Re: [patch] Fix debug info for modified parameter

[Eric Botcazou <ebotcazou at adacore dot com>]

> Changing the coalesce cost to MUST_COALESCE_COST for
> PARM_DECLs and RESULT_DECLs should add the required
> checking (we'll ICE if we cannot honor that coalescing request).
> Note that the code also has a !DECL_IGNORED_P check, so
> eventually the expansion code change should restrict itself to
> that as well.
> 
> But the patch is ok, you can followup with the coalesce cost change
> and the !DECL_IGNORED_P checking (if you are not confident enough it
> works as-is).

Even if I were, I strongly believe in consistency and the changes you sketched 
here are the ones I should have come up with, had I known my way around the 
coalescing machinery...  So, thanks again for the tip, patch attached, tested 
with no regressions on x86_64-suse-linux.


2013-11-13  Eric Botcazou  <ebotcazou@adacore.com>

	* cfgexpand.c (expand_used_vars): Allocate space for partitions based
	on PARM_DECLs or RESULT_DECLs only if they are ignored for debug info
	or if optimization is enabled.
	* tree-ssa-coalesce.c (coalesce_ssa_name): If optimization is disabled,
	require that all the names based on a PARM_DECL or a RESULT_DECL that
	isn't ignored for debug info be coalesced.


-- 
Eric Botcazou

Index: cfgexpand.c
===================================================================
--- cfgexpand.c	(revision 204678)
+++ cfgexpand.c	(working copy)
@@ -1610,9 +1610,15 @@ expand_used_vars (void)
 	  replace_ssa_name_symbol (var, (tree) *slot);
 	}
 
+      /* Always allocate space for partitions based on VAR_DECLs.  But for
+	 those based on PARM_DECLs or RESULT_DECLs and which matter for the
+	 debug info, there is no need to do so if optimization is disabled
+	 because all the SSA_NAMEs based on these DECLs have been coalesced
+	 into a single partition, which is thus assigned the canonical RTL
+	 location of the DECLs.  */
       if (TREE_CODE (SSA_NAME_VAR (var)) == VAR_DECL)
 	expand_one_var (var, true, true);
-      else
+      else if (DECL_IGNORED_P (SSA_NAME_VAR (var)) || optimize)
 	{
 	  /* This is a PARM_DECL or RESULT_DECL.  For those partitions that
 	     contain the default def (representing the parm or result itself)
Index: tree-ssa-coalesce.c
===================================================================
--- tree-ssa-coalesce.c	(revision 204678)
+++ tree-ssa-coalesce.c	(working copy)
@@ -1256,8 +1256,8 @@ coalesce_ssa_name (void)
   cl = create_coalesce_list ();
   map = create_outofssa_var_map (cl, used_in_copies);
 
-  /* We need to coalesce all names originating same SSA_NAME_VAR
-     so debug info remains undisturbed.  */
+  /* If optimization is disabled, we need to coalesce all the names originating
+     from the same SSA_NAME_VAR so debug info remains undisturbed.  */
   if (!optimize)
     {
       hash_table <ssa_name_var_hash> ssa_name_hash;
@@ -1278,8 +1278,16 @@ coalesce_ssa_name (void)
 		*slot = a;
 	      else
 		{
-		  add_coalesce (cl, SSA_NAME_VERSION (a), SSA_NAME_VERSION (*slot),
-				MUST_COALESCE_COST - 1);
+		  /* If the variable is a PARM_DECL or a RESULT_DECL, we
+		     _require_ that all the names originating from it be
+		     coalesced, because there must be a single partition
+		     containing all the names so that it can be assigned
+		     the canonical RTL location of the DECL safely.  */
+		  const int cost
+		    = TREE_CODE (SSA_NAME_VAR (a)) == VAR_DECL
+		      ? MUST_COALESCE_COST - 1 : MUST_COALESCE_COST;
+		  add_coalesce (cl, SSA_NAME_VERSION (a),
+				SSA_NAME_VERSION (*slot), cost);
 		  bitmap_set_bit (used_in_copies, SSA_NAME_VERSION (a));
 		  bitmap_set_bit (used_in_copies, SSA_NAME_VERSION (*slot));
 		}