[llvm-bugs] [Bug 46476] New: Failure to optimize tautological comparisons of comparisons to a single one

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=46476

            Bug ID: 46476
           Summary: Failure to optimize tautological comparisons of
                    comparisons to a single one
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: gabravier at gmail.com
                CC: llvm-bugs at lists.llvm.org

int f(int a, int b)
{
    return (((b != 0) & (a == 0)) | ((a != 0) & (b == 0)));
}

This can be optimized to `(a != 0) ^ (b != 0)`. I originally found this while
compiling this code :

inline bool nand(bool a, bool b)
{
    return !(a && b);
}

int f(int a, int b)
{
    return nand(nand(b, nand(a, a)), nand(a, nand(b, b)));
}

Which GCC compiles to the above example, and that LLVM optimizes with the
transformation I gave (strangely, LLVM does not seem to optimize the example at
the top of this bug report to the transformed version if directly given the top
example, which is why I'm giving these details as I'm thinking there could be
some kind of UB weirdness or something like that with the transformations here)

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