Compiler hazard on `1ULL << (63 - clz(d))) / d`

[Validark]

Godbolt link

#include <stdint.h>

uint64_t bar(uint32_t divisor) {
    return (1ULL << (63 - __builtin_clz(divisor))) / divisor;
}

This compiled to:

bar:
        lzcnt   edx, edi
        mov     eax, 1
        movabs  rsi, -4294967296
        mov     ecx, edi
        xor     dl, 63
        shlx    rax, rax, rdx
        movzx   edx, dl
        bt      rsi, rdx
        jae     .LBB0_1
        xor     edx, edx
        div     rcx
        ret
.LBB0_1:
        xor     edx, edx
        div     ecx
        ret

I was expecting:

bar:
    lzcnt   ecx, edi
    not     cl
    mov     eax, 1
    shlx    rax, rax, rcx
    xor     rdx, rdx
    div     rdi
    ret

This snippet seems fine:

uint64_t bar(uint32_t divisor) {
    return (1ULL << (__builtin_clz(divisor))) / divisor;
}

baz:
        lzcnt   eax, edi
        mov     ecx, 1
        xor     edx, edx
        shlx    eax, ecx, eax
        div     edi
        ret

[efriedma-quic]

The two divide instructions is an intentional transform. See #90985 etc.

The way it's computing the condition is a little ugly.