https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114622
Bug ID: 114622 Summary: memcmp -Wstringop-overread false positive Product: gcc Version: 13.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: arnaud.lb at gmail dot com Target Milestone: --- The following code: ``` inline __attribute__((always_inline)) int g(const char *haystack, const char *needle, long unsigned int needle_len) { if (needle_len == 1 || needle_len == 0) { return 0; } return __builtin_memcmp(needle, haystack, needle_len-2); } int f(const char *c) { long unsigned int len = 1; return g(c, "=", len); } ``` Results in the following warning: ``` In function 'int g(const char*, const char*, long unsigned int)', inlined from 'int f(const char*)' at <source>:13:10: <source>:8:32: warning: 'int __builtin_memcmp(const void*, const void*, long unsigned int)' specified bound 18446744073709551615 exceeds maximum object size 9223372036854775807 [-Wstringop-overread] 8 | return __builtin_memcmp(needle, haystack, needle_len-2); | ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ``` >From my understanding, the compiler knows needle_len to be in range [1,1] when inlining g() in f(), but it also believes that line 8 (the memcmp call) is feasible, so needle_len-2 is (uint64_t)1-2, which is 18446744073709551615. Local gcc version 13.2.1 20240316 (Red Hat 13.2.1-7) (GCC) Reproducible since 12.x on godbolt