[gcc r12-3313] Refine fix for PR78185, improve LIM for code after inner loops

[gcc r12-3313] Refine fix for PR78185, improve LIM for code after inner loops

[Richard Biener]

https://gcc.gnu.org/g:483e400870601f650c80f867ec781cd5f83507d6

commit r12-3313-g483e400870601f650c80f867ec781cd5f83507d6
Author: Richard Biener <rguenther@suse.de>
Date:   Thu Sep 2 10:47:35 2021 +0200

    Refine fix for PR78185, improve LIM for code after inner loops
    
    This refines the fix for PR78185 after understanding that the code
    regarding to the comment 'In a loop that is always entered we may
    proceed anyway.  But record that we entered it and stop once we leave
    it.' was supposed to protect us from leaving possibly infinite inner
    loops.  The simpler fix of moving the misplaced stopping code
    can then be refined to continue processing when the exited inner
    loop is finite, improving invariant motion for cases like in the
    added testcase.
    
    2021-09-02  Richard Biener  <rguenther@suse.de>
    
            * tree-ssa-loop-im.c (fill_always_executed_in_1): Refine
            fix for PR78185 and continue processing when leaving
            finite inner loops.
    
            * gcc.dg/tree-ssa/ssa-lim-16.c: New testcase.

Diff:
---
 gcc/testsuite/gcc.dg/tree-ssa/ssa-lim-16.c | 19 +++++++++++++++++
 gcc/tree-ssa-loop-im.c                     | 33 +++++++++++++++++-------------
 2 files changed, 38 insertions(+), 14 deletions(-)

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-lim-16.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-lim-16.c
new file mode 100644
index 00000000000..741582b745f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-lim-16.c
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-lim2-details" } */
+
+volatile int flag, bar;
+double foo (double *valp)
+{
+  double sum = 0;
+  for (int i = 0; i < 256; ++i)
+    {
+      for (int j = 0; j < 256; ++j)
+        bar = flag;
+      if (flag)
+        sum += 1.;
+      sum += *valp; // we should move the load of *valp out of the loop
+    }
+  return sum;
+}
+
+/* { dg-final { scan-tree-dump-times "Moving statement" 1 "lim2" } } */
diff --git a/gcc/tree-ssa-loop-im.c b/gcc/tree-ssa-loop-im.c
index d9f75d5025e..01f3fc2f7f0 100644
--- a/gcc/tree-ssa-loop-im.c
+++ b/gcc/tree-ssa-loop-im.c
@@ -3044,23 +3044,27 @@ fill_always_executed_in_1 (class loop *loop, sbitmap contains_call)
 	  edge_iterator ei;
 	  bb = bbs[i];
 
+	  if (!flow_bb_inside_loop_p (inn_loop, bb))
+	    {
+	      /* When we are leaving a possibly infinite inner loop
+		 we have to stop processing.  */
+	      if (!finite_loop_p (inn_loop))
+		break;
+	      /* If the loop was finite we can continue with processing
+		 the loop we exited to.  */
+	      inn_loop = bb->loop_father;
+	    }
+
 	  if (dominated_by_p (CDI_DOMINATORS, loop->latch, bb))
 	    last = bb;
 
 	  if (bitmap_bit_p (contains_call, bb->index))
 	    break;
 
+	  /* If LOOP exits from this BB stop processing.  */
 	  FOR_EACH_EDGE (e, ei, bb->succs)
-	    {
-	      /* If there is an exit from this BB.  */
-	      if (!flow_bb_inside_loop_p (loop, e->dest))
-		break;
-	      /* Or we enter a possibly non-finite loop.  */
-	      if (flow_loop_nested_p (bb->loop_father,
-				      e->dest->loop_father)
-		  && ! finite_loop_p (e->dest->loop_father))
-		break;
-	    }
+	    if (!flow_bb_inside_loop_p (loop, e->dest))
+	      break;
 	  if (e)
 	    break;
 
@@ -3069,22 +3073,23 @@ fill_always_executed_in_1 (class loop *loop, sbitmap contains_call)
 	  if (bb->flags & BB_IRREDUCIBLE_LOOP)
 	    break;
 
-	  if (!flow_bb_inside_loop_p (inn_loop, bb))
-	    break;
-
 	  if (bb->loop_father->header == bb)
 	    {
 	      if (!dominated_by_p (CDI_DOMINATORS, loop->latch, bb))
 		break;
 
 	      /* In a loop that is always entered we may proceed anyway.
-		 But record that we entered it and stop once we leave it.  */
+		 But record that we entered it and stop once we leave it
+		 since it might not be finite.  */
 	      inn_loop = bb->loop_father;
 	    }
 	}
 
       while (1)
 	{
+	  if (dump_enabled_p ())
+	    dump_printf (MSG_NOTE, "BB %d is always executed in loop %d\n",
+			 last->index, loop->num);
 	  SET_ALWAYS_EXECUTED_IN (last, loop);
 	  if (last == loop->header)
 	    break;