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.

4 files changed, 38 insertions, 0 deletions

diff --git a/string/Versions b/string/Versions
index 298ecd401a..864c4cf7a4 100644
--- a/string/Versions
+++ b/string/Versions
@@ -89,4 +89,7 @@ libc {
     sigdescr_np; sigabbrev_np;
     strerrordesc_np; strerrorname_np;
   }
+  GLIBC_2.35 {
+    __memcmpeq;
+  }
 }
diff --git a/string/memcmp.c b/string/memcmp.c
index 9b46d7a905..eac4112530 100644
--- a/string/memcmp.c
+++ b/string/memcmp.c
@@ -359,3 +359,6 @@ libc_hidden_builtin_def(memcmp)
 # undef bcmp
 weak_alias (memcmp, bcmp)
 #endif
+
+#undef __memcmpeq
+strong_alias (memcmp, __memcmpeq)
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++"
diff --git a/string/tester.c b/string/tester.c
index 778160ae6e..605b3f00f9 100644
--- a/string/tester.c
+++ b/string/tester.c
@@ -1450,6 +1450,19 @@ test_bcmp (void)
 }
 
 static void
+test_memcmpeq (void)
+{
+  it = "__memcmpeq";
+  check (__memcmpeq ("a", "a", 1) == 0, 1); /* Identity.  */
+  check (__memcmpeq ("abc", "abc", 3) == 0, 2); /* Multicharacter.  */
+  check (__memcmpeq ("abcd", "abce", 4) != 0, 3); /* Honestly unequal.  */
+  check (__memcmpeq ("abce", "abcd", 4) != 0, 4);
+  check (__memcmpeq ("alph", "beta", 4) != 0, 5);
+  check (__memcmpeq ("abce", "abcd", 3) == 0, 6); /* Count limited.  */
+  check (__memcmpeq ("abc", "def", 0) == 0, 8); /* Zero count.  */
+}
+
+static void
 test_strerror (void)
 {
   it = "strerror";
@@ -1611,6 +1624,9 @@ main (void)
   /* bcmp - somewhat like memcmp.  */
   test_bcmp ();
 
+  /* __memcmpeq - somewhat like memcmp.  */
+  test_memcmpeq ();
+
   /* strndup.  */
   test_strndup ();