[1/2] Fix bogus double reduction (PR 86725)

[Richard Sandiford <richard dot sandiford at arm dot com>]

This patch is the first part of the fix for PR 86725.  We would
treat x_1 in:

    outer1:
      x_1 = PHI <x_4(outer2), ...>;
      ...

    inner:
      x_2 = ...x_1...;
      ...
      x_3 = ...;
      ...

    outer2:
      x_4 = PHI <x_3(inner)>;
      ...

as a double reduction without checking what kind of statement x_2 is.
In practice it has to be a phi, since for other x_2, x_1 would simply
be a loop invariant that gets used for every inner loop iteration.

The idea with doing this patch first is that, by checking x_2 really
is a phi, we can hand off the validation of the rest of the reduction
to the phi analysis in the inner loop.

The test case is a variant of the one in the PR.

Tested on aarch64-linux-gnu (with and without SVE), aarch64_be-elf
and x86_64-linux-gnu.  OK for trunk and GCC 8?

Richard


2018-08-22  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/86725
	* tree-vect-loop.c (vect_is_simple_reduction): When treating
	an outer loop phi as a double reduction, make sure that the
	single user of the phi result is an inner loop phi.

gcc/testsuite/
	PR tree-optimization/86725
	* gcc.dg/vect/no-scevccp-pr86725-1.c: New test.

Index: gcc/tree-vect-loop.c
===================================================================
--- gcc/tree-vect-loop.c	2018-08-21 14:47:07.047174151 +0100
+++ gcc/tree-vect-loop.c	2018-08-22 10:25:06.682099699 +0100
@@ -2992,6 +2992,7 @@ vect_is_simple_reduction (loop_vec_info
           && loop->inner
           && flow_bb_inside_loop_p (loop->inner, gimple_bb (def1))
           && is_gimple_assign (def1)
+	  && is_a <gphi *> (phi_use_stmt)
 	  && flow_bb_inside_loop_p (loop->inner, gimple_bb (phi_use_stmt)))
         {
           if (dump_enabled_p ())
Index: gcc/testsuite/gcc.dg/vect/no-scevccp-pr86725-1.c
===================================================================
--- /dev/null	2018-07-26 10:26:13.137955424 +0100
+++ gcc/testsuite/gcc.dg/vect/no-scevccp-pr86725-1.c	2018-08-22 10:25:06.682099699 +0100
@@ -0,0 +1,26 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-O -w" } */
+
+int foo;
+int
+nr (int xe, int z)
+{
+  int oo, wo = 0;
+
+  for (oo = 0; oo < 4; ++oo)
+    {
+      int qq;
+
+      int old_wo = wo;
+      for (qq = 0; qq < 2; ++qq)
+        {
+          wo = z + qq + old_wo;
+          xe += wo;
+        }
+    }
+  foo = wo;
+  return xe;
+}
+
+/* { dg-final { scan-tree-dump-not "double reduction: wo" "vect" } } */
+/* { dg-final { scan-tree-dump-not "OUTER LOOP VECTORIZED" "vect" } } */