From: Denis Chertykov <chertykov@gmail.com>
Date: Mon, 20 Jan 2025 20:27:04 +0000 (+0400)
Subject: [PR117868][LRA]: Restrict the reuse of spill slots
X-Git-Tag: basepoints/gcc-16~2475
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=5cd4605141b8b45cab95e4de8005c69273071107;p=thirdparty%2Fgcc.git

[PR117868][LRA]: Restrict the reuse of spill slots

This is an LRA bug derived from reuse spilling slots after frame pointer spilling.
The slot was created for QImode (1 byte) and it was reused after spilling of the
frame pointer for TImode register (16 bytes long) and it overlaps other slots.

Wrong things happened while `lra_spill ()'
---------------------------- part of lra-spills.cc ----------------------------
  n = assign_spill_hard_regs (pseudo_regnos, n);
  slots_num = 0;
  assign_stack_slot_num_and_sort_pseudos (pseudo_regnos, n);  <--- first call ---
  for (i = 0; i < n; i++)
    if (pseudo_slots[pseudo_regnos[i]].mem == NULL_RTX)
      assign_mem_slot (pseudo_regnos[i]);
  if ((n2 = lra_update_fp2sp_elimination (pseudo_regnos)) > 0)
    {
      /* Assign stack slots to spilled pseudos assigned to fp.  */
      assign_stack_slot_num_and_sort_pseudos (pseudo_regnos, n2);  <--- second call ---
      for (i = 0; i < n2; i++)
	if (pseudo_slots[pseudo_regnos[i]].mem == NULL_RTX)
	  assign_mem_slot (pseudo_regnos[i]);
    }
------------------------------------------------------------------------------

In a first call of `assign_stack_slot_num_and_sort_pseudos(...)' LRA allocates slot #17
for r93 (QImode - 1 byte).
In a second call of `assign_stack_slot_num_and_sort_pseudos(...)' LRA reuse slot #17 for
r114 (TImode - 16 bytes).
It's wrong. We can't reuse 1 byte slot #17 for 16 bytes register.

The code in patch does reuse slots only without allocated memory or only with equal or
smaller registers with equal or smaller alignment.
Also, a small fix for debugging output of slot width.
Print slot size as width, not a 0 as a size of (mem/c:BLK (...)).

	PR rtl-optimization/117868
gcc/
	* lra-spills.cc (assign_stack_slot_num_and_sort_pseudos): Reuse slots
	only without allocated memory or only with equal or smaller registers
	with equal or smaller alignment.
	(lra_spill): Print slot size as width.
---

diff --git a/gcc/lra-spills.cc b/gcc/lra-spills.cc
index db78dcd28a3..93a0c92db9f 100644
--- a/gcc/lra-spills.cc
+++ b/gcc/lra-spills.cc
@@ -386,7 +386,18 @@ assign_stack_slot_num_and_sort_pseudos (int *pseudo_regnos, int n)
 		&& ! (lra_intersected_live_ranges_p
 		      (slots[j].live_ranges,
 		       lra_reg_info[regno].live_ranges)))
-	      break;
+	      {
+		/* A slot without allocated memory can be shared.  */
+		if (slots[j].mem == NULL_RTX)
+		  break;
+
+		/* A slot with allocated memory can be shared only with equal
+		   or smaller register with equal or smaller alignment.  */
+		if (slots[j].align >= spill_slot_alignment (mode)
+		    && compare_sizes_for_sort (slots[j].size,
+					       GET_MODE_SIZE (mode)) != -1)
+		  break;
+	      }
 	}
       if (j >= slots_num)
 	{
@@ -656,8 +667,7 @@ lra_spill (void)
       for (i = 0; i < slots_num; i++)
 	{
 	  fprintf (lra_dump_file, "  Slot %d regnos (width = ", i);
-	  print_dec (GET_MODE_SIZE (GET_MODE (slots[i].mem)),
-		     lra_dump_file, SIGNED);
+	  print_dec (slots[i].size, lra_dump_file, SIGNED);
 	  fprintf (lra_dump_file, "):");
 	  for (curr_regno = slots[i].regno;;
 	       curr_regno = pseudo_slots[curr_regno].next - pseudo_slots)