[gcc/devel/omp/gcc-11] openacc: Warn about "independent" "kernels" loops with data-dependences

[Frederik Harwath <frederik@gcc.gnu.org>]

https://gcc.gnu.org/g:a4ce3665319fbf3d5c1547e9985e64a9ff98ced8

commit a4ce3665319fbf3d5c1547e9985e64a9ff98ced8
Author: Frederik Harwath <frederik@codesourcery.com>
Date:   Tue Nov 16 16:20:15 2021 +0100

    openacc: Warn about "independent" "kernels" loops with data-dependences
    
    This commit concerns loops in OpenACC "kernels" region that have been marked
    up with an explicit "independent" clause by the user, but for which Graphite
    found data dependences.  A discussion on the private internal OpenACC mailing
    list suggested that warning the user about the dependences woud be a more
    acceptable solution than reverting the user's decision. This behavior is
    implemented by the present commit.
    
    gcc/ChangeLog:
    
            * common.opt: Add flag Wopenacc-false-independent.
            * omp-offload.c (oacc_loop_warn_if_false_independent): New function.
            (oacc_loop_fixed_partitions): Call from here.

Diff:
---
 gcc/common.opt    |  5 +++++
 gcc/omp-offload.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+)

diff --git a/gcc/common.opt b/gcc/common.opt
index aa695e56dc4..4c38ed5cf9a 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -838,6 +838,11 @@ Wtsan
 Common Var(warn_tsan) Init(1) Warning
 Warn about unsupported features in ThreadSanitizer.
 
+Wopenacc-false-independent
+Common Var(warn_openacc_false_independent) Init(1) Warning
+Warn in case a loop in an OpenACC \"kernels\" region has an \"independent\"
+clause but analysis shows that it has loop-carried dependences.
+
 Xassembler
 Driver Separate
 
diff --git a/gcc/omp-offload.c b/gcc/omp-offload.c
index 94a975a8866..b806e36ef51 100644
--- a/gcc/omp-offload.c
+++ b/gcc/omp-offload.c
@@ -2043,6 +2043,51 @@ oacc_loop_transform_auto_into_independent (oacc_loop *loop)
   return true;
 }
 
+/* Emit a warning if LOOP has an "independent" clause but Graphite's
+   analysis shows that it has data dependences. Note that we respect
+   the user's explicit decision to parallelize the loop but we
+   nevertheless warn that this decision could be wrong. */
+
+static void
+oacc_loop_warn_if_false_independent (oacc_loop *loop)
+{
+  if (!optimize)
+    return;
+
+  if (loop->routine)
+    return;
+
+  /* TODO Warn about "auto" & "independent" in "parallel" regions? */
+  if (!oacc_parallel_kernels_graphite_fun_p ())
+    return;
+
+  if (!(loop->flags & OLF_INDEPENDENT))
+    return;
+
+  bool analyzed = false;
+  bool can_be_parallel = oacc_loop_can_be_parallel_p (loop, analyzed);
+  loop_p cfg_loop = oacc_loop_get_cfg_loop (loop);
+
+  if (cfg_loop && cfg_loop->inner && !analyzed)
+    {
+      if (dump_enabled_p ())
+	{
+	  const dump_user_location_t loc
+	    = dump_user_location_t::from_location_t (loop->loc);
+	  dump_printf_loc (MSG_MISSED_OPTIMIZATION, loc,
+			   "'independent' loop in 'kernels' region has not been "
+			   "analyzed (cf. 'graphite' "
+			   "dumps for more information).\n");
+	}
+      return;
+    }
+
+  if (!can_be_parallel)
+    warning_at (loop->loc, 0,
+                "loop has \"independent\" clause but data dependences were "
+                "found.");
+}
+
 /* Walk the OpenACC loop hierarchy checking and assigning the
    programmer-specified partitionings.  OUTER_MASK is the partitioning
    this loop is contained within.  Return mask of partitioning
@@ -2094,6 +2139,10 @@ oacc_loop_fixed_partitions (oacc_loop *loop, unsigned outer_mask)
 	    }
 	}
 
+      /* TODO Is this flag needed? Perhaps use -Wopenacc-parallelism? */
+      if (warn_openacc_false_independent)
+        oacc_loop_warn_if_false_independent (loop);
+
       if (maybe_auto && (loop->flags & OLF_INDEPENDENT))
 	{
 	  loop->flags |= OLF_AUTO;