[openmp, simt] Disable SIMT for user-defined reduction

The test-case included in this patch contains this target region:
...
  for (int i0 = 0 ; i0 < N0 ; i0++ )
    counter_N0.i += 1;
...

When running with nvptx accelerator, the counter variable is expected to
be N0 after the region, but instead is N0 / 32.  The problem is that rather
than getting the result for all warp lanes, we get it for just one lane.

This is caused by the implementation of SIMT being incomplete.  It handles
regular reductions, but appearantly not user-defined reductions.

For now, handle this by disabling SIMT in this case, specifically by setting
sctx->max_vf to 1.

Tested libgomp on x86_64-linux with nvptx accelerator.

gcc/ChangeLog:

2021-05-03  Tom de Vries  <tdevries@suse.de>

	PR target/100321
	* omp-low.c (lower_rec_input_clauses): Disable SIMT for user-defined
	reduction.

libgomp/ChangeLog:

2021-05-03  Tom de Vries  <tdevries@suse.de>

	PR target/100321
	* testsuite/libgomp.c/target-44.c: New test.

diff --git a/gcc/omp-low.c b/gcc/omp-low.c
index 7b122059c6e83033b7277e714ebe2e407390f401..1f14c4b1d6946cf486cdb979e3ed036cc3296178 100644 (file)
--- a/gcc/omp-low.c
+++ b/gcc/omp-low.c
@@ -4385,6 +4385,19 @@ lower_rec_simd_input_clauses (tree new_var, omp_context *ctx,
                sctx->max_vf = lower_bound (sctx->max_vf, safe_len);
            }
        }
+      if (sctx->is_simt && !known_eq (sctx->max_vf, 1U))
+       {
+         for (tree c = gimple_omp_for_clauses (ctx->stmt); c;
+              c = OMP_CLAUSE_CHAIN (c))
+           if (OMP_CLAUSE_CODE (c) == OMP_CLAUSE_REDUCTION
+               && OMP_CLAUSE_REDUCTION_PLACEHOLDER (c))
+             {
+               /* UDR reductions are not supported yet for SIMT, disable
+                  SIMT.  */
+               sctx->max_vf = 1;
+               break;
+             }
+       }
       if (maybe_gt (sctx->max_vf, 1U))
        {
          sctx->idx = create_tmp_var (unsigned_type_node);

diff --git a/libgomp/testsuite/libgomp.c/target-44.c b/libgomp/testsuite/libgomp.c/target-44.c
new file mode 100644 (file)
index 0000000..13e0c75
--- /dev/null
+++ b/libgomp/testsuite/libgomp.c/target-44.c
@@ -0,0 +1,27 @@
+/* { dg-additional-options "-foffload=-latomic" { target { offload_target_nvptx } } } */
+
+#include <stdlib.h>
+
+struct s
+{
+  int i;
+};
+
+#pragma omp declare reduction(+: struct s: omp_out.i += omp_in.i)
+
+int
+main (void)
+{
+  const int N0 = 32768;
+
+  struct s counter_N0 = { 0 };
+#pragma omp target
+#pragma omp for simd reduction(+: counter_N0)
+  for (int i0 = 0 ; i0 < N0 ; i0++ )
+    counter_N0.i += 1;
+
+  if (counter_N0.i != N0)
+    abort ();
+
+  return 0;
+}