[PATCH] Shortcut identity VEC_PERM expansion [PR94710]

[Jakub Jelinek via Gcc-patches]

Hi!

This PR is about the rs6000 backend emitting wrong assembly
for whole vector shift by 0, and while I think it is desirable
to fix the backend, I don't see a point why the expander should
try to emit that, whole vector shift by 0 is identity, we can just
return the operand.

Bootstrapped/regtested on powerpc64{,le}-linux, ok for trunk?

2020-04-23  Jakub Jelinek  <ja...@redhat.com>

        PR target/94710
        * optabs.c (expand_vec_perm_const): For shift_amt const0_rtx
        just return v2.

--- gcc/optabs.c.jj     2020-04-17 14:18:44.380437703 +0200
+++ gcc/optabs.c        2020-04-23 11:50:07.931780323 +0200
@@ -5627,6 +5627,8 @@ expand_vec_perm_const (machine_mode mode
       if (shift_amt)
        {
          class expand_operand ops[3];
+         if (shift_amt == const0_rtx)
+           return v2;
          if (shift_code != CODE_FOR_nothing)
            {
              create_output_operand (&ops[0], target, mode);

        Jakub