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From: "congh at google dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/58728] New: [missed optimization] == or != comparisons may affect range test optimization.
Date: Mon, 14 Oct 2013 20:04:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58728

            Bug ID: 58728
           Summary: [missed optimization] == or != comparisons may affect
                    range test optimization.
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: congh at google dot com

Created attachment 31002
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31002&action=edit
Patch

Look at the following code:


int foo(unsigned int n)
{
  if (n != 0)
  if (n != 1)
  if (n != 2)
  if (n != 3)
  if (n != 4)
    return ++n;
  return n;
}


Those five comparisons should be able to be merged into one during range test
optimization but they are not. The reason is that GCC checks the phi args of n
after the branch to make sure two false edges of two neighboring ifs define the
same phi arg at the join node (thus guarantees side-effect free). However, the
"vrp" pass replaced the phi arg by the identical value of the original phi arg
deducted from == or != comparisons, hence preventing the range test
optimization.

The same case is in if-combine pass.

I made a patch for this issue which is attached here.