String: Add support for __memcmpeq() ABI on all targets

No bug.

This commit adds support for __memcmpeq() as a new ABI for all
targets. In this commit __memcmpeq() is implemented only as an alias
to the corresponding targets memcmp() implementation. __memcmpeq() is
added as a new symbol starting with GLIBC_2.35 and defined in string.h
with comments explaining its behavior. Basic tests that it is callable
and works where added in string/tester.c

As discussed in the proposal "Add new ABI '__memcmpeq()' to libc"
__memcmpeq() is essentially a reserved namespace for bcmp(). The means
is shares the same specifications as memcmp() except the return value
for non-equal byte sequences is any non-zero value. This is less
strict than memcmp()'s return value specification and can be better
optimized when a boolean return is all that is needed.

__memcmpeq() is meant to only be called by compilers if they can prove
that the return value of a memcmp() call is only used for its boolean
value.

All tests in string/tester.c passed. As well build succeeds on
x86_64-linux-gnu target.

1 files changed, 2 insertions, 0 deletions

diff --git a/sysdeps/x86_64/memcmp.S b/sysdeps/x86_64/memcmp.S
index 870e15c5a0..1b351ee092 100644
--- a/sysdeps/x86_64/memcmp.S
+++ b/sysdeps/x86_64/memcmp.S
@@ -358,4 +358,6 @@ END(memcmp)
 
 #undef bcmp
 weak_alias (memcmp, bcmp)
+#undef __memcmpeq
+strong_alias (memcmp, __memcmpeq)
 libc_hidden_builtin_def (memcmp)