On Mon, 29 Sep 2025, Avinash Jayakar wrote:
> On Thu, 2025-09-25 at 20:18 +0530, Avinash Jayakar wrote:
> > On Wed, 2025-09-24 at 08:30 +0200, Richard Biener via Gcc wrote:
> > >
> > >
> > > > 2. Implement mulv2di3 for this specific target (which does
> > > > exactly
> > > > what
> > > > scalar code would do), and let expand pass (expand_mult) take
> > > > care
> > > > of
> > > > converting mult to shift/add/sub.
> > >
> > > The expand pass wouldn't do this when the target implements
> > > mulv2di3.
> > >
> >
> > Actually I tried it by adding define_expand "mulv2di3", and the
> > following code in expmed.cc from the function expand_mult
> > 3550 {
> > 3551 int shift = wi::exact_log2 (rtx_mode_t (scalar_op1,
> > mode));
> > 3552 /* Perfect power of 2 (other than 1, which is handled
> > above). */
> > 3553 if (shift > 0)
> > 3554 return expand_shift (LSHIFT_EXPR, mode, op0,
> > 3555 shift, target, unsignedp);
> > 3556 else
> > 3557 goto skip_synth;
> >
> > Does a shift if operand 1 is power of 2. Also there are other
> > optimizations applied similar to vector pattern recog.
> > So instead of adding similar thing in expand_vector_operations, I was
> > thinking of adding mulv2di3 define_expand so that it goes through the
> > vectorizer and let expand pass handle the optimization of
> > multiplication.
> >
>
> The second approach is a bit hacky, there is a particular test that
> fails pr25413a.c, which checks that if target does not support
> vector_double multiply then loop must not be vectorized. But by adding
> mulv2di3 the vector reports the loop has been vectorized. So I think
> this approach would be incorrect from the perspective of vectorizer's
> output.
>
> Because in case of checking support if op is supported for target, the
> predicate can_implement_p is used. It only checks if the code for the
> target is available or not and not how it is implemented, so the target
> has to implement the code in vectorized way and not emulate it using
> scalar instructions to guarantee the vectorizer's output is correct.
>
> Would it be ok to reuse some of the code already written in expand_mult
> for constant multiplications? As you mentioned if we have sth like
> can_vec_mult_p, then we can use the same in define_expand as well.
Sure, as suggested both the existing scalar vector pattern and the
vector lowering and the expand code could use common functionality.
Richard.
> Thanks and regards,
> Avinash Jayakar
>
--
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
