openacc: Remove unnecessary barriers (gimple worker partitioning/broadcast)

This is an optimisation for middle-end worker-partitioning support (used
to support multiple workers on AMD GCN).  At present, barriers may be
emitted in cases where they aren't needed and cannot be optimised away.
This patch stops the extraneous barriers from being emitted in the
first place.

One exception to the above (where the barrier is still needed) is for
predicated blocks of code that perform a write to gang-private shared
memory from one worker.  We must execute a barrier before other workers
read that shared memory location.

gcc/
	* config/gcn/gcn.c (gimple.h): Include.
	(gcn_fork_join): Emit barrier for worker-level joins.
	* omp-oacc-neuter-broadcast.cc (find_local_vars_to_propagate): Add
	writes_gang_private bitmap parameter. Set bit for blocks
	containing gang-private variable writes.
	(worker_single_simple): Don't emit barrier after predicated block.
	(worker_single_copy): Don't emit barrier if we're not broadcasting
	anything and the block contains no gang-private writes.
	(neuter_worker_single): Don't predicate blocks that only contain
	NOPs or internal marker functions.  Pass has_gang_private_write
	argument to worker_single_copy.
	(oacc_do_neutering): Add writes_gang_private bitmap handling.

diff --git a/gcc/config/gcn/gcn.c b/gcc/config/gcn/gcn.c
index b1bfdeac7b64082cb3930fa0c7f6f23534ba3f93..2a3fc96c1ee0fd905ecbd5a3d4af27e716d6a2c1 100644 (file)
--- a/gcc/config/gcn/gcn.c
+++ b/gcc/config/gcn/gcn.c
@@ -51,6 +51,7 @@
 #include "intl.h"
 #include "rtl-iter.h"
 #include "dwarf2.h"
+#include "gimple.h"
 
 /* This file should be included last.  */
 #include "target-def.h"
@@ -5133,10 +5134,14 @@ gcn_oacc_dim_pos (int dim)
 /* Implement TARGET_GOACC_FORK_JOIN.  */
 
 static bool
-gcn_fork_join (gcall *ARG_UNUSED (call), const int *ARG_UNUSED (dims),
-              bool ARG_UNUSED (is_fork))
+gcn_fork_join (gcall *call, const int dims[], bool is_fork)
 {
-  /* GCN does not need to expand fork/join markers at the RTL level.  */
+  tree arg = gimple_call_arg (call, 2);
+  unsigned axis = TREE_INT_CST_LOW (arg);
+
+  if (!is_fork && axis == GOMP_DIM_WORKER && dims[axis] != 1)
+    return true;
+
   return false;
 }
 

diff --git a/gcc/omp-oacc-neuter-broadcast.cc b/gcc/omp-oacc-neuter-broadcast.cc
index e0bd01311ee1245509edc6142241e509029d3162..aa5990ed7a12fd127efaec66f8fc43905db5e1d4 100644 (file)
--- a/gcc/omp-oacc-neuter-broadcast.cc
+++ b/gcc/omp-oacc-neuter-broadcast.cc
@@ -769,16 +769,19 @@ static void
 find_local_vars_to_propagate (parallel_g *par, unsigned outer_mask,
                              hash_set<tree> *partitioned_var_uses,
                              hash_set<tree> *gang_private_vars,
+                             bitmap writes_gang_private,
                              vec<propagation_set *> *prop_set)
 {
   unsigned mask = outer_mask | par->mask;
 
   if (par->inner)
     find_local_vars_to_propagate (par->inner, mask, partitioned_var_uses,
-                                 gang_private_vars, prop_set);
+                                 gang_private_vars, writes_gang_private,
+                                 prop_set);
   if (par->next)
     find_local_vars_to_propagate (par->next, outer_mask, partitioned_var_uses,
-                                 gang_private_vars, prop_set);
+                                 gang_private_vars, writes_gang_private,
+                                 prop_set);
 
   if (!(mask & GOMP_DIM_MASK (GOMP_DIM_WORKER)))
     {
@@ -799,8 +802,7 @@ find_local_vars_to_propagate (parallel_g *par, unsigned outer_mask,
                  if (!VAR_P (var)
                      || is_global_var (var)
                      || AGGREGATE_TYPE_P (TREE_TYPE (var))
-                     || !partitioned_var_uses->contains (var)
-                     || gang_private_vars->contains (var))
+                     || !partitioned_var_uses->contains (var))
                    continue;
 
                  if (stmt_may_clobber_ref_p (stmt, var))
@@ -814,6 +816,14 @@ find_local_vars_to_propagate (parallel_g *par, unsigned outer_mask,
                          fprintf (dump_file, "\n");
                        }
 
