Re: Using POPCNT and other advanced bit manipulation instructions

Andres Freund <andres@anarazel.de> writes:
> On 2019-02-14 15:47:13 -0300, Alvaro Herrera wrote:
>> Hah, I just realized you have to add -mlzcnt in order for these builtins
>> to use the lzcnt instructions.  It goes from something like
>> 
>> bsrq    %rax, %rax
>> xorq    $63, %rax

> I'm confused how this is a general count leading zero operation? Did you
> use constants or something that allowed ot infer a range in the test? If
> so the compiler probably did some optimizations allowing it to do the
> above.

No.  If you compile

int myclz(unsigned long long x)
{
  return __builtin_clzll(x);
}

at -O2, on just about any x86_64 gcc, you will get

myclz:
.LFB1:
        .cfi_startproc
        bsrq    %rdi, %rax
        xorq    $63, %rax
        ret
        .cfi_endproc

            regards, tom lane