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, 16 insertions, 0 deletions

diff --git a/string/string.h b/string/string.h
index b1b083edce..e70feeaeea 100644
--- a/string/string.h
+++ b/string/string.h
@@ -64,6 +64,22 @@ extern void *memset (void *__s, int __c, size_t __n) __THROW __nonnull ((1));
 extern int memcmp (const void *__s1, const void *__s2, size_t __n)
      __THROW __attribute_pure__ __nonnull ((1, 2));
 
+/* Compare N bytes of S1 and S2.  Return zero if S1 and S2 are equal.
+   Return some non-zero value otherwise.
+
+   Essentially __memcmpeq has the exact same semantics as memcmp
+   except the return value is less constrained.  memcmp is always a
+   correct implementation of __memcmpeq.  As well !!memcmp, -memcmp,
+   or bcmp are correct implementations.
+
+   __memcmpeq is meant to be used by compilers when memcmp return is
+   only used for its bolean value.
+
+   __memcmpeq is declared only for use by compilers.  Programs should
+   continue to use memcmp.  */
+extern int __memcmpeq (const void *__s1, const void *__s2, size_t __n)
+     __THROW __attribute_pure__ __nonnull ((1, 2));
+
 /* Search N bytes of S for C.  */
 #ifdef __CORRECT_ISO_CPP_STRING_H_PROTO
 extern "C++"