Shortcut identity VEC_PERM expansion [PR94710]

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.

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

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

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 5ddb433..54e207c 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,5 +1,9 @@
 2020-04-23  Jakub Jelinek  <jakub@redhat.com>
 
+       PR target/94710
+       * optabs.c (expand_vec_perm_const): For shift_amt const0_rtx
+       just return v2.
+
        PR middle-end/94724
        * tree.c (get_narrower): Instead of creating COMPOUND_EXPRs
        temporarily with non-final second operand and updating it later,

diff --git a/gcc/optabs.c b/gcc/optabs.c
index 1456e59..d85ce47 100644 (file)
--- a/gcc/optabs.c
+++ b/gcc/optabs.c
@@ -5627,6 +5627,8 @@ expand_vec_perm_const (machine_mode mode, rtx v0, rtx v1,
       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);