Hi,
Can you point to where in the C standard this is treated as undefined behavior?
Best regards, Kjetil Oftedal
On Tue, 7 Jan 2025 at 13:10, Marcus Haehnel marcus.haehnel@kernkonzept.com wrote:
From: Frank Mehnert frank.mehnert@kernkonzept.com
It is undefined behavior to compare two pointers belonging to different objects. This includes the case where the addition overflows. Clang-20 seems to follow this rule more eagerly and optimizes away the old test.
Fix the test by performing the addition on uintptr_t values rather than on on char pointers.
See also https://github.com/llvm/llvm-project/issues/121909.
Signed-off-by: Marcus Haehnel marcus.haehnel@kernkonzept.com
libc/string/generic/strnlen.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/libc/string/generic/strnlen.c b/libc/string/generic/strnlen.c index 4d4cde84f..82d4122ec 100644 --- a/libc/string/generic/strnlen.c +++ b/libc/string/generic/strnlen.c @@ -29,15 +29,17 @@ '\0' terminator is found in that many characters, return MAXLEN. */ size_t strnlen (const char *str, size_t maxlen) {
- const char *char_ptr, *end_ptr = str + maxlen;
const char *char_ptr, *end_ptr; const unsigned long int *longword_ptr; unsigned long int longword, himagic, lomagic;
if (maxlen == 0) return 0;
- if (__builtin_expect (end_ptr < str, 0))
if (__builtin_expect ((uintptr_t)str + maxlen < (uintptr_t)str, 0)) end_ptr = (const char *) ~0UL;
else
end_ptr = str + maxlen;
/* Handle the first few characters by reading one character at a time. Do this until CHAR_PTR is aligned on a longword boundary. */
-- 2.47.1
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