https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110499
Bug ID: 110499 Summary: malloc branch predictor is broken Product: gcc Version: 13.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: hubicka at gcc dot gnu.org Target Milestone: --- Malloc branch predictor currently predicts that malloc() call likely returns 1. This is good for NULL pointer checks, but not good for checking pointers for equality: #include <malloc.h> void test() { if (malloc(10) == malloc(20)) printf ("Impossible!\n"); } gets predicted as: void test () { void * _1; void * _2; <bb 2> [local count: 1073741824]: _1 = malloc (10); _2 = malloc (20); if (_1 == _2) goto <bb 3>; [99.96%] else goto <bb 4>; [0.04%] <bb 3> [local count: 1073312329]: __builtin_puts (&"Impossible!"[0]); <bb 4> [local count: 1073741824]: return; } So we think that Impossible is output with 99.96 probability.