[llvm-bugs] [Bug 31810] New: llvm.bitreverse.i8 on x86 lowers to questionable assembly

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31810

            Bug ID: 31810
           Summary: llvm.bitreverse.i8 on x86 lowers to questionable
                    assembly
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: gonzalobg88 at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

On x86 the 32 and 64 bit versions of bitreverse look very good, but the
generated code for the 8-bit version looks worse than the naive implementation:

https://godbolt.org/g/kAlOC7

@bitrev_8_naive(unsigned char)
        mov     eax, 2149582850
        imul    rax, rdi
        movabs  rcx, 36578664720
        and     rcx, rax
        movabs  rax, 4311810305
        imul    rax, rcx
        shr     rax, 32
        ret

vs llvm.bitreverse.i8:

@bitrev_8(unsigned char)
        rol     dil, 4
        mov     eax, edi
        and     al, 51
        shl     al, 2
        and     dil, -52
        shr     dil, 2
        or      dil, al
        mov     eax, edi
        and     al, 85
        add     al, al
        and     dil, -86
        shr     dil
        or      dil, al
        mov     eax, edi
        ret

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