+                     if (gang_private_vars->contains (var))
+                       {
+                         /* If we write a gang-private variable, we want a
+                            barrier at the end of the block.  */
+                         bitmap_set_bit (writes_gang_private, block->index);
+                         continue;
+                       }
+
                      if (!(*prop_set)[block->index])
                        (*prop_set)[block->index] = new propagation_set;
 
@@ -925,14 +935,6 @@ worker_single_simple (basic_block from, basic_block to,
            }
        }
     }
-
-  gsi = gsi_start_bb (skip_block);
-
-  decl = builtin_decl_explicit (BUILT_IN_GOACC_BARRIER);
-  gimple *acc_bar = gimple_build_call (decl, 0);
-
-  gsi_insert_before (&gsi, acc_bar, GSI_SAME_STMT);
-  update_stmt (acc_bar);
 }
 
 /* Build COMPONENT_REF and set TREE_THIS_VOLATILE and TREE_READONLY on it
@@ -1011,7 +1013,7 @@ worker_single_copy (basic_block from, basic_block to,
                    hash_set<tree> *worker_partitioned_uses,
                    tree record_type, record_field_map_t *record_field_map,
                    unsigned HOST_WIDE_INT placement,
-                   bool isolate_broadcasts)
+                   bool isolate_broadcasts, bool has_gang_private_write)
 {
   /* If we only have virtual defs, we'll have no record type, but we still want
      to emit single_copy_start and (particularly) single_copy_end to act as
@@ -1094,14 +1096,19 @@ worker_single_copy (basic_block from, basic_block to,
   edge ef = make_edge (from, barrier_block, EDGE_FALSE_VALUE);
   ef->probability = et->probability.invert ();
 
-  decl = builtin_decl_explicit (BUILT_IN_GOACC_BARRIER);
-  gimple *acc_bar = gimple_build_call (decl, 0);
-
   gimple_stmt_iterator bar_gsi = gsi_start_bb (barrier_block);
-  gsi_insert_before (&bar_gsi, acc_bar, GSI_NEW_STMT);
-
   cond = gimple_build_cond (NE_EXPR, recv_tmp, zero_ptr, NULL_TREE, NULL_TREE);
-  gsi_insert_after (&bar_gsi, cond, GSI_NEW_STMT);
+
+  if (record_type != char_type_node || has_gang_private_write)
+    {
+      decl = builtin_decl_explicit (BUILT_IN_GOACC_BARRIER);
+      gimple *acc_bar = gimple_build_call (decl, 0);
+
+      gsi_insert_before (&bar_gsi, acc_bar, GSI_NEW_STMT);
+      gsi_insert_after (&bar_gsi, cond, GSI_NEW_STMT);
+    }
+  else
+    gsi_insert_before (&bar_gsi, cond, GSI_NEW_STMT);
 
   edge et2 = split_block (barrier_block, cond);
   et2->flags &= ~EDGE_FALLTHRU;
@@ -1263,7 +1270,8 @@ neuter_worker_single (parallel_g *par, unsigned outer_mask,
                      vec<propagation_set *> *prop_set,
                      hash_set<tree> *partitioned_var_uses,
                      record_field_map_t *record_field_map,
-                     blk_offset_map_t *blk_offset_map)
+                     blk_offset_map_t *blk_offset_map,
+                     bitmap writes_gang_private)
 {
   unsigned mask = outer_mask | par->mask;
 
@@ -1349,10 +1357,57 @@ neuter_worker_single (parallel_g *par, unsigned outer_mask,
              (*prop_set)[block->index] = 0;
            }
 
