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From: "fweimer at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/55616] New: bogus warning about undefined overflow after overflow check
Date: Fri, 07 Dec 2012 14:03:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55616

             Bug #: 55616
           Summary: bogus warning about undefined overflow after overflow
                    check
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: fweimer@redhat.com
                CC: nils@redhat.com


Created attachment 28893
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28893
overflow-check.c

Code which performs proper overflow checking results in a compiler warning (at
-O2, without -fwrapv):

overflow-check.c:9:6: warning: assuming signed overflow does not occur when
assuming that (X + c) < X is always false [-Wstrict-overflow]
   if (a > b)
      ^

I'm surprised that the call to something from f is not optimized away with
-fwrapv.  Shouldn't VRP catch this?  Addressing this missed optimization
opportunity might also fix the warning.

The g function in the test case intends to make the if statement in r actually
relevant.

This is not a regression, it happens with 4.7.2 as well.