[committed][nvptx] Handle unused result in nvptx_unisimt_handle_set

[Tom de Vries via Gcc-patches]

Hi,

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.

Committed to trunk.

Thanks,
- Tom

[nvptx] Handle unused result in nvptx_unisimt_handle_set

gcc/ChangeLog:

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

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

gcc/testsuite/ChangeLog:

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

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

---
 gcc/config/nvptx/nvptx.cc                       |  4 +++-
 gcc/testsuite/gcc.target/nvptx/uniform-simt-4.c | 22 ++++++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/gcc/config/nvptx/nvptx.cc b/gcc/config/nvptx/nvptx.cc
index 14911bd15f1..c41e305a34f 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);
diff --git a/gcc/testsuite/gcc.target/nvptx/uniform-simt-4.c 
b/gcc/testsuite/gcc.target/nvptx/uniform-simt-4.c
new file mode 100644
index 00000000000..c33de7a4111
--- /dev/null
+++ b/gcc/testsuite/gcc.target/nvptx/uniform-simt-4.c
@@ -0,0 +1,22 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -muniform-simt -mptx=_" } */
+
+enum memmodel
+{
+  MEMMODEL_RELAXED = 0
+};
+
+unsigned long long int *p64;
+unsigned long long int v64;
+
+int
+main()
+{
+  __atomic_fetch_add (p64, v64, MEMMODEL_RELAXED);
+
+  return 0;
+}
+
+/* { dg-final { scan-assembler-times "atom.add.u64\[\t \]+_," 1 } } */
+/* { dg-final { scan-assembler-times "bar.warp.sync" 1 } } */
+/* { dg-final { scan-assembler-not "shfl.sync.idx" } } */