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From: "meissner at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/51274] New: Starting with GCC 4.5, powerpc generated different code for x != 0.
Date: Tue, 22 Nov 2011 23:42:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51274

             Bug #: 51274
           Summary: Starting with GCC 4.5, powerpc generated different
                    code for x != 0.
    Classification: Unclassified
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: meissner@gcc.gnu.org
              Host: powerpc64-linux power-linux
            Target: powerpc64-linux
             Build: powerpc64-linux


In doing some investigation for optimizations of comparisons for power7, I
noticed that starting with GCC 4.5, the compiler no longer generates the code
it used to for setting x = (y != 0).  In the rs6000.md file, these
optimizations start with the insn "ne0si".

In the 4.4 time frame, for -m32 (and for -m32 -mpowerpc64, which is PR 36557),
gcc would generate:
        addic 9,3,-1
        subfe 0,9,3

However, it wouldn't generate this code for -m64.  Starting with 4.5, it
generates the code for !(x == 0), or:
        cntlzw 3,3
        srwi 3,3,5
        xori 3,3,1

We should either remove the insns in rs6000.md that no longer are matching, or
fix them so they do match.  I suspect that the longer code sequence is actually
faster on the newer processors, since you don't need to track the carry between
the two insns.