Hi!
On Wed, Jun 02, 2021 at 09:07:35AM +0200, Richard Biener wrote:
> On Wed, Jun 2, 2021 at 7:41 AM liuhongt via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> > For i386, it will enable below opt
> >
> > from
> > notl %edi
> > vpbroadcastd %edi, %xmm0
> > vpand %xmm1, %xmm0, %xmm0
> > to
> > vpbroadcastd %edi, %xmm0
> > vpandn %xmm1, %xmm0, %xmm0
>
> There will be cases where (vec_duplicate (not A)) is better
> than (not (vec_duplicate A)), so I'm not sure it is a good idea
> to forcefully canonicalize unary operations.
It is two unaries in sequence, where the order does not matter either.
As in all such cases you either have to handle both cases everywhere, or
have a canonical order.
> I suppose the
> simplification happens inside combine
combine uses simplify-rtx for most cases (it is part of combine, but
used in quite a few other places these days).
> - doesn't combine
> already have code to try variants of an expression and isn't
> this a good candidate that can be added there, avoiding
> the canonicalization?
As I mentioned, this is done in simplify-rtx in cases that do not have a
canonical representation. This is critical because it prevents loops.
A very typical example is how UMIN is optimised:
case UMIN:
if (trueop1 == CONST0_RTX (mode) && ! side_effects_p (op0))
return op1;
if (rtx_equal_p (trueop0, trueop1) && ! side_effects_p (op0))
return op0;
tem = simplify_associative_operation (code, mode, op0, op1);
if (tem)
return tem;
break;
(the stuff using "tem").
Hongtao, can we do something similar here? Does that work well? Please
try it out :-)
Segher
