Patchwork [PR,tree-optimization/52558] : RFC: questions on store data race

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Submitter Aldy Hernandez
Date May 31, 2012, 7:52 p.m.
Message ID <4FC7CC0E.6020805@redhat.com>
Download mbox | patch
Permalink /patch/162220/
State New
Headers show

Comments

Aldy Hernandez - May 31, 2012, 7:52 p.m.
On 05/29/12 06:13, Richard Guenther wrote:
> On Mon, 21 May 2012, Aldy Hernandez wrote:
>
>> On 05/16/12 07:53, Richard Guenther wrote:
>>> On Mon, 7 May 2012, Aldy Hernandez wrote:

>
> (flag_tm&&  loop_preheader_edge (loop)->src->flags&  BB_IN_TRANSACTION)
>
> can you encapsulate this into a predicate?  Like block_in_transaction ()
> that also checks flag_tm?

Done.. whoops, forgot to check for flag_tm.  I will move this into the 
predicate after I do another round of testing.  I missed this bit, and 
I've just committed.

>
> +  /* ?? FIXME TESTING TESTING ?? */
> +  multi_threaded_model_p=true;
> +  /* ?? FIXME TESTING TESTING ?? */
>
> that of course needs fixing ;)  (and re-testing)

This was here on purpose, so you'd see how I was testing.  I have 
committed the attached patch, not before testing with _and_ without the 
above FIXME.

Thanks so much for the review.  I will follow up with the flag_tm 
abstraction.

Closing the PR...

Aldy
PR tree-optimization/52558
	* cfg.c (alloc_aux_for_edge): Fix comment.
	(alloc_aux_for_edge): Remove static.
	* basic-block.h (alloc_aux_for_edge): Protoize.
	* tree-ssa-loop-im.c (execute_sm_if_changed): New.
	(execute_sm_if_changed_flag): New.
	(execute_sm_if_changed_flag_set): New.
	(execute_sm): Do not generate data races unless requested.
	(tree_ssa_lim_initialize): Call alloc_aux_for_edges.
	(tree_ssa_lim_finalize): Call free_aux_for_edges.
	* gimple.h (block_in_transaction): New.
	(gimple_in_transaction): Use block_in_transaction.
Tobias Burnus - June 1, 2012, 6:22 a.m.
Aldy Hernandez wrote:
> 	PR tree-optimization/52558
> 	* cfg.c (alloc_aux_for_edge): Fix comment.
> 	(alloc_aux_for_edge): Remove static.
> 	* basic-block.h (alloc_aux_for_edge): Protoize.
> 	* tree-ssa-loop-im.c (execute_sm_if_changed): New.
> 	(execute_sm_if_changed_flag): New.
> 	(execute_sm_if_changed_flag_set): New.
> 	(execute_sm): Do not generate data races unless requested.
> 	(tree_ssa_lim_initialize): Call alloc_aux_for_edges.
> 	(tree_ssa_lim_finalize): Call free_aux_for_edges.
> 	* gimple.h (block_in_transaction): New.
> 	(gimple_in_transaction): Use block_in_transaction.

> Index: gimple.h
> ===================================================================
> --- gimple.h	(revision 188080)
> +++ gimple.h	(working copy)

> +/* Return true if BB is in a transaction.  */
> +
> +static inline bool
> +block_in_transaction (basic_block bb)
> +{
> +  return bb->flags & BB_IN_TRANSACTION;

The compile does not seem to like the latter very much and, thus, warns 
for every file which includes gimple.h:

gcc/gimple.h: In function 'block_in_transaction':
gcc/gimple.h:1596:20: warning: overflow in implicit constant conversion 
[-Woverflow]
    return bb->flags & BB_IN_TRANSACTION;
                     ^

Tobias
Aldy Hernandez - June 1, 2012, 3:11 p.m.
On 06/01/12 01:22, Tobias Burnus wrote:

> gcc/gimple.h: In function 'block_in_transaction':
> gcc/gimple.h:1596:20: warning: overflow in implicit constant conversion
> [-Woverflow]
> return bb->flags & BB_IN_TRANSACTION;
> ^

Is this still the case with the code currently in mainline:

   return flag_tm && bb->flags & BB_IN_TRANSACTION;

