Hi!
On Wed, Jul 01, 2020 at 06:43:32PM -0400, Michael Meissner wrote:
> This patch fixes a PR that I noticed several years ago during power8
> development. I noticed that the compiler would often create a two element
> vector and store the vector.
>
> Particularly for DImode on power8, this could involve two direct moves and a
> XXPERMDI to glue the two parts together. On power9, there a single direct
> move
> instruction that combines the two elements.
But on p9 it reduces path length (and actual latency) as well, while not
taking more insns.
>
> Originally I had the optimization for DFmode as well as DImode. I found if
> the
> values were already in vector registers, that generally it was faster to do
> the
> XXPERMDI and vector store.
I don't see that? How is DF different from DI in any way here?
> +(define_insn_and_split "*concatv2di_store"
> + [(set (match_operand:V2DI 0 "memory_operand" "=m,m,m,m")
Should this just require ds_form_memory, instead of having a fallback
later?
> + /* Because we are creating scalar stores, we don't have to swap the order
> + of the elements and then swap the stores to get the right order on
> + little endian systems. */
Or any other system. Yes.
> +;; Optimize creating a vector with 2 duplicate DImode elements and storing
> it.
> +(define_insn_and_split "*dupv2di_store"
Hrm, can't this just use *concatv2di_store some way?
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/powerpc/pr81594.c
> @@ -0,0 +1,61 @@
> +/* { dg-do compile { target { powerpc-*-* && ilp64 } } } */
> +/* { dg-require-effective-target powerpc_p8vector_ok } */
> +/* { dg-options "-mdejagnu-cpu=power8 -O2" } */
This hasn't been tested.
> +/* PR target/81594. Optimize creating a vector of 2 64-bit elements and then
> + storing the vector into separate stores. */
Please mention the PR # in the subject and the changelog.
> +/* { dg-final { scan-assembler-not {\mstxv\M} } } */
> +/* { dg-final { scan-assembler-not {\mstxvx\M} } } */
> +/* { dg-final { scan-assembler-not {\mmfvsrd\M} } } */
> +/* { dg-final { scan-assembler-not {\mmtvsrd\M} } } */
> +/* { dg-final { scan-assembler-not {\mmtvsrdd\M} } } */
> +/* { dg-final { scan-assembler-not {\mxxpermdi\M} } } */
There are other ways to write the stores (older insns, and prefixed
insns, for example). xxpermdi has many extenden mnemonics (and we
actually use those, they improve readability quite a bit).
It helps if you also test what insns *should* be there, the testcase
will not so easily silently pass (while the generated code is not what
it wants) that way.
Segher
