On Wed, 4 May 2022, Richard Earnshaw wrote:
>
>
> On 04/05/2022 12:14, Richard Biener wrote:
> > On Wed, May 4, 2022 at 12:16 PM Richard Earnshaw via Gcc-patches
> > <gcc-patches@gcc.gnu.org> wrote:
> >>
> >>
> >> Gimple isel already handles x CMP y ? -1 : 0 when lowering vector cond
> >> operations, but this can be generalized further when the comparison
> >> forms a natural mask so that we can also handle x CMP y ? z : 0 by
> >> transforming it into (x CMP y) & z. This will, in most cases save
> >> having to load a register with the zero vector.
> >
> > Hmm, I'm not sure why exactly the existing case is in ISEL but if
> > we want to extend it there we migh also want to handle ? 0 : -1
> > as BIT_NOT.
>
> I was assuming that there had already been some level of canonicalization at
> this point, but maybe that's an incorrect assumption. There are quite a few
> transforms that would avoid a select operation, but at some point the bit
> manipulations may well become more expensive than the select itself.
>
> >
> > For the case in question it looks like it might better fit as optimization
> > in match.pd? It also lacks a query for present and<mode>3 support.
> > For match.pd it might be nice to handle x CMP y ? 0 : z as well
> > if we can invert x CMP y (and it has only a single use). When in
> > ISEL we might as well check for andn<mode>3 support and go with
> > BIT_NOT + BIT_AND ...
>
> I'll have to think about that a bit, the approach was inspired by the hunk
> just a bit earlier that starts:
>
> /* Lower mask typed, non-vector mode VEC_COND_EXPRs to bitwise operations.
> Those can end up generated by folding and at least for integer mode masks
> we cannot expect vcond expanders to exist. We lower a ? b : c
> to (b & a) | (c & ~a). */
> if (VECTOR_BOOLEAN_TYPE_P (TREE_TYPE (lhs))
> && !VECTOR_MODE_P (mode))
> {
> gcc_assert (types_compatible_p (TREE_TYPE (op0), TREE_TYPE (op1)));
> gimple_seq stmts = NULL;
> tree type = TREE_TYPE (lhs);
> location_t loc = gimple_location (stmt);
> tree tem0 = gimple_build (&stmts, loc, BIT_AND_EXPR, type, op1, op0);
>
> There's no test for and<mode>3 support there, but I guess that's working on
> integral modes rather than vectors.
Yes, that's for word_mode or AVX512 mask modes (QImode or HImode).
> >
> > Do you have a testcase where it improves code generation btw?
>
> It was originally inspired by:
>
> void f (int * __restrict__ dest, int *a, int *b)
> {
> int i;
> for (i=0; i<4; i++) {
> dest[i] = a[i] == b[i];
> }
> }
>
> which with Neon on Arm was producing a vsel sequence rather than a mask
> operation because the backend doesn't handle this case directly during expand
> (unlike some other targets); but it seemed wrong for every target to have to
> handle this in the back-end when it's a pretty generic optimization.
But of course whether the AND or the VSEL is more efficient depends
on the target so in match.pd it would be canonicalization while in
ISEL (aka instruction selection) we should look at what the target
prefers. ISEL is supposed to become a place where we do all the
non-trivial RTL expansion tricks.
The existing trick just drops an operation which should be always
profitable, evading the problem of deciding. If we add things
here we should either reason that by all means profitability
is guaranteed (with a comment) or somehow query the target
(I'm not sure how - how does RTL expansion do it? IIRC that
might also just pick the first "working" expansion?)
I understand your target can do the vcond as well, but for
the testcases it seems we have used_vec_cond_exprs == 1
and thus the vcond would also do the compare and thus to me
it looks like the single .VCOND is going to be cheaper
than a compare to produce the mask and the then AND?
If the VSEL doesn't actually perform the comparison then I think
you shouldn't have vcond{[u],eq} patterns but just comparison
patterns and vcond_mask? For that I'd then say the target
could choose to expand vcond_mask with same mode (not
mixed FP/INT modes) as AND if profitable?
Richard.
> A more generalized case would be something like
>
> void f (int * __restrict__ dest, int *a, int *b)
> {
> int i;
> for (i=0; i<4; i++) {
> dest[i] = a[i] ? b[i] : 0;
> }
> }
>
> R.
>
> >
> > Richard.
> >
> >>gcc/ChangeLog:
> >> * gimple-isel.cc (gimple_expand_vec_cond_expr): Handle
> >> x CMP y ? z : 0.
> >>---
> >> gcc/gimple-isel.cc | 29 +++++++++++++++++++++++------
> >> 1 file changed, 23 insertions(+), 6 deletions(-)
> >>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Ivo Totev; HRB 36809 (AG Nuernberg)