??
Aldy Hernandez - June 1, 2012, 3:45 p.m.
On 06/01/12 10:11, Aldy Hernandez wrote:
> On 06/01/12 01:22, Tobias Burnus wrote:
>
>> gcc/gimple.h: In function 'block_in_transaction':
>> gcc/gimple.h:1596:20: warning: overflow in implicit constant conversion
>> [-Woverflow]
>> return bb->flags & BB_IN_TRANSACTION;
>> ^
>
> Is this still the case with the code currently in mainline:
>
> return flag_tm && bb->flags & BB_IN_TRANSACTION;
>
> ??

Whoops, I forgot to commit that last patch.  Check now.
Eric Botcazou - June 15, 2012, 11:40 a.m.
> Whoops, I forgot to commit that last patch.  Check now.

The warning is there on the 4.7 branch now.

Patch

Index: testsuite/gcc.dg/pr52558-1.c
===================================================================
--- testsuite/gcc.dg/pr52558-1.c	(revision 0)
+++ testsuite/gcc.dg/pr52558-1.c	(revision 0)
@@ -0,0 +1,22 @@ 
+/* { dg-do compile } */
+/* { dg-options "--param allow-store-data-races=0 -O2 -fdump-tree-lim1" } */
+
+/* Test that `count' is not written to unless p->data > 0.  */
+
+int count;
+
+struct obj {
+    int data;
+    struct obj *next;
+} *q;
+
+void func()
+{
+  struct obj *p;
+  for (p = q; p; p = p->next)
+    if (p->data > 0)
+      count++;
+}
+
+/* { dg-final { scan-tree-dump-times "MEM count_lsm.. count_lsm_flag" 1 "lim1" } } */
+/* { dg-final { cleanup-tree-dump "lim1" } } */
Index: testsuite/gcc.dg/pr52558-2.c
===================================================================
--- testsuite/gcc.dg/pr52558-2.c	(revision 0)
+++ testsuite/gcc.dg/pr52558-2.c	(revision 0)
@@ -0,0 +1,23 @@ 
+/* { dg-do compile } */
+/* { dg-options "--param allow-store-data-races=0 -O2 -fdump-tree-lim1" } */
+
+/* Test that g_2 is not written to unless !g_1.  */
+
+int g_1 = 1;
+int g_2 = 0;
+
+int func_1(void)
+{
+ int l;
+ for (l = 0; l < 1234; l++)
+ {
+   if (g_1)
+     return l;
+   else
+     g_2 = 0;
+ }
+ return 999;
+}
+
+/* { dg-final { scan-tree-dump-times "MEM.*g_2_lsm_flag" 1 "lim1" } } */
+/* { dg-final { cleanup-tree-dump "lim1" } } */
Index: testsuite/gcc.dg/tm/reg-promotion.c
===================================================================
--- testsuite/gcc.dg/tm/reg-promotion.c	(revision 0)
+++ testsuite/gcc.dg/tm/reg-promotion.c	(revision 0)
@@ -0,0 +1,24 @@ 
+/* { dg-do compile } */
+/* { dg-options "-fgnu-tm -O2 -fdump-tree-lim1" } */
+
+/* Test that `count' is not written to unless p->data>0.  */
+
+int count;
+
+struct obj {
+    int data;
+    struct obj *next;
+} *q;
+
+void func()
+{
+  struct obj *p;
+  __transaction_atomic {
+    for (p = q; p; p = p->next)
+      if (p->data > 0)
+	count++;
+  }
+}
+
+/* { dg-final { scan-tree-dump-times "MEM count_lsm.. count_lsm_flag" 1 "lim1" } } */
+/* { dg-final { cleanup-tree-dump "lim1" } } */
Index: tree-ssa-loop-im.c
===================================================================
--- tree-ssa-loop-im.c	(revision 188080)
+++ tree-ssa-loop-im.c	(working copy)
@@ -52,7 +52,7 @@  along with GCC; see the file COPYING3.
 	 }
      }
 
