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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
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.1.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110499

            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 return=
s 1.
This is good for NULL pointer checks, but not good for checking pointers for
equality:

#include <malloc.h>
void
test()
{
         if (malloc(10) =3D=3D malloc(20))
                 printf ("Impossible!\n");
}

gets predicted as:

void test ()
{
  void * _1;
  void * _2;

  <bb 2> [local count: 1073741824]:
  _1 =3D malloc (10);
  _2 =3D malloc (20);
  if (_1 =3D=3D _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.=