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From: "hubicka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/110499] New: malloc branch predictor is broken Date: Fri, 30 Jun 2023 13:53:01 +0000 [thread overview] Message-ID: <bug-110499-4@http.gcc.gnu.org/bugzilla/> (raw) 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.
next reply other threads:[~2023-06-30 13:53 UTC|newest] Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-06-30 13:53 hubicka at gcc dot gnu.org [this message] 2023-07-01 20:53 ` [Bug tree-optimization/110499] " pinskia at gcc dot gnu.org
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