-   Where COND and INV are is invariants, but evaluating INV may trap or be
+   Where COND and INV are invariants, but evaluating INV may trap or be
    invalid from some other reason if !COND.  This may be transformed to
 
    if (cond)
@@ -1626,6 +1626,7 @@  gather_mem_refs_stmt (struct loop *loop,
 	  fprintf (dump_file, "\n");
 	}
     }
+
   if (is_stored)
     mark_ref_stored (ref, loop);
 
@@ -1956,6 +1957,173 @@  get_lsm_tmp_name (tree ref, unsigned n)
   return lsm_tmp_name;
 }
 
+struct prev_flag_edges {
+  /* Edge to insert new flag comparison code.  */
+  edge append_cond_position;
+
+  /* Edge for fall through from previous flag comparison.  */
+  edge last_cond_fallthru;
+};
+
+/* Helper function for execute_sm.  Emit code to store TMP_VAR into
+   MEM along edge EX.
+
+   The store is only done if MEM has changed.  We do this so no
+   changes to MEM occur on code paths that did not originally store
+   into it.
+
+   The common case for execute_sm will transform:
+
+     for (...) {
+       if (foo)
+         stuff;
+       else
+         MEM = TMP_VAR;
+     }
+
+   into:
+
+     lsm = MEM;
+     for (...) {
+       if (foo)
+         stuff;
+       else
+         lsm = TMP_VAR;
+     }
+     MEM = lsm;
+
+  This function will generate:
+
+     lsm = MEM;
+
+     lsm_flag = false;
+     ...
+     for (...) {
+       if (foo)
+         stuff;
+       else {
+         lsm = TMP_VAR;
+         lsm_flag = true;
+       }
+     }
+     if (lsm_flag)	<--
+       MEM = lsm;	<--
+*/
+
+static void
+execute_sm_if_changed (edge ex, tree mem, tree tmp_var, tree flag)
+{
+  basic_block new_bb, then_bb, old_dest;
+  bool loop_has_only_one_exit;
+  edge then_old_edge, orig_ex = ex;
+  gimple_stmt_iterator gsi;
+  gimple stmt;
+  struct prev_flag_edges *prev_edges = (struct prev_flag_edges *) ex->aux;
+
+  /* ?? Insert store after previous store if applicable.  See note
+     below.  */
+  if (prev_edges)
+    ex = prev_edges->append_cond_position;
+
+  loop_has_only_one_exit = single_pred_p (ex->dest);
+
+  if (loop_has_only_one_exit)
+    ex = split_block_after_labels (ex->dest);
+
+  old_dest = ex->dest;
+  new_bb = split_edge (ex);
+  then_bb = create_empty_bb (new_bb);
+  if (current_loops && new_bb->loop_father)
+    add_bb_to_loop (then_bb, new_bb->loop_father);
+
+  gsi = gsi_start_bb (new_bb);
+  stmt = gimple_build_cond (NE_EXPR, flag, boolean_false_node,
+			    NULL_TREE, NULL_TREE);
+  gsi_insert_after (&gsi, stmt, GSI_CONTINUE_LINKING);
+
+  gsi = gsi_start_bb (then_bb);
+  /* Insert actual store.  */
+  stmt = gimple_build_assign (unshare_expr (mem), tmp_var);
+  gsi_insert_after (&gsi, stmt, GSI_CONTINUE_LINKING);
+
+  make_edge (new_bb, then_bb, EDGE_TRUE_VALUE);
+  make_edge (new_bb, old_dest, EDGE_FALSE_VALUE);
+  then_old_edge = make_edge (then_bb, old_dest, EDGE_FALLTHRU);
+
+  set_immediate_dominator (CDI_DOMINATORS, then_bb, new_bb);
+
+  if (prev_edges)
+    {
