[PATCH] FIx endless match.pd recursion on cst1 + cst2 + cst3 (PR tree-optimization/84334)

[Jakub Jelinek]

Hi!

On the following testcase, we recurse infinitely, because
we have float re-association enabled, but also rounding-math, so
we try to optimize (cst1 + cst2) + cst3 as (cst2 + cst3) + cst1
but (cst2 + cst3) doesn't simplify and we try again and optimize
it as (cst3 + cst1) + cst2 and then (cst1 + cst2) + cst3 and so on
forever.  If @0 is not a CONSTANT_CLASS_P, there is not a problem,
if it is, the code just checks if we can actually simplify the
operation between cst2 and cst3 into a constant.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2018-02-13  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/84334
	* match.pd ((A +- CST1) +- CST2 -> A + CST3): If A is
	also a CONSTANT_CLASS_P, only optimize if we can fold the
	operation between CST1 and CST2 into a constant.

	* gcc.dg/pr84334.c: New test.

--- gcc/match.pd.jj	2018-02-13 09:33:31.000000000 +0100
+++ gcc/match.pd	2018-02-13 12:14:08.108314686 +0100
@@ -1733,9 +1733,20 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
 	       CONSTANT_CLASS_P@2)
      /* If one of the types wraps, use that one.  */
      (if (!ANY_INTEGRAL_TYPE_P (type) || TYPE_OVERFLOW_WRAPS (type))
-      (if (outer_op == PLUS_EXPR)
-       (plus (view_convert @0) (inner_op @2 (view_convert @1)))
-       (minus (view_convert @0) (neg_inner_op @2 (view_convert @1))))
+      /* If all 3 captures are CONSTANT_CLASS_P, only optimize if we
+	 can simplify @2 with @1 into a constant, otherwise we might recurse
+	 forever.  */
+      (if (CONSTANT_CLASS_P (@0))
+       (with { tree cst = fold_unary (VIEW_CONVERT_EXPR, type, @1);
+	       if (cst && CONSTANT_CLASS_P (cst))
+		 cst = const_binop (outer_op == PLUS_EXPR
+				    ? inner_op : neg_inner_op, type,
+				    @2, cst); }
+	(if (cst)
+	 (outer_op (view_convert @0) { cst; })))
+       (if (outer_op == PLUS_EXPR)
+	(plus (view_convert @0) (inner_op @2 (view_convert @1)))
+	(minus (view_convert @0) (neg_inner_op @2 (view_convert @1)))))
       (if (!ANY_INTEGRAL_TYPE_P (TREE_TYPE (@0))
 	   || TYPE_OVERFLOW_WRAPS (TREE_TYPE (@0)))
        (if (outer_op == PLUS_EXPR)
--- gcc/testsuite/gcc.dg/pr84334.c.jj	2018-02-13 12:18:12.765463667 +0100
+++ gcc/testsuite/gcc.dg/pr84334.c	2018-02-13 11:36:56.019632428 +0100
@@ -0,0 +1,12 @@
+/* PR tree-optimization/84334 */
+/* { dg-do compile } */
+/* { dg-options "-Ofast -frounding-math" } */
+
+float
+foo (void)
+{
+  float a = 9.999999974752427078783512115478515625e-7f;
+  float b = 1.999999994950485415756702423095703125e-6f;
+  float c = 4.999999873689375817775726318359375e-6f;
+  return a + b + c;
+}

	Jakub