Andres Freund <and...@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