+      basic_block prevbb = prev_edges->last_cond_fallthru->src;
+      redirect_edge_succ (prev_edges->last_cond_fallthru, new_bb);
+      set_immediate_dominator (CDI_DOMINATORS, new_bb, prevbb);
+      set_immediate_dominator (CDI_DOMINATORS, old_dest,
+			       recompute_dominator (CDI_DOMINATORS, old_dest));
+    }
+
+  /* ?? Because stores may alias, they must happen in the exact
+     sequence they originally happened.  Save the position right after
+     the (_lsm) store we just created so we can continue appending after
+     it and maintain the original order.  */
+  {
+    struct prev_flag_edges *p;
+
+    if (orig_ex->aux)
+      orig_ex->aux = NULL;
+    alloc_aux_for_edge (orig_ex, sizeof (struct prev_flag_edges));
+    p = (struct prev_flag_edges *) orig_ex->aux;
+    p->append_cond_position = then_old_edge;
+    p->last_cond_fallthru = find_edge (new_bb, old_dest);
+    orig_ex->aux = (void *) p;
+  }
+
+  if (!loop_has_only_one_exit)
+    for (gsi = gsi_start_phis (old_dest); !gsi_end_p (gsi); gsi_next (&gsi))
+      {
+	gimple phi = gsi_stmt (gsi);
+	unsigned i;
+
+	for (i = 0; i < gimple_phi_num_args (phi); i++)
+	  if (gimple_phi_arg_edge (phi, i)->src == new_bb)
+	    {
+	      tree arg = gimple_phi_arg_def (phi, i);
+	      add_phi_arg (phi, arg, then_old_edge, UNKNOWN_LOCATION);
+	      update_stmt (phi);
+	    }
+      }
+  /* Remove the original fall through edge.  This was the
+     single_succ_edge (new_bb).  */
+  EDGE_SUCC (new_bb, 0)->flags &= ~EDGE_FALLTHRU;
+}
+
+/* Helper function for execute_sm.  On every location where REF is
+   set, set an appropriate flag indicating the store.  */
+
+static tree
+execute_sm_if_changed_flag_set (struct loop *loop, mem_ref_p ref)
+{
+  unsigned i;
+  mem_ref_loc_p loc;
+  tree flag;
+  VEC (mem_ref_loc_p, heap) *locs = NULL;
+  char *str = get_lsm_tmp_name (ref->mem, ~0);
+
+  lsm_tmp_name_add ("_flag");
+  flag = make_rename_temp (boolean_type_node, str);
+  get_all_locs_in_loop (loop, ref, &locs);
+  FOR_EACH_VEC_ELT (mem_ref_loc_p, locs, i, loc)
+    {
+      gimple_stmt_iterator gsi;
+      gimple stmt;
+
+      gsi = gsi_for_stmt (loc->stmt);
+      stmt = gimple_build_assign (flag, boolean_true_node);
+      gsi_insert_after (&gsi, stmt, GSI_CONTINUE_LINKING);
+    }
+  VEC_free (mem_ref_loc_p, heap, locs);
+  return flag;
+}
+
 /* Executes store motion of memory reference REF from LOOP.
    Exits from the LOOP are stored in EXITS.  The initialization of the
    temporary variable is put to the preheader of the loop, and assignments
@@ -1964,12 +2132,13 @@  get_lsm_tmp_name (tree ref, unsigned n)
 static void
 execute_sm (struct loop *loop, VEC (edge, heap) *exits, mem_ref_p ref)
 {
-  tree tmp_var;
+  tree tmp_var, store_flag;
   unsigned i;
-  gimple load, store;
+  gimple load;
   struct fmt_data fmt_data;
-  edge ex;
+  edge ex, latch_edge;
   struct lim_aux_data *lim_data;
+  bool multi_threaded_model_p = false;
 
