[SMS, DDG] Additional edges to instructions with clobber

[Roman Zhuykov <zhroma at ispras dot ru>]

This letter contains the first separate patch for ddg.cIt creates
additional nessesary anti-dep edges in data dependency graph.
2011/10/12 Ayal Zaks <ayal.zaks@gmail.com>:>>> The following situation
happens when SMS is enabled without register renaming>>>
(-fno-modulo-sched-allow-regmoves). ?When data dependency graph is
built, there>>> is a step when we generate anti-dependencies from last
register use to first>>> write of this register at the next
iteration.>> is a step when we generate anti-dependencies from all
last registers> uses (i.e., of last register def) to first write of
this register at> the next iteration.>
Right.
>>> At this moment we should also>>> create such dependencies to all instructions which clobber the register to>>> prevent this clobbers being before last use is new schedule.>>>>> well, we simply need to connect these last uses to either the first> write *or* the first clobber of this register at the next iteration,> according to whichever is first, no?
No, is works now just how you describe. Clobber is considered as a
write,and last uses are connected with first write or first
clobber.But this is not sufficient: similarly to how there's no
dependenciesbetween last uses,there shall be no dependency between
"first" clobbers (i.e. clobbersof a registercan be permuted freely).
So at least we have to make an additional dependencyedge to each
clobber before first write.
>>> Here is an model of example:>>>>>> loop {>>> set1 regR>>> use1 regR>>> clobber regR>>> set2 regR>>> use2 regR>>> }>>>>>> If we create only use2->set1 anti-dependency (and no use2->cloober) the>>> following wrong schedule is possible:>>>>>> prologue {>>> set1 regR>>> use1 regR>>> clobber regR>>> }>>> kernel {>>> set2 regR>>> clobber regR (instruction from next iteration in terms of old loop kernel)>>> use2 regR>>> set1 regR (also from next iteration)>>> use1 regR (also from next iteration)>>> }>>> epilogue {>>> set2 regR>>> use2 regR>>> }>>>>> strange; according to prolog (and epilog), clobber should appear after> use1 in the kernel, no? Aren't there (intra-iteration) dependencies> preventing the clobber to skip over use1 and/or set1?>
Yeah, this is bad example - I wrongly suggested that there is
noanti-dep between use1 and clobber.
New example:loop {?clobber1?clobber2?set?use}When there is no
"use->clobber2" anti-dep - the following wrong scheduleis possible.
Clobber2 is on stage0, other instructions are on stage1.prologue {
clobber2}kernel {?clobber1?set?clobber2?use}epilogue {?clobber1?set
use}
>>> This problem was succesfully fixed by creating a vector of all clobbering>>> instructions together with first write and adding all needed dependencies.>>>>> seems like an overkill to me; we didn't draw an edge between every> last use and every write, because writes are kept in order by having> output dependences between them. So should be the case with clobbers.
Clobbers themselves aren't kept in order - there are no output
dependencesbetween them. They may be scheduled in any order.
> Presumably, the ddg already has all intra-iteration edges preventing> motion of clobbering instructions within an iteration, and we are only> missing inter-iteration deps or deps surfaced by eliminating> anti-deps, right?
I think the new version of a fix suits this reasoning.Now it creates
dependencies only to clobber instructions before first write.And
certainly to first write instruction itself.
--Roman Zhuykovzhroma@ispras.ru

2011-12-07  Roman Zhuykov  <zhroma@ispras.ru>
	* ddg.c: New VEC.
	(add_cross_iteration_register_deps): Store information about
	clobbers before first write to a register.  Use collected
	information to create anti-dependencies from last uses.
---

diff --git a/gcc/ddg.c b/gcc/ddg.c
index 2b1cfe8..e3e065c 100644
--- a/gcc/ddg.c
+++ b/gcc/ddg.c
@@ -49,6 +49,10 @@ along with GCC; see the file COPYING3.  If not see
 /* A flag indicating that a ddg edge belongs to an SCC or not.  */
 enum edge_flag {NOT_IN_SCC = 0, IN_SCC};
 
+/* A vector of dependencies needed while processing clobbers.  */
+DEF_VEC_P(df_ref);
+DEF_VEC_ALLOC_P(df_ref,heap);
+
 /* Forward declarations.  */
 static void add_backarc_to_ddg (ddg_ptr, ddg_edge_ptr);
 static void add_backarc_to_scc (ddg_scc_ptr, ddg_edge_ptr);
@@ -273,24 +277,52 @@ create_ddg_dep_no_link (ddg_ptr g, ddg_node_ptr from, ddg_node_ptr to,
 static void
 add_cross_iteration_register_deps (ddg_ptr g, df_ref last_def)
 {
-  int regno = DF_REF_REGNO (last_def);
+  unsigned int regno = DF_REF_REGNO (last_def);
   struct df_link *r_use;
   int has_use_in_bb_p = false;
-  rtx def_insn = DF_REF_INSN (last_def);
+  rtx insn, def_insn = DF_REF_INSN (last_def);
   ddg_node_ptr last_def_node = get_node_of_insn (g, def_insn);
   ddg_node_ptr use_node;
 #ifdef ENABLE_CHECKING
   struct df_rd_bb_info *bb_info = DF_RD_BB_INFO (g->bb);
 #endif
-  df_ref first_def = df_bb_regno_first_def_find (g->bb, regno);
+  static VEC(df_ref,heap) *clobbers;
+  df_ref first_write_def = NULL;
+  df_ref *def_rec;
+  unsigned int uid;
+  
+  clobbers = VEC_alloc (df_ref, heap, 0);
+
+  FOR_BB_INSNS (g->bb, insn)
+    {
+      if (!INSN_P (insn))
+        continue;
+      uid = INSN_UID (insn);
+      for (def_rec = DF_INSN_UID_DEFS (uid); *def_rec; def_rec++)
+        {
+          df_ref def = *def_rec;
+          if (DF_REF_REGNO (def) == regno)
+            {
+	      VEC_safe_push (df_ref, heap, clobbers, def);
+	      if (!(DF_REF_FLAGS (def)
+		    & (DF_REF_MUST_CLOBBER | DF_REF_MAY_CLOBBER)))
+		{
+		  first_write_def = def;
+		  break;
+		}
+	    }
+        }
+      if (first_write_def != NULL)
+	break;
+    }
 
