Allow inner-loop reductions with variable-length vectors

[Richard Sandiford]

While working on PR 86871, I noticed we were being overly restrictive
when handling variable-length vectors.  For:

  for (i : ...)
    {
      res = ...;
      for (j : ...)
        res op= ...;
      a[i] = res;
    }

we don't need a reduction operation (although we do for double
reductions like:

  res = ...;
  for (i : ...)
    for (j : ...)
      res op= ...;
  a[i] = res;

which must still be rejected).

Tested on aarch64-linux-gnu (with and without SVE), aarch64_be-elf and
x86_64-linux-gnu.  OK to install?

Richard


2018-08-09  Richard Sandiford  <richard.sandif...@arm.com>

gcc/
        * tree-vect-loop.c (vectorizable_reduction): Allow inner-loop
        reductions for variable-length vectors.

gcc/testsuite/
        * gcc.target/aarch64/sve/reduc_8.c: New test.

Index: gcc/tree-vect-loop.c
===================================================================
--- gcc/tree-vect-loop.c        2018-08-01 16:14:50.227052736 +0100
+++ gcc/tree-vect-loop.c        2018-08-09 15:38:35.230258362 +0100
@@ -6711,6 +6711,7 @@ vectorizable_reduction (stmt_vec_info st
     }
 
   if (reduction_type != EXTRACT_LAST_REDUCTION
+      && (!nested_cycle || double_reduc)
       && reduc_fn == IFN_LAST
       && !nunits_out.is_constant ())
     {
Index: gcc/testsuite/gcc.target/aarch64/sve/reduc_8.c
===================================================================
--- /dev/null   2018-07-26 10:26:13.137955424 +0100
+++ gcc/testsuite/gcc.target/aarch64/sve/reduc_8.c      2018-08-09 
15:38:35.230258362 +0100
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -ftree-vectorize" } */
+
+int
+reduc (int *restrict a, int *restrict b, int *restrict c)
+{
+  for (int i = 0; i < 100; ++i)
+    {
+      int res = 0;
+      for (int j = 0; j < 100; ++j)
+       if (b[i + j] != 0)
+         res = c[i + j];
+      a[i] = res;
+    }
+}
+
+/* { dg-final { scan-assembler-times {\tcmpne\tp[0-9]+\.s, } 1 } } */
+/* We ought to use the CMPNE result for the SEL too.  */
+/* { dg-final { scan-assembler-not {\tcmpeq\tp[0-9]+\.s, } { xfail *-*-* } } } 
*/
+/* { dg-final { scan-assembler-times {\tsel\tz[0-9]+\.s, } 1 } } */