   if (dump_file && (dump_flags & TDF_DETAILS))
     {
@@ -1985,23 +2154,47 @@  execute_sm (struct loop *loop, VEC (edge
   fmt_data.orig_loop = loop;
   for_each_index (&ref->mem, force_move_till, &fmt_data);
 
+  if ((flag_tm && block_in_transaction (loop_preheader_edge (loop)->src))
+      || !PARAM_VALUE (PARAM_ALLOW_STORE_DATA_RACES))
+    multi_threaded_model_p = true;
+
+  if (multi_threaded_model_p)
+    store_flag = execute_sm_if_changed_flag_set (loop, ref);
+
   rewrite_mem_refs (loop, ref, tmp_var);
 
-  /* Emit the load & stores.  */
+  /* Emit the load code into the latch, so that we are sure it will
+     be processed after all dependencies.  */
+  latch_edge = loop_latch_edge (loop);
+
+  /* FIXME/TODO: For the multi-threaded variant, we could avoid this
+     load altogether, since the store is predicated by a flag.  We
+     could, do the load only if it was originally in the loop.  */
   load = gimple_build_assign (tmp_var, unshare_expr (ref->mem));
   lim_data = init_lim_data (load);
   lim_data->max_loop = loop;
   lim_data->tgt_loop = loop;
+  gsi_insert_on_edge (latch_edge, load);
 
-  /* Put this into the latch, so that we are sure it will be processed after
-     all dependencies.  */
-  gsi_insert_on_edge (loop_latch_edge (loop), load);
-
-  FOR_EACH_VEC_ELT (edge, exits, i, ex)
+  if (multi_threaded_model_p)
     {
-      store = gimple_build_assign (unshare_expr (ref->mem), tmp_var);
-      gsi_insert_on_edge (ex, store);
+      load = gimple_build_assign (store_flag, boolean_false_node);
+      lim_data = init_lim_data (load);
+      lim_data->max_loop = loop;
+      lim_data->tgt_loop = loop;
+      gsi_insert_on_edge (latch_edge, load);
     }
+
+  /* Sink the store to every exit from the loop.  */
+  FOR_EACH_VEC_ELT (edge, exits, i, ex)
+    if (!multi_threaded_model_p)
+      {
+	gimple store;
+	store = gimple_build_assign (unshare_expr (ref->mem), tmp_var);
+	gsi_insert_on_edge (ex, store);
+      }
+    else
+      execute_sm_if_changed (ex, ref->mem, tmp_var, store_flag);
 }
 
 /* Hoists memory references MEM_REFS out of LOOP.  EXITS is the list of exit
@@ -2410,6 +2603,8 @@  tree_ssa_lim_initialize (void)
 
   if (flag_tm)
     compute_transaction_bits ();
+
+  alloc_aux_for_edges (0);
 }
 
 /* Cleans up after the invariant motion pass.  */
@@ -2421,6 +2616,8 @@  tree_ssa_lim_finalize (void)
   unsigned i;
   bitmap b;
 
+  free_aux_for_edges ();
+
   FOR_EACH_BB (bb)
     SET_ALWAYS_EXECUTED_IN (bb, NULL);
 
Index: gimple.h
===================================================================
--- gimple.h	(revision 188080)
+++ gimple.h	(working copy)
@@ -1588,12 +1588,20 @@  gimple_set_has_volatile_ops (gimple stmt
     stmt->gsbase.has_volatile_ops = (unsigned) volatilep;
 }
 
+/* Return true if BB is in a transaction.  */
+
+static inline bool
+block_in_transaction (basic_block bb)
+{
+  return bb->flags & BB_IN_TRANSACTION;
+}
+
 /* Return true if STMT is in a transaction.  */
 
 static inline bool
 gimple_in_transaction (gimple stmt)
 {
-  return gimple_bb (stmt)->flags & BB_IN_TRANSACTION;
+  return block_in_transaction (gimple_bb (stmt));
 }
 
 /* Return true if statement STMT may access memory.  */
Index: basic-block.h
===================================================================
--- basic-block.h	(revision 188080)
+++ basic-block.h	(working copy)
@@ -802,6 +802,7 @@  extern basic_block alloc_block (void);
 extern void alloc_aux_for_blocks (int);
 extern void clear_aux_for_blocks (void);
 extern void free_aux_for_blocks (void);
+extern void alloc_aux_for_edge (edge, int);
 extern void alloc_aux_for_edges (int);
 extern void clear_aux_for_edges (void);
 extern void free_aux_for_edges (void);
Index: cfg.c
===================================================================
--- cfg.c	(revision 188080)
+++ cfg.c	(working copy)
@@ -814,10 +814,10 @@  free_aux_for_blocks (void)
   clear_aux_for_blocks ();
 }
 
-/* Allocate a memory edge of SIZE as BB->aux.  The obstack must
+/* Allocate a memory edge of SIZE as E->aux.  The obstack must
    be first initialized by alloc_aux_for_edges.  */
 
-static void
+void
 alloc_aux_for_edge (edge e, int size)
 {
   /* Verify that aux field is clear.  */