[nvptx] Handle unused result in nvptx_unisimt_handle_set

For an example:
...
  #pragma omp target map(tofrom: counter_N0)
  #pragma omp simd
  for (int i = 0 ; i < 1 ; i++ )
    {
      #pragma omp atomic update
      counter_N0 = counter_N0 + 1 ;
    }
...
I noticed that the result of the atomic update (%r30) is propagated:
...
  @%r33 atom.add.u32		_, [%r29], 1;
	shfl.sync.idx.b32	%r30, %r30, %r32, 31, 0xffffffff;
...
even though it is unused (which is why the bit bucket operand _ is used).

Fix this by not emitting the shuffle in this case, such that we have instead:
...
  @%r33 atom.add.u32	_, [%r29], 1;
	bar.warp.sync	0xffffffff;
...

Tested on nvptx.

gcc/ChangeLog:

2022-03-07  Tom de Vries  <tdevries@suse.de>

	* config/nvptx/nvptx.cc (nvptx_unisimt_handle_set): Handle unused
	result.

gcc/testsuite/ChangeLog:

2022-03-07  Tom de Vries  <tdevries@suse.de>

	* gcc.target/nvptx/uniform-simt-4.c: New test.

1 files changed, 3 insertions, 1 deletions

diff --git a/gcc/config/nvptx/nvptx.cc b/gcc/config/nvptx/nvptx.cc
index 14911bd..c41e305 100644
--- a/gcc/config/nvptx/nvptx.cc
+++ b/gcc/config/nvptx/nvptx.cc
@@ -3274,7 +3274,9 @@ static bool
 nvptx_unisimt_handle_set (rtx set, rtx_insn *insn, rtx master)
 {
   rtx reg;
-  if (GET_CODE (set) == SET && REG_P (reg = SET_DEST (set)))
+  if (GET_CODE (set) == SET
+      && REG_P (reg = SET_DEST (set))
+      && find_reg_note (insn, REG_UNUSED, reg) == NULL_RTX)
     {
       emit_insn_after (nvptx_gen_shuffle (reg, reg, master, SHUFFLE_IDX),
 		       insn);