Re: [PATCH] Fix combiner (PRs rtl-optimization/46034, rtl-optimization/46212, rtl-optimization/46248)

[Eric Botcazou <ebotcazou@adacore.com>]

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> There are two issues, one is that the earlier
> newpat = subst (newpat, i0dest, i0src, ...);
> might have (but not necessarily) have changed i1src and so when i1dest
> is first replaced with i1src that way modified and then i0dest is replaced
> with i0src, the replacements are already wrong and as testcases show
> self-referential.

FWIW I also debugged this (and spotted a pasto in the 4-insn combiner patch 
that I'll fix after your fixes, patch attached).  I also roughly came up with:

+      /* Following subst may modify i1src, make a copy of it
+        before it is for added_sets_2 handling if needed.  */
+      if (added_sets_2
+         && i0dest_in_i0src
+         && i0_feeds_i1_n
+         && (i1_feeds_i2_n || i0_feeds_i2_n))
+       i1src_copy = copy_rtx (i1src);

but why not just

 if (i0_feeds_i1_n && added_sets_2 && i1_feeds_i2_n)

i.e. make a copy if substituting I0 will clobber I1SRC and I1SRC will be re- 
substituted in I2PAT?

> The other issue is that if we are to apply more than 
> one substitution on i2pat and i0dest_in_i0src, then we need to pass
> 1 as last argument to the first subst in order to avoid unwanted
> rtl sharing (which again can lead to self-referential rtl).
> Another issue is that if all of i0_feeds_i2_n, i0_feeds_i1_n and
> i1_feeds_i2_n is true, then we'd be substituting i0dest with i0src
> in i2pat twice.

This part looks OK to me.


	* combine.c (try_combine): Fix formatting issues and a pasto.


-- 
Eric Botcazou

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Index: combine.c
===================================================================
--- combine.c	(revision 166059)
+++ combine.c	(working copy)
@@ -3071,23 +3071,23 @@ try_combine (rtx i3, rtx i2, rtx i1, rtx
 	}
 
       n_occurrences = 0;		/* `subst' counts here */
-
-      /* If I1 feeds into I2 and I1DEST is in I1SRC, we need to make a
-	 unique copy of I2SRC each time we substitute it to avoid
-	 self-referential rtl.  */
-
       subst_low_luid = DF_INSN_LUID (i2);
+
+      /* If I1 feeds into I2 and I1DEST is in I1SRC, we need to make a unique
+	 copy of I2SRC each time we substitute it, in order to avoid creating
+	 self-referential RTL when we will be substituting I1SRC for I1DEST
+	 later.  Likewise if I0 feeds into I2 and I0DEST is in I0SRC.  */
       newpat = subst (PATTERN (i3), i2dest, i2src, 0,
-		      ((i1_feeds_i2_n && i1dest_in_i1src)
-		       || (i0_feeds_i2_n && i0dest_in_i0src)));
+		      (i1_feeds_i2_n && i1dest_in_i1src)
+		      || (i0_feeds_i2_n && i0dest_in_i0src));
       substed_i2 = 1;
 
-      /* Record whether i2's body now appears within i3's body.  */
+      /* Record whether I2's body now appears within I3's body.  */
       i2_is_used = n_occurrences;
     }
 
-  /* If we already got a failure, don't try to do more.  Otherwise,
-     try to substitute in I1 if we have it.  */
+  /* If we already got a failure, don't try to do more.  Otherwise, try to
+     substitute I1 if we have it.  */
 
   if (i1 && GET_CODE (newpat) != CLOBBER)
     {
@@ -3098,10 +3098,10 @@ try_combine (rtx i3, rtx i2, rtx i1, rtx
 	   && i1_feeds_i2_n
 	   && dead_or_set_p (i2, i1dest)
 	   && !reg_overlap_mentioned_p (i1dest, newpat))
-	  /* Before we can do this substitution, we must redo the test done
-	     above (see detailed comments there) that ensures  that I1DEST
-	     isn't mentioned in any SETs in NEWPAT that are field assignments.  */
-          || !combinable_i3pat (NULL_RTX, &newpat, i1dest, NULL_RTX, NULL_RTX,
+	   /* Before we can do this substitution, we must redo the test done
+	      above (see detailed comments there) that ensures I1DEST isn't
+	      mentioned in any SETs in NEWPAT that are field assignments.  */
+	  || !combinable_i3pat (NULL_RTX, &newpat, i1dest, NULL_RTX, NULL_RTX,
 				0, 0, 0))
 	{
 	  undo_all ();
@@ -3110,18 +3110,28 @@ try_combine (rtx i3, rtx i2, rtx i1, rtx
 
       n_occurrences = 0;
       subst_low_luid = DF_INSN_LUID (i1);
+
+      /* If I0 feeds into I1 and I0DEST is in I0SRC, we need to make a unique
+	 copy of I1SRC each time we substitute it, in order to avoid creating
+	 self-referential RTL when we will be substituting I0SRC for I0DEST
+	 later.  */
       newpat = subst (newpat, i1dest, i1src, 0,
 		      i0_feeds_i1_n && i0dest_in_i0src);
       substed_i1 = 1;
+
+      /* Record whether I1's body now appears within I3's body.  */
       i1_is_used = n_occurrences;
     }
+
+  /* Likewise for I0 if we have it.  */
+
   if (i0 && GET_CODE (newpat) != CLOBBER)
     {
       if ((FIND_REG_INC_NOTE (i0, NULL_RTX) != 0
 	   && ((i0_feeds_i2_n && dead_or_set_p (i2, i0dest))
 	       || (i0_feeds_i1_n && dead_or_set_p (i1, i0dest)))
 	   && !reg_overlap_mentioned_p (i0dest, newpat))
-          || !combinable_i3pat (NULL_RTX, &newpat, i0dest, NULL_RTX, NULL_RTX,
+	  || !combinable_i3pat (NULL_RTX, &newpat, i0dest, NULL_RTX, NULL_RTX,
 				0, 0, 0))
 	{
 	  undo_all ();
@@ -3130,8 +3140,7 @@ try_combine (rtx i3, rtx i2, rtx i1, rtx
 
       n_occurrences = 0;
       subst_low_luid = DF_INSN_LUID (i0);
-      newpat = subst (newpat, i0dest, i0src, 0,
-		      i0_feeds_i1_n && i0dest_in_i0src);
+      newpat = subst (newpat, i0dest, i0src, 0, 0);
       substed_i0 = 1;
     }