On 14/03/2024 6:04 am, Alexander Monakov wrote:
> On Thu, 14 Mar 2024, Andrew Cooper via Gcc wrote:
>
>> I suppose that what I'm looking for is something a little like
>> __builtin_constant_p() which can either be used in a straight if(), or
>> in a __builtin_choose_expr().
>>
>> Anyway - is there a way of doing this that I've managed to overlook?
> I am missing what is lacking for you with __builtin_constant_p, I would
> do it like this:
>
> unsigned ffs(unsigned x)
> {
> unsigned res;
> unsigned nonzero = x != 0;
> if (__builtin_constant_p(nonzero) && nonzero)
> asm("bsf %1, %0" : "=r"(res) : "rm"(x));
> else {
> res = -1;
> asm("bsf %1, %0" : "+r"(res) : "rm"(x));
> }
> return res;
> }
Oh - so it does. I'd not considered that expressing it like that would
still work.
>
> or with handling known-zero-input case like this:
>
> unsigned ffs(unsigned x)
> {
> unsigned res;
> unsigned nonzero = x != 0;
> if (!__builtin_constant_p(nonzero)) {
> res = -1;
> asm("bsf %1, %0" : "+r"(res) : "rm"(x));
> } else if (nonzero) {
> asm("bsf %1, %0" : "=r"(res) : "rm"(x));
> } else {
> res = -1;
> }
> return res;
> }
>
>
> Does it work for you?
I simplified things when asking the question. The real implementation
has a general
if (__builtin_constant_p(x))
return __builtin_ffs(x);
so any known-constant value can be folded. What I'm dealing with is the
remainder of the cases.
Anyway - thankyou for your help.
~Andrew
