Re: [PATCH] Canonicalize vec_merge when mask is constant.

[Hongtao Liu <crazylht@gmail.com>]

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



-- 
BR,
Hongtao