[llvm-dev] Where's the optimiser gone? (part 6): "rotation" idiom not recognised for 64-bit value on 32-bit target

[Stefan Kanthak via llvm-dev]

unsigned long long rotate(unsigned long long value, unsigned int count)
{
    return (value << count) | (value >> (64 - count));
}

Compiled with "-O3 -m32" clang produces the following unoptimised code
(see <https://godbolt.org/z/OksNnO>): OUCH!

rotate: # @rotate
    push   ebp                        |
    push   ebx                        |    push   ebx
    push   edi                        |
    push   esi                        |
    mov    eax, dword ptr [esp + 20]  |    mov    eax, dword ptr [esp + 8]
    mov    esi, dword ptr [esp + 24]  |    mov    edx, dword ptr [esp + 12]
    mov    ecx, dword ptr [esp + 28]  |    mov    ecx, dword ptr [esp + 16]
    xor    edx, edx                   |    mov    ebx, edx
    mov    ebx, 64                    |
    mov    edi, eax                   |
    mov    ebp, esi                   |
    shl    edi, cl                    |
    shld   ebp, eax, cl               |
    test   cl, 32                     |
    cmovne ebp, edi                   |
    cmovne edi, edx                   |
    sub    ebx, ecx                   |
    mov    edx, esi                   |
    mov    ecx, ebx                   |
    shr    edx, cl                    |
    shrd   eax, esi, cl               |
    test   bl, 32                     |    test   cl, 32
    mov    ecx, 0                     |    cmovne edx, eax
    cmovne eax, edx                   |    cmovne eax, ebx
    cmovne edx, ecx                   |    cmovne ebx, edx
    or     eax, edi                   |    shld   edx, eax, cl
    or     edx, ebp                   |    shld   eax, ebx, cl
    pop    esi                        |
    pop    edi                        |
    pop    ebx                        |    pop    ebx
    pop    ebp                        |
    ret                               |    ret