Re: [PATCH] match.pd: Fix up the recent bitmask_inv_cst_vector_p simplification [PR103417]

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Nov 25, 2021 at 08:23:50AM +0000, Tamar Christina wrote:
> > But, IMNSHO while it isn't incorrect to handle le and gt there, it is
> > unnecessary.  Because (x & cst) <= 0U and (x & cst) > 0U should never appear,
> > again in
> > /* Non-equality compare simplifications from fold_binary  */ we have a
> > simplification for it:
> >        (if (cmp == LE_EXPR)
> >         (eq @2 @1))
> >        (if (cmp == GT_EXPR)
> >         (ne @2 @1))))
> > This is done for
> >   (cmp (convert?@2 @0) uniform_integer_cst_p@1) and so should be done
> > for both integers and vectors.
> > As the bitmask_inv_cst_vector_p simplification only handles eq and ne for
> > signed types, I think it can be simplified to just following patch.
> 
> As I mentioned on the PR I don't think LE and GT should be removed, the patch
> Is attempting to simplify the bitmask used because most vector ISAs can create
> the simpler mask much easier than the complex mask.
> 
> It. 0xFFFFFF00 is harder to create than 0xFF.   So while for scalar it doesn't matter
> as much, it does for vector code.

What I'm trying to explain is that you should never see those le or gt cases
with TYPE_UNSIGNED (especially when the simplification is moved after those
/* Non-equality compare simplifications from fold_binary  */
I've mentioned), because if you try:
typedef unsigned V __attribute__((vector_size (4)));

unsigned f1 (unsigned x) { unsigned z = 0; return x > z; }
unsigned f2 (unsigned x) { unsigned z = 0; return x <= z; }
V f3 (V x) { V z = (V) {}; return x > z; }
V f4 (V x) { V z = (V) {}; return x <= z; }
you'll see that this is at ccp1 when the constants propagate simplified
using the rules I mentioned into x != 0U, x == 0U, x != (V) {} and x == (V) {}.

The important rule of match.pd is composability, the simplifications should
rely on other simplifications and not repeating all their decisions because
that makes the *match.c larger and more expensive (and a source of extra
possible bugs).

	Jakub