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From: "aldyh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/52548] New: missed PRE optimization when function call follows to-be hoisted variable
Date: Fri, 09 Mar 2012 21:09:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52548

             Bug #: 52548
           Summary: missed PRE optimization when function call follows
                    to-be hoisted variable
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: aldyh@gcc.gnu.org


For the following code (for -O2):

int flag, hoist, y, z;

void
foo (void)
{
  if (flag)
    y = hoist + 4;
  else
    flag = 888;
  z = hoist + 4;
  bark ();
}

...PRE should be moving "hoist + 4" to the else arm, but it fails to do so.

If you remove the call to bark(), [hoist + 4] gets moved appropriately.

The bark() function call is in the same basic block as "z = hoist + 4".  I wild
guess is that "hoist" isn't anticipatable at the *end* of the BB beginning with
"z = hoist + 4".  Splitting BB's at function calls may improve PRE.  Just a
guess...