https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90626
Bug ID: 90626 Summary: fold strcmp(a, b) == 0 to zero when one string length is exact and the other is unequal Product: gcc Version: 9.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: msebor at gcc dot gnu.org Target Milestone: --- Similar to pr90625 but simpler, the strcmp equality in the function below can safely be folded to zero because the two strings are of unequal lengths. The code in handle_builtin_string_cmp() in strlen.c (committed in r261039) should have all it needs to implement this optimization, it just also needs to consider minimum string lengths. $ cat b.c && gcc -O2 -S -Wall -fdump-tree-strlen=/dev/stdout b.c int f (char * restrict a, char * restrict b) { __builtin_memcpy (a, "1234", 4); // length >= 4 __builtin_strcpy (b, "123"); // length == 3 return __builtin_strcmp (a, b) == 0; // must be false } ;; Function f (f, funcdef_no=0, decl_uid=1907, cgraph_uid=1, symbol_order=0) f (char * restrict a, char * restrict b) { int _1; _Bool _2; int _8; <bb 2> [local count: 1073741824]: __builtin_memcpy (a_4(D), "1234", 4); __builtin_memcpy (b_6(D), "123", 4); _1 = __builtin_strcmp (a_4(D), b_6(D)); _2 = _1 == 0; _8 = (int) _2; return _8; }