[llvm-bugs] [Bug 42696] New: Missed optimization for (n << 1) ? X : Y

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42696

            Bug ID: 42696
           Summary: Missed optimization for (n << 1) ? X : Y
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

int test(int n)
{
    return (2*n) ? n << 1 : 0;
}


int test2(int n)
{
    return (n << 1) ? 2*n : 0;
}

Clang -O3 trunk:
test(int):                               # @test(int)
        lea     eax, [rdi + rdi]
        ret
test2(int):                              # @test2(int)
        lea     eax, [rdi + rdi]
        mov     ecx, edi
        and     ecx, 2147483647
        test    ecx, ecx
        cmove   eax, ecx
        ret

GCC -O3 trunk:
test(int):
        lea     eax, [rdi+rdi]
        ret
test2(int):
        lea     eax, [rdi+rdi]
        ret


Also, GCC's add should be better than TEST with imm?


int test2(int n)
{
    return (n << 1) ? 1 : 0;
}

Clang:
test2(int):                              # @test2(int)
        xor     eax, eax
        test    edi, 2147483647
        setne   al
        ret

GCC:

test3(int):
        xor     eax, eax
        add     edi, edi
        setne   al
        ret

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