Re: [PATCH] vect: Missed opportunity to use [SU]ABD

[Oluwatamilore Adebayo <oluwatamilore.adebayo@arm.com>]

> > +  if (TYPE_PRECISION (unprom_diff.type) != TYPE_PRECISION (abs_type)
> > +      && TYPE_UNSIGNED (unprom_diff.type)
> > +      && TYPE_UNSIGNED (abs_type))
> 
> The last line is now redundant, since TYPE_UNSIGNED was checked above.

Done.

> > +  // Failed to find a widen operation so we check for a regular MINUS_EXPR
> 
> Nit: please use /* ... */ for files that currently use that style.

Done.

> > +  if (diff_oprnds && diff
> > +      && gimple_assign_rhs_code (diff) == MINUS_EXPR)
> 
> We also need to check TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (abs_oprnd)),
> since this case exploits undefined behaviour for signed subtraction.

Done.

> > +      UABD expressions are used when the input types are
> > +      narrower than the output types or the output type is narrower
> > +      than 32 bits
> 
> Isn't it only the first (input types narrower than the output types)?
> I guess the second part is referring to the fact that, in C, an operation
> on sub-32-bit values will happen in int and then be narrowed back down.
> But in gimple that's all explicit, and it's the widening to int that
> allows UABD to be used.  (And UABD can then be used regardless of whether
> the result is narrowed back down.)

Dropped this block as it is no longer applicable. Must've been from a much
earlier version of this patch.

> Might as well drop signed_out_vectype and use vectype directly.

Done.

> > +      if (!SAME_TYPE (TREE_TYPE (diff_oprnds[0]), TREE_TYPE (abd_oprnds[0])))
> > +	return NULL;
> 
> This shouldn't be necesary.  The transformation would be correct
> even if the subtraction is widened before the ABS, since we already
> checked for signed promotion, and since we're only trying SABD.

Done.

Patch is in the next response.