085 100755
--- a/configure
+++ b/configure
@@ -6261,37 +6261,6 @@  if test $libc_cv_have_section_quotes = yes; then
 
 fi
 
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for __builtin_memset" >&5
-$as_echo_n "checking for __builtin_memset... " >&6; }
-if ${libc_cv_gcc_builtin_memset+:} false; then :
-  $as_echo_n "(cached) " >&6
-else
-  cat > conftest.c <<\EOF
-void zero (void *x)
-{
-  __builtin_memset (x, 0, 1000);
-}
-EOF
-if { ac_try='${CC-cc} -O3 -S conftest.c -o - | grep -F "memset" > /dev/null'
-  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
-  (eval $ac_try) 2>&5
-  ac_status=$?
-  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
-  test $ac_status = 0; }; };
-then
-  libc_cv_gcc_builtin_memset=no
-else
-  libc_cv_gcc_builtin_memset=yes
-fi
-rm -f conftest*
-fi
-{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_gcc_builtin_memset" >&5
-$as_echo "$libc_cv_gcc_builtin_memset" >&6; }
-if test "$libc_cv_gcc_builtin_memset" = yes ; then
-  $as_echo "#define HAVE_BUILTIN_MEMSET 1" >>confdefs.h
-
-fi
-
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for redirection of built-in functions" >&5
 $as_echo_n "checking for redirection of built-in functions... " >&6; }
 if ${libc_cv_gcc_builtin_redirection+:} false; then :
diff --git a/configure.ac b/configure.ac
index 34ecbba54054..0c5ee6623c4c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1508,25 +1508,6 @@  if test $libc_cv_have_section_quotes = yes; then
   AC_DEFINE(HAVE_SECTION_QUOTES)
 fi
 
-AC_CACHE_CHECK(for __builtin_memset, libc_cv_gcc_builtin_memset, [dnl
-cat > conftest.c <<\EOF
-void zero (void *x)
-{
-  __builtin_memset (x, 0, 1000);
-}
-EOF
-dnl
-if AC_TRY_COMMAND([${CC-cc} -O3 -S conftest.c -o - | grep -F "memset" > /dev/null]);
-then
-  libc_cv_gcc_builtin_memset=no
-else
-  libc_cv_gcc_builtin_memset=yes
-fi
-rm -f conftest* ])
-if test "$libc_cv_gcc_builtin_memset" = yes ; then
-  AC_DEFINE(HAVE_BUILTIN_MEMSET)
-fi
-
 AC_CACHE_CHECK(for redirection of built-in functions, libc_cv_gcc_builtin_redirection, [dnl
 cat > conftest.c <<\EOF
 extern char *strstr (const char *, const char *) __asm ("my_strstr");
diff --git a/elf/rtld.c b/elf/rtld.c
index d733359eaf80..d0da99bd6d78 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -526,19 +526,12 @@  _dl_start (void *arg)
 
   /* Partly clean the `bootstrap_map' structure up.  Don't use
      `memset' since it might not be built in or inlined and we cannot
-     make function calls at this point.  Use '__builtin_memset' if we
-     know it is available.  We do not have to clear the memory if we
-     do not have to use the temporary bootstrap_map.  Global variables
-     are initialized to zero by default.  */
+     make function calls at this point.  Use '__builtin_memset' instead.
+     We do not have to clear the memory if we do not have to use the
+     temporary bootstrap_map.  Global variables are initialized to zero
+     by default.  */
 #ifndef DONT_USE_BOOTSTRAP_MAP
-# ifdef HAVE_BUILTIN_MEMSET
   __builtin_memset (bootstrap_map.l_info, '\0', sizeof (bootstrap_map.l_info));
-# else
-  for (size_t cnt = 0;
-       cnt < sizeof (bootstrap_map.l_info) / sizeof (bootstrap_map.l_info[0]);
-       ++cnt)
-    bootstrap_map.l_info[cnt] = 0;
-# endif
 #endif
 
   /* Figure out the run-time load address of the dynamic linker itself.  */