[gcc r13-2514] tree-optimization/106860 - fix profile scaling in split_loop

[gcc r13-2514] tree-optimization/106860 - fix profile scaling in split_loop

[Richard Biener]

https://gcc.gnu.org/g:0386609923577e07354ee63754795b2f729e7e00

commit r13-2514-g0386609923577e07354ee63754795b2f729e7e00
Author: Richard Biener <rguenther@suse.de>
Date:   Wed Sep 7 10:44:33 2022 +0200

    tree-optimization/106860 - fix profile scaling in split_loop
    
    The following fixes a mistake in loop splitting which assumes loop
    latches have a single predecessor and that edge is from the exit
    test.  Instead work from the single exit edge we have to find the
    edge towards the latch.
    
            PR tree-optimization/106860
            * tree-ssa-loop-split.cc (split_loop): Find the exit to
            latch edge from the loop exit edge instead of from the
            latch.  Verify we're going to find it.
    
            * g++.dg/opt/pr106860.C: New testcase.

Diff:
---
 gcc/testsuite/g++.dg/opt/pr106860.C | 23 +++++++++++++++++++++++
 gcc/tree-ssa-loop-split.cc          | 16 ++++++++++------
 2 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/gcc/testsuite/g++.dg/opt/pr106860.C b/gcc/testsuite/g++.dg/opt/pr106860.C
new file mode 100644
index 00000000000..a0209dcf9da
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/pr106860.C
@@ -0,0 +1,23 @@
+// { dg-do compile }
+// { dg-options "-Ofast -ftrapv -fnon-call-exceptions -fno-tree-fre" }
+
+static const int N = 12;
+int nSlip;
+
+int main ()
+{
+  int i, j, k, fdot = 0;
+  int a[N][N];
+
+  for ( i = 1; i < nSlip; i++)
+    {
+      for ( j = i+1; j < nSlip; j++)
+        {
+          for ( k = 0; k < i; k++)
+            fdot += a[i][k] * a[k][j];
+          a[i][j] = a[i][j] - fdot;
+        }
+   }
+
+  return 0;
+}
diff --git a/gcc/tree-ssa-loop-split.cc b/gcc/tree-ssa-loop-split.cc
index bccf621493b..fad4e8361b0 100644
--- a/gcc/tree-ssa-loop-split.cc
+++ b/gcc/tree-ssa-loop-split.cc
@@ -531,16 +531,17 @@ split_loop (class loop *loop1)
   tree guard_iv;
   tree border = NULL_TREE;
   affine_iv iv;
+  edge exit1;
 
-  if (!single_exit (loop1)
+  if (!(exit1 = single_exit (loop1))
+      || EDGE_COUNT (exit1->src->succs) != 2
       /* ??? We could handle non-empty latches when we split the latch edge
 	 (not the exit edge), and put the new exit condition in the new block.
 	 OTOH this executes some code unconditionally that might have been
 	 skipped by the original exit before.  */
       || !empty_block_p (loop1->latch)
       || !easy_exit_values (loop1)
-      || !number_of_iterations_exit (loop1, single_exit (loop1), &niter,
-				     false, true)
+      || !number_of_iterations_exit (loop1, exit1, &niter, false, true)
       || niter.cmp == ERROR_MARK
       /* We can't yet handle loops controlled by a != predicate.  */
       || niter.cmp == NE_EXPR)
@@ -644,10 +645,13 @@ split_loop (class loop *loop1)
 	fix_loop_bb_probability (loop1, loop2, true_edge, false_edge);
 
 	/* Fix first loop's exit probability after scaling.  */
-	edge exit_to_latch1 = single_pred_edge (loop1->latch);
+	edge exit_to_latch1;
+	if (EDGE_SUCC (exit1->src, 0) == exit1)
+	  exit_to_latch1 = EDGE_SUCC (exit1->src, 1);
+	else
+	  exit_to_latch1 = EDGE_SUCC (exit1->src, 0);
 	exit_to_latch1->probability *= true_edge->probability;
-	single_exit (loop1)->probability
-	  = exit_to_latch1->probability.invert ();
+	exit1->probability = exit_to_latch1->probability.invert ();
 
 	/* Finally patch out the two copies of the condition to be always
 	   true/false (or opposite).  */