[gcc/devel/omp/gcc-11] Disable SIMT for user-defined reduction

[Tobias Burnus <burnus@gcc.gnu.org>]

https://gcc.gnu.org/g:6723e6700a3178f7e0fe5696325b25ccb81e5d09

commit 6723e6700a3178f7e0fe5696325b25ccb81e5d09
Author: Tom de Vries <tdevries@suse.de>
Date:   Fri May 14 09:24:47 2021 +0200

    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.
    
    (cherry picked from commit f87990a2a8fc9e20d30462a0a4c9047582af0cd9)

Diff:
---
 gcc/ChangeLog.omp                       |  9 +++++++++
 gcc/omp-low.c                           | 13 +++++++++++++
 libgomp/ChangeLog.omp                   |  8 ++++++++
 libgomp/testsuite/libgomp.c/target-44.c | 27 +++++++++++++++++++++++++++
 4 files changed, 57 insertions(+)

diff --git a/gcc/ChangeLog.omp b/gcc/ChangeLog.omp
index 407ecb406f5..8e306a70480 100644
--- a/gcc/ChangeLog.omp
+++ b/gcc/ChangeLog.omp
@@ -1,3 +1,12 @@
+2021-05-14  Tobias Burnus  <tobias@codesourcery.com>
+
+	Backported from master:
+	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.
+
 2021-05-14  Tobias Burnus  <tobias@codesourcery.com>
 
 	Backported from master:
diff --git a/gcc/omp-low.c b/gcc/omp-low.c
index f0cc49c2048..5eb8aee9c5e 100644
--- a/gcc/omp-low.c
+++ b/gcc/omp-low.c
@@ -4710,6 +4710,19 @@ 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/ChangeLog.omp b/libgomp/ChangeLog.omp
index 96fc5ead0a0..e0f1ec0b50c 100644
--- a/libgomp/ChangeLog.omp
+++ b/libgomp/ChangeLog.omp
@@ -1,3 +1,11 @@
+2021-05-14  Tobias Burnus  <tobias@codesourcery.com>
+
+	Backported from master:
+	2021-05-03  Tom de Vries  <tdevries@suse.de>
+
+	PR target/100321
+	* testsuite/libgomp.c/target-44.c: New test.
+
 2021-05-14  Tobias Burnus  <tobias@codesourcery.com>
 
 	Backported from master:
diff --git a/libgomp/testsuite/libgomp.c/target-44.c b/libgomp/testsuite/libgomp.c/target-44.c
new file mode 100644
index 00000000000..13e0c757845
--- /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;
+}