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From: "vermaelen.wouter at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/53979] New: (a^b^b) not simplified to (a)   (in combination with CSE??)
Date: Mon, 16 Jul 2012 10:41:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53979

             Bug #: 53979
           Summary: (a^b^b) not simplified to (a)   (in combination with
                    CSE??)
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vermaelen.wouter@gmail.com


The following 3 functions should ideally generate identical code (and they do
when using clang).

int f1(int a, int b) {
    int c = b;
    return (a ^ b ^ c) | (a ^ b) | a;
}
int f2(int a, int b) {
    return (a ^ b) | a;
}
int f3(int a, int b) {
    return a | b;
}

I tested gcc revision trunk@189510 and it shows 2 missed optimizations:

1) (a ^ b ^ b) not simplified to (a)
Normally gcc performs this optimization, but I *guess* it misses it here
because of the CSE opportunity with (a ^ b).

2) ((a ^ b) | a) not simplified to (a | b)


Of course in this example it's easy to manually rewrite the code. But in my
original code this function was actually a template and for some instantiations
the expression for the variable 'c' simplified to just 'b'. So the first missed
optimization is something I saw in real code. The second missed optimization
only occurs in this (much) simplified variant of the function.