Canonicalize vec_merge when mask is constant.

  Use swap_communattive_operands_p for canonicalization. When both value
has same operand precedence value, then first bit in the mask should
select first operand.

The canonicalization should help backends for pattern match. .i.e. x86
backend has lots of vec_merge patterns, combine will create any form
of vec_merge(mask, or inverted mask), then backend need to add 2
patterns to match exact 1 instruction. The canonicalization can
simplify 2 patterns to 1.

Bootstrapped and regtested on x86_64-pc-linux-gnu{-m32,}, aarch64-linux-gnu.
Ok for trunk?

gcc/ChangeLog:

	* combine.cc (maybe_swap_commutative_operands): Canonicalize
	vec_merge when mask is constant.
---
 gcc/combine.cc | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)
  

On 4/19/23 21:58, liuhongt via Gcc-patches wrote:
> Use swap_communattive_operands_p for canonicalization. When both value
> has same operand precedence value, then first bit in the mask should
> select first operand.
> 
> The canonicalization should help backends for pattern match. .i.e. x86
> backend has lots of vec_merge patterns, combine will create any form
> of vec_merge(mask, or inverted mask), then backend need to add 2
> patterns to match exact 1 instruction. The canonicalization can
> simplify 2 patterns to 1.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu{-m32,}, aarch64-linux-gnu.
> Ok for trunk?
> 
> gcc/ChangeLog:
> 
> 	* combine.cc (maybe_swap_commutative_operands): Canonicalize
> 	vec_merge when mask is constant.
ISTM that if we're going to call this the canonical form, then we should 
document it in rtl.texi.

Otherwise it looks pretty good to me.  So let's get the docs updated and 
get this installed.

jeff
  

On Mon, May 1, 2023 at 3:37 AM Jeff Law <jeffreyalaw@gmail.com> wrote:
>
>
>
> On 4/19/23 21:58, liuhongt via Gcc-patches wrote:
> > Use swap_communattive_operands_p for canonicalization. When both value
> > has same operand precedence value, then first bit in the mask should
> > select first operand.
> >
> > The canonicalization should help backends for pattern match. .i.e. x86
> > backend has lots of vec_merge patterns, combine will create any form
> > of vec_merge(mask, or inverted mask), then backend need to add 2
> > patterns to match exact 1 instruction. The canonicalization can
> > simplify 2 patterns to 1.
> >
> > Bootstrapped and regtested on x86_64-pc-linux-gnu{-m32,}, aarch64-linux-gnu.
> > Ok for trunk?
> >
> > gcc/ChangeLog:
> >
> >       * combine.cc (maybe_swap_commutative_operands): Canonicalize
> >       vec_merge when mask is constant.
> ISTM that if we're going to call this the canonical form, then we should
> document it in rtl.texi.
Yes., how about the wording:

1 file changed, 7 insertions(+)
gcc/doc/md.texi | 7 +++++++

modified   gcc/doc/md.texi
@@ -8215,6 +8215,13 @@ second operand.  If a machine only supports a
constant as the second
 operand, only patterns that match a constant in the second operand need
 be supplied.

+@cindex @code{vec_merge}, canonicalization of
+@item
+For the @code{vec_merge} with constant mask(the third operand), the first
+and the second operand can be exchanged by inverting the mask. In such cases,
+a constant is always made the second operand, otherwise the least significant
+bit of the mask is always set(select the first operand first).
+
 @item
 For associative operators, a sequence of operators will always chain
 to the left; for instance, only the left operand of an integer @code{plus}


>
> Otherwise it looks pretty good to me.  So let's get the docs updated and
> get this installed.
>
> jeff