tree-optimization/113080 - missing final value replacement

When performing final value replacement we guard against exponential
(temporary) code growth due to unsharing of trees (SCEV heavily
relies on tree sharing).  The following relaxes this a tiny bit
to cover some more optimizations and puts in comments as to what
the real fix would be.

	PR tree-optimization/113080
	* tree-scalar-evolution.cc (expression_expensive_p): Allow
	a tiny bit of growth due to expansion of shared trees.
	(final_value_replacement_loop): Add comment.

	* gcc.dg/tree-ssa/sccp-3.c: New testcase.

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/sccp-3.c b/gcc/testsuite/gcc.dg/tree-ssa/sccp-3.c
new file mode 100644 (file)
index 0000000..b8c6742
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/sccp-3.c
@@ -0,0 +1,20 @@
+/* PR/113080 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+int a,b,n;
+int w;
+void fun1(int t)
+{
+  for(int i=0;i<100;i++)
+    {
+      a+=w;
+      b-=w;
+      t+=a+b;
+    }
+  n=t;
+}
+
+/* We should apply final value replacement to all reductions and
+   elide the loop.  */
+/* { dg-final { scan-tree-dump-times "<bb" 1 "optimized" } } */

diff --git a/gcc/tree-scalar-evolution.cc b/gcc/tree-scalar-evolution.cc
index 94250b143b36dffdfe36e9551660503a21554210..d743402d1c8640b043b9d3de12f2a4f73d41546b 100644 (file)
--- a/gcc/tree-scalar-evolution.cc
+++ b/gcc/tree-scalar-evolution.cc
@@ -3529,7 +3529,12 @@ expression_expensive_p (tree expr, bool *cond_overflow_p)
   uint64_t expanded_size = 0;
   *cond_overflow_p = false;
   return (expression_expensive_p (expr, cond_overflow_p, cache, expanded_size)
-         || expanded_size > cache.elements ());
+         /* ???  Both the explicit unsharing and gimplification of expr will
+            expand shared trees to multiple copies.
+            Guard against exponential growth by counting the visits and
+            comparing againt the number of original nodes.  Allow a tiny
+            bit of duplication to catch some additional optimizations.  */
+         || expanded_size > (cache.elements () + 1));
 }
 
 /* Match.pd function to match bitwise inductive expression.
@@ -3867,6 +3872,10 @@ final_value_replacement_loop (class loop *loop)
          fprintf (dump_file, "\n");
        }
       any = true;
+      /* ???  Here we'd like to have a unshare_expr that would assign
+        shared sub-trees to new temporary variables either gimplified
+        to a GIMPLE sequence or to a statement list (keeping this a
+        GENERIC interface).  */
       def = unshare_expr (def);
       remove_phi_node (&psi, false);