[llvm-bugs] [Bug 43658] New: Improve codegen for std::midpoint

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43658

            Bug ID: 43658
           Summary: Improve codegen for std::midpoint
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

When one argument is a known constant value, gcc produces better code.

int foo(int a) {
    return std::midpoint(a,100);
}

Clang trunk -O3

foo(int):                                # @foo(int)
        cmp     edi, 100
        mov     ecx, 100
        mov     eax, edi
        cmovg   eax, ecx
        cmp     edi, 99
        cmovg   ecx, edi
        sub     ecx, eax
        shr     ecx
        mov     eax, ecx
        neg     eax
        cmp     edi, 100
        cmovle  eax, ecx
        add     eax, edi
        ret

Block RThroughput: 2.7

GCC -O3
foo(int):
        mov     edx, 100
        cmp     edi, 101
        mov     eax, edx
        cmovl   edx, edi
        setl    cl
        cmovge  eax, edi
        movzx   ecx, cl
        lea     ecx, [rcx-1+rcx]
        sub     eax, edx
        shr     eax
        imul    eax, ecx
        add     eax, edi
        ret

Block RThroughput: 2.5

https://godbolt.org/z/F6PE2w

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