For
long test(long a, long b)
{
        if (a > 65535 || a < 0)
                __builtin_unreachable ();
        if (b > 65535 || b < 0)
                __builtin_unreachable ();
        return a/b;
}

we produce
test:
.LFB0:
        .cfi_startproc
        movq    %rdi, %rax
        cqto
        idivq   %rsi
        ret

while clang does:

test:                                   # @test
        .cfi_startproc
# %bb.0:
        movq    %rdi, %rax
                                        # kill: def $ax killed $ax killed $rax
        xorl    %edx, %edx
        divw    %si
        movzwl  %ax, %eax
        retq

clang also by default adds 32bit divide path even when value range is not known

long test(long a, long b)
{
        return a/b;
}

compiles as

test:                                   # @test
        .cfi_startproc
# %bb.0:
        movq    %rdi, %rax
        movq    %rdi, %rcx
        orq     %rsi, %rcx
        shrq    $32, %rcx
        je      .LBB0_1
# %bb.2:
        cqto
        idivq   %rsi
        retq