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From: "vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/67955] New: tree-dse does not use pointer info
Date: Tue, 13 Oct 2015 13:57:00 -0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 6.0
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X-Bugzilla-Severity: enhancement
X-Bugzilla-Who: vries at gcc dot gnu.org
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67955

            Bug ID: 67955
           Summary: tree-dse does not use pointer info
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

Consider this testcase, with dead store '*p = 1':
...
int
f ()
{
  int a;
  int *p = &a;
  *p = 1;
  a = 2;
  return a;
}
...

We compile with -O2 -fno-tree-ccp -fno-tree-forwprop -fno-tree-fre.

At dse1 we have
...
f ()
{
  int * p;
  int a;
  int _5;

  <bb 2>:
  p_1 = &a;
  *p_1 = 1;
  a = 2;
  _5 = a;
  a ={v} {CLOBBER};
  return _5;

}
...

So, the dead store '*p_1 = 1' has not been eliminated.

However, at ealias we already know:
...
Alias information for f

Aliased symbols

a, UID D.1757, int, is addressable

Call clobber information

ESCAPED, points-to non-local, points-to vars: { }

Flow-insensitive points-to information

p_1, points-to vars: { D.1757 }
...

So, we could use points-to info in dce.