[llvm-bugs] [Bug 48999] New: Failure to optimize combined popcount pattern

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48999

            Bug ID: 48999
           Summary: Failure to optimize combined popcount pattern
           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(unsigned int a, unsigned int b)
{
    return __builtin_popcount(a & 8) + __builtin_popcount(b & 2);
}

This can be optimized to `return __builtin_popcount((a & 8) | (b & 2));`. This
transformation is done by GCC, but not by LLVM.

Godbolt comparison: https://godbolt.org/z/zjP3Ys
(If someone can tell me how to use ctpop in alive2 that'd be nice btw, I
couldn't get it to work personally)

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