[llvm-bugs] [Bug 49866] New: Optimize C++ spaceship comparison

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49866

            Bug ID: 49866
           Summary: Optimize C++ spaceship comparison
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: marc.glisse at normalesup.org
                CC: llvm-bugs at lists.llvm.org

Similar to gcc's https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94589 , it would
be nice to optimize (x<=>y)@0 to x at y for all comparisons @, and clang/llvm
seems to manage for some but not all.

#include <compare>
int n(int x){return (x<=>0)<0;}

Debian clang version 13.0.0-++20210129063721+010b176cdefb-1~exp1
x86_64-pc-linux-gnu

        movl    %edi, %eax
        shrl    $24, %eax
        testl   %edi, %edi
        setne   %cl
        testb   %al, %al
        sets    %al
        andb    %cl, %al
        movzbl  %al, %eax
        retq

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