Re: [PR103302] skip multi-word pre-move clobber during lra

[Alexandre Oliva <oliva@adacore.com>]

On Feb 23, 2022, Alexandre Oliva <oliva@adacore.com> wrote:

> On Feb 21, 2022, Richard Biener <richard.guenther@gmail.com> wrote:
>>> Ok to revert commit r12-5852-g50e8b0c9bca6cdc57804f860ec5311b641753fbb

>> OK.  Please re-open the bug as appropriate.

> Thanks.  I've reopened it.  Here's what I'm installing.  I'm not
> reverting the testcase, since it stopped failing even before the patch
> was put in.

And now here's a patch that fixes the underlying issue.


Undo multi-word optional reloads correctly

From: Alexandre Oliva <oliva@adacore.com>

Unlike e.g. remove_inheritance_pseudos, undo_optional_reloads didn't
deal with subregs, so instead of removing multi-word moves, it
replaced the reload pseudo with the original pseudo.  Besides the
redundant move, that retained the clobber of the dest, that starts a
multi-word move.  After the remap, the sequence that should have
become a no-op move starts by clobbering the original pseudo and then
moving its pieces onto themselves.  The problem is the clobber: it
makes earlier sets of the original pseudo to be regarded as dead: if
the optional reload sequence was an output reload, the insn for which
the output reload was attempted may be regarded as dead and deleted.

I've arranged for undo_optional_reloads to accept SUBREGs and use
get_regno, like remove_inheritance_pseudo, adjusted its insn-removal
loop to tolerate iterating over a removed clobber, and added logic to
catch any left-over reload clobbers that could trigger the problem.


for  gcc/ChangeLog

	* lra-constraints.cc (undo_optional_reloads): Recognize and
	drop insns of multi-word move sequences, tolerate removal
	iteration on an already-removed clobber, and refuse to
	substitute original pseudos into clobbers.
---
 gcc/lra-constraints.cc |   37 ++++++++++++++++++++++++-------------
 1 file changed, 24 insertions(+), 13 deletions(-)

diff --git a/gcc/lra-constraints.cc b/gcc/lra-constraints.cc
index b2c4590153c4c..5cd89e7eeddc2 100644
--- a/gcc/lra-constraints.cc
+++ b/gcc/lra-constraints.cc
@@ -7261,15 +7261,17 @@ undo_optional_reloads (void)
 	      continue;
 	    src = SET_SRC (set);
 	    dest = SET_DEST (set);
-	    if (! REG_P (src) || ! REG_P (dest))
+	    if ((! REG_P (src) && ! SUBREG_P (src))
+		|| (! REG_P (dest) && ! SUBREG_P (dest)))
 	      continue;
-	    if (REGNO (dest) == regno
+	    if (get_regno (dest) == (int) regno
 		/* Ignore insn for optional reloads itself.  */
-		&& REGNO (lra_reg_info[regno].restore_rtx) != REGNO (src)
+		&& (get_regno (lra_reg_info[regno].restore_rtx)
+		    != get_regno (src))
 		/* Check only inheritance on last inheritance pass.  */
-		&& (int) REGNO (src) >= new_regno_start
+		&& get_regno (src) >= new_regno_start
 		/* Check that the optional reload was inherited.  */
-		&& bitmap_bit_p (&lra_inheritance_pseudos, REGNO (src)))
+		&& bitmap_bit_p (&lra_inheritance_pseudos, get_regno (src)))
 	      {
 		keep_p = true;
 		break;
@@ -7291,18 +7293,22 @@ undo_optional_reloads (void)
       bitmap_copy (insn_bitmap, &lra_reg_info[regno].insn_bitmap);
       EXECUTE_IF_SET_IN_BITMAP (insn_bitmap, 0, uid, bi2)
 	{
+	  /* We may have already removed a clobber.  */
+	  if (!lra_insn_recog_data[uid])
+	    continue;
 	  insn = lra_insn_recog_data[uid]->insn;
 	  if ((set = single_set (insn)) != NULL_RTX)
 	    {
 	      src = SET_SRC (set);
 	      dest = SET_DEST (set);
-	      if (REG_P (src) && REG_P (dest)
-		  && ((REGNO (src) == regno
-		       && (REGNO (lra_reg_info[regno].restore_rtx)
-			   == REGNO (dest)))
-		      || (REGNO (dest) == regno
-			  && (REGNO (lra_reg_info[regno].restore_rtx)
-			      == REGNO (src)))))
+	      if ((REG_P (src) || SUBREG_P (src))
+		  && (REG_P (dest) || SUBREG_P (dest))
+		  && ((get_regno (src) == (int) regno
+		       && (get_regno (lra_reg_info[regno].restore_rtx)
+			   == get_regno (dest)))
+		      || (get_regno (dest) == (int) regno
+			  && (get_regno (lra_reg_info[regno].restore_rtx)
+			      == get_regno (src)))))
 		{
 		  if (lra_dump_file != NULL)
 		    {
@@ -7310,7 +7316,7 @@ undo_optional_reloads (void)
 			       INSN_UID (insn));
 		      dump_insn_slim (lra_dump_file, insn);
 		    }
-		  delete_move_and_clobber (insn, REGNO (dest));
+		  delete_move_and_clobber (insn, get_regno (dest));
 		  continue;
 		}
 	      /* We should not worry about generation memory-memory
@@ -7319,6 +7325,11 @@ undo_optional_reloads (void)
 		 we remove the inheritance pseudo and the optional
 		 reload.  */
 	    }
+	  if (GET_CODE (PATTERN (insn)) == CLOBBER
+	      && REG_P (SET_DEST (insn))
+	      && get_regno (SET_DEST (insn)) == (int) regno)
+	    /* Refuse to remap clobbers to preexisting pseudos.  */
+	    gcc_unreachable ();
 	  lra_substitute_pseudo_within_insn
 	    (insn, regno, lra_reg_info[regno].restore_rtx, false);
 	  lra_update_insn_regno_info (insn);


-- 
Alexandre Oliva, happy hacker                https://FSFLA.org/blogs/lxo/
   Free Software Activist                       GNU Toolchain Engineer
Disinformation flourishes because many people care deeply about injustice
but very few check the facts.  Ask me about <https://stallmansupport.org>