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

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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.
This commit is contained in:
Noah Goldstein
2021-10-21 15:54:57 -05:00
parent 8438135d34
commit 44829b3ddb
64 changed files with 120 additions and 0 deletions
Download Patch File Download Diff File Expand all files Collapse all files

3
string/Versions
View File

@@ -89,4 +89,7 @@ libc {
sigdescr_np; sigabbrev_np;
strerrordesc_np; strerrorname_np;
}
GLIBC_2.35 {
__memcmpeq;
}
}

3
string/memcmp.c
View File

@@ -359,3 +359,6 @@ libc_hidden_builtin_def(memcmp)
# undef bcmp
weak_alias (memcmp, bcmp)
#endif
#undef __memcmpeq
strong_alias (memcmp, __memcmpeq)

16
string/string.h
View File

@@ -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++"

16
string/tester.c
View File

@@ -1449,6 +1449,19 @@ test_bcmp (void)
check(bcmp("abc", "def", 0) == 0, 8); /* Zero count. */
}
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)
{
@@ -1611,6 +1624,9 @@ main (void)
/* bcmp - somewhat like memcmp. */
test_bcmp ();
/* __memcmpeq - somewhat like memcmp. */
test_memcmpeq ();
/* strndup. */
test_strndup ();