Builtin for consulting value analysis (better ffs() code gen)

[Andrew Cooper]

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