-         tree record_type = (tree) block->aux;
+         bool only_marker_fns = true;
+         bool join_block = false;
+
+         for (gimple_stmt_iterator gsi = gsi_start_bb (block);
+              !gsi_end_p (gsi);
+              gsi_next (&gsi))
+           {
+             gimple *stmt = gsi_stmt (gsi);
+             if (gimple_code (stmt) == GIMPLE_CALL
+                 && gimple_call_internal_p (stmt, IFN_UNIQUE))
+               {
+                 enum ifn_unique_kind k = ((enum ifn_unique_kind)
+                   TREE_INT_CST_LOW (gimple_call_arg (stmt, 0)));
+                 if (k != IFN_UNIQUE_OACC_PRIVATE
+                     && k != IFN_UNIQUE_OACC_JOIN
+                     && k != IFN_UNIQUE_OACC_FORK
+                     && k != IFN_UNIQUE_OACC_HEAD_MARK
+                     && k != IFN_UNIQUE_OACC_TAIL_MARK)
+                   only_marker_fns = false;
+                 else if (k == IFN_UNIQUE_OACC_JOIN)
+                   /* The JOIN marker is special in that it *cannot* be
+                      predicated for worker zero, because it may be lowered
+                      to a barrier instruction and all workers must typically
+                      execute that barrier.  We shouldn't be doing any
+                      broadcasts from the join block anyway.  */
+                   join_block = true;
+               }
+             else if (gimple_code (stmt) == GIMPLE_CALL
+                      && gimple_call_internal_p (stmt, IFN_GOACC_LOOP))
+               /* Empty.  */;
+             else if (gimple_nop_p (stmt))
+               /* Empty.  */;
+             else
+               only_marker_fns = false;
+           }
+
+         /* We can skip predicating this block for worker zero if the only
+            thing it contains is marker functions that will be removed in the
+            oaccdevlow pass anyway.
+            Don't do this if the block has (any) phi nodes, because those
+            might define SSA names that need broadcasting.
+            TODO: We might be able to skip transforming blocks that only
+            contain some other trivial statements too.  */
+         if (only_marker_fns && !phi_nodes (block))
+           continue;
+
+         gcc_assert (!join_block);
 
          if (has_defs)
            {
+             tree record_type = (tree) block->aux;
              std::pair<unsigned HOST_WIDE_INT, bool> *off_rngalloc
                = blk_offset_map->get (block);
              gcc_assert (!record_type || off_rngalloc);
@@ -1360,10 +1415,13 @@ neuter_worker_single (parallel_g *par, unsigned outer_mask,
                = off_rngalloc ? off_rngalloc->first : 0;
              bool range_allocated
                = off_rngalloc ? off_rngalloc->second : true;
+             bool has_gang_private_write
+               = bitmap_bit_p (writes_gang_private, block->index);
              worker_single_copy (block, block, &def_escapes_block,
                                  &worker_partitioned_uses, record_type,
                                  record_field_map,
-                                 offset, !range_allocated);
+                                 offset, !range_allocated,
+                                 has_gang_private_write);
            }
          else
            worker_single_simple (block, block, &def_escapes_block);
@@ -1401,11 +1459,11 @@ neuter_worker_single (parallel_g *par, unsigned outer_mask,
   if (par->inner)
     neuter_worker_single (par->inner, mask, worker_single, vector_single,
                          prop_set, partitioned_var_uses, record_field_map,
-                         blk_offset_map);
+                         blk_offset_map, writes_gang_private);
   if (par->next)
     neuter_worker_single (par->next, outer_mask, worker_single, vector_single,
                          prop_set, partitioned_var_uses, record_field_map,
-                         blk_offset_map);
+                         blk_offset_map, writes_gang_private);
 }
 
 static void
@@ -1596,11 +1654,13 @@ oacc_do_neutering (unsigned HOST_WIDE_INT bounds_lo,
 
   hash_set<tree> partitioned_var_uses;
   hash_set<tree> gang_private_vars;
+  auto_bitmap writes_gang_private;
 
   find_gang_private_vars (&gang_private_vars);
   find_partitioned_var_uses (par, mask, &partitioned_var_uses);
   find_local_vars_to_propagate (par, mask, &partitioned_var_uses,
-                               &gang_private_vars, &prop_set);
+                               &gang_private_vars, writes_gang_private,
+                               &prop_set);
 
   record_field_map_t record_field_map;
 
@@ -1757,7 +1817,7 @@ oacc_do_neutering (unsigned HOST_WIDE_INT bounds_lo,
 
   neuter_worker_single (par, mask, worker_single, vector_single, &prop_set,
                        &partitioned_var_uses, &record_field_map,
-                       &blk_offset_map);
+                       &blk_offset_map, writes_gang_private);
 
   for (auto it : record_field_map)
     delete it.second;