   gcc_assert (last_def_node);
-  gcc_assert (first_def);
+  gcc_assert (first_write_def);
 
 #ifdef ENABLE_CHECKING
-  if (DF_REF_ID (last_def) != DF_REF_ID (first_def))
+  if (DF_REF_ID (last_def) != DF_REF_ID (first_write_def))
     gcc_assert (!bitmap_bit_p (&bb_info->gen,
-			       DF_REF_ID (first_def)));
+			       DF_REF_ID (first_write_def)));
 #endif
 
   /* Create inter-loop true dependences and anti dependences.  */
@@ -316,29 +348,35 @@ add_cross_iteration_register_deps (ddg_ptr g, df_ref last_def)
 	}
       else if (!DEBUG_INSN_P (use_insn))
 	{
+	  unsigned int i;
+	  df_ref curr_def;
 	  /* Add anti deps from last_def's uses in the current iteration
-	     to the first def in the next iteration.  We do not add ANTI
-	     dep when there is an intra-loop TRUE dep in the opposite
-	     direction, but use regmoves to fix such disregarded ANTI
-	     deps when broken.	If the first_def reaches the USE then
-	     there is such a dep.  */
-	  ddg_node_ptr first_def_node = get_node_of_insn (g,
-							  DF_REF_INSN (first_def));
-
-	  gcc_assert (first_def_node);
-
-         /* Always create the edge if the use node is a branch in
-            order to prevent the creation of reg-moves.  
-            If the address that is being auto-inc or auto-dec in LAST_DEF
-            is used in USE_INSN then do not remove the edge to make sure
-            reg-moves will not be created for that address.  */
-          if (DF_REF_ID (last_def) != DF_REF_ID (first_def)
-              || !flag_modulo_sched_allow_regmoves
-	      || JUMP_P (use_node->insn)
-              || autoinc_var_is_used_p (DF_REF_INSN (last_def), use_insn))
-            create_ddg_dep_no_link (g, use_node, first_def_node, ANTI_DEP,
-                                    REG_DEP, 1);
-
+	     to the first def and all clobbers before it in the next iteration.
+	     We do not add ANTI dep when there is an intra-loop TRUE dep
+	     in the opposite direction, but use regmoves to fix such
+	     disregarded ANTI deps when broken.	If the curr_def reaches
+	     the USE then there is such a dep.  */
+	  FOR_EACH_VEC_ELT (df_ref, clobbers, i, curr_def)
+	    {
+	      if (DF_REF_ID (last_def) != DF_REF_ID (curr_def)
+		  /* Some hard regs (for ex. CC-flags) can't be renamed.  
+                  || HARD_REGISTER_P (DF_REF_REG (last_def)) */
+		  || !flag_modulo_sched_allow_regmoves
+		  /* If the address that is being auto-inc or auto-dec in LAST_DEF
+		     is used in USE_INSN then do not remove the edge to make sure
+		     reg-moves will not be created for that address.  */
+		  || autoinc_var_is_used_p (DF_REF_INSN (last_def), use_insn)
+		  /* Always create the edge if the use node is a branch in
+		     order to prevent the creation of reg-moves.  */
+		  || JUMP_P (use_node->insn))
+		{
+	          ddg_node_ptr curr_def_node = get_node_of_insn (g,
+						DF_REF_INSN (curr_def));
+		  gcc_assert (curr_def_node);
+		  create_ddg_dep_no_link (g, use_node, curr_def_node,
+					  ANTI_DEP, REG_DEP, 1);
+	        }
+	    }
 	}
     }
   /* Create an inter-loop output dependence between LAST_DEF (which is the
@@ -352,14 +390,16 @@ add_cross_iteration_register_deps (ddg_ptr g, df_ref last_def)
     {
       ddg_node_ptr dest_node;
 
-      if (DF_REF_ID (last_def) == DF_REF_ID (first_def))
+      if (DF_REF_ID (last_def) == DF_REF_ID (first_write_def))
 	return;
 
-      dest_node = get_node_of_insn (g, DF_REF_INSN (first_def));
+      dest_node = get_node_of_insn (g, DF_REF_INSN (first_write_def));
       gcc_assert (dest_node);
       create_ddg_dep_no_link (g, last_def_node, dest_node,
 			      OUTPUT_DEP, REG_DEP, 1);
     }
+
+  VEC_free (df_ref, heap, clobbers);
 }
 /* Build inter-loop dependencies, by looking at DF analysis backwards.  */
 static void