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From: "law at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/64700] New: Sink common code through PHI
Date: Tue, 20 Jan 2015 22:01:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64700

            Bug ID: 64700
           Summary: Sink common code through PHI
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: law at redhat dot com

Created attachment 34507
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34507&action=edit
Testcode

Originally from BZ 64081....


We do miss some interesting kind of optimization opportunities like
transforming

  if (prephitmp_87 == 1)
    goto <bb 9>;
  else
    goto <bb 10>;

  <bb 9>:
  _24 = arr1.5_23 + _62;
  pos.6_25 = *_24;
  goto <bb 11>;

  <bb 10>:
  _28 = arr2.7_27 + _62;
  pos.8_29 = *_28;

  <bb 11>:
  # prephitmp_89 = PHI <pos.6_25(9), pos.8_29(10)>

to

  if (prephitmp_87 == 1)
    goto <bb 9>;
  else
    goto <bb 11>;

  <bb 9>:
  goto <bb 11>;

  <bb 11>:
  # _24 = PHI <arr1.5_23, arr2.7_27>
  _28 = _24 + _62;
  prephitmp_89 = *_28;

sinking common computations through a PHI.

With the followup optimization in out-of-SSA to coalesce arr1.5_23 and
arr2.7_27 which means we can drop the conditional entirely.