[Bug rtl-optimization/63259] New: Detecting byteswap sequence

[bisqwit at iki dot fi]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63259

            Bug ID: 63259
           Summary: Detecting byteswap sequence
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bisqwit at iki dot fi

This is just silly. GCC optimizes the first function into single opcode
(bswap), but not the other. For Clang, it's the other way around.

unsigned byteswap_gcc(unsigned result)
{
    result = ((result & 0xFFFF0000u) >>16) | ((result & 0x0000FFFFu) <<16);
    result = ((result & 0xFF00FF00u) >> 8) | ((result & 0x00FF00FFu) << 8);
    return result;
}
unsigned byteswap_clang(unsigned result)
{
    result = ((result & 0xFF00FF00u) >> 8) | ((result & 0x00FF00FFu) << 8);
    result = ((result & 0xFFFF0000u) >>16) | ((result & 0x0000FFFFu) <<16);
    return result;
}

unsigned byteswap(unsigned v)
{
    #ifdef __clang__
     return byteswap_clang(v);
    #else
     return byteswap_gcc(v);
    #endif
}

GCC output:

    byteswap_gcc:
        movl    %edi, %eax
        bswap   %eax
        ret

    byteswap_clang:
        movl    %edi, %eax
        andl    $-16711936, %eax
        shrl    $8, %eax
        movl    %eax, %edx
        movl    %edi, %eax
        andl    $16711935, %eax
        sall    $8, %eax
        orl     %edx, %eax
        roll    $16, %eax
        ret

    byteswap:
        movl    %edi, %eax
        bswap   %eax
        ret

Clang output:

    byteswap_gcc:                           # @byteswap_gcc
        roll    $16, %edi
        movl    %edi, %eax
        shrl    $8, %eax
        andl    $16711935, %eax         # imm = 0xFF00FF
        shll    $8, %edi
        andl    $-16711936, %edi        # imm = 0xFFFFFFFFFF00FF00
        orl     %eax, %edi
        movl    %edi, %eax
        retq

    byteswap_clang:                         # @byteswap_clang
        bswapl  %edi
        movl    %edi, %eax
        retq

    byteswap:                               # @byteswap
        bswapl  %edi
        movl    %edi, %eax
        retq


Tested both -m32 and -m64, with options: -Ofast -S
Tested versions:
- gcc (Debian 4.9.1-11) 4.9.1  Target: x86_64-linux-gnu
- Debian clang version 3.5.0-+rc1-2 (tags/RELEASE_35/rc1) (based on LLVM 3.5.0)
 Target: x86_64-pc-linux-gnu