Don't use permutes for single-element accesses (PR83753)

After cunrolling the inner loop, the remaining loop in the testcase
has a single 32-bit access and a group of 64-bit accesses.  We first
try to vectorise at 128 bits (VF 4), but decide not to for cost reasons.
We then try with 64 bits (VF 2) instead.  This means that the group
of 64-bit accesses uses a single-element vector, which is deliberately
supported as of r251538.  We then try to create "permutes" for these
single-element vectors and fall foul of:

	      for (i = 0; i < 6; i++)
		sel[i] += exact_div (nelt, 2);

in vect_grouped_store_supported, since nelt==1.

Maybe we shouldn't even be trying to vectorise statements in the
single-element case, and instead just copy the scalar statement
for each member of the group.  But until then, this patch treats
non-strided grouped accesses as VMAT_CONTIGUOUS if no permutation
is necessary.

2018-01-10  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/83753
	* tree-vect-stmts.c (get_group_load_store_type): Use VMAT_CONTIGUOUS
	for non-strided grouped accesses if the number of elements is 1.

gcc/testsuite/
	PR tree-optimization/83753
	* gcc.dg/torture/pr83753.c: New test.

From-SVN: r256427

4 files changed, 40 insertions, 4 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index a30e285..f0e9e59 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2018-01-10  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	PR tree-optimization/83753
+	* tree-vect-stmts.c (get_group_load_store_type): Use VMAT_CONTIGUOUS
+	for non-strided grouped accesses if the number of elements is 1.
+
 2018-01-10  Jan Hubicka  <hubicka@ucw.cz>
 
 	PR target/81616
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 32a1460..2fda017 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2018-01-10  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	PR tree-optimization/83753
+	* gcc.dg/torture/pr83753.c: New test.
+
 2018-01-09  Jan Hubicka  <hubicka@ucw.cz>
 
 	* gcc.target/i386/avx2-gather-1.c: Add -march.
diff --git a/gcc/testsuite/gcc.dg/torture/pr83753.c b/gcc/testsuite/gcc.dg/torture/pr83753.c
new file mode 100644
index 0000000..54aafc0
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr83753.c
@@ -0,0 +1,19 @@
+/* { dg-do compile } */
+/* { dg-options "-mcpu=xgene1" { target aarch64*-*-* } } */
+
+typedef struct {
+  int m1[10];
+  double m2[10][8];
+} blah;
+
+void
+foo (blah *info) {
+  int i, d;
+
+  for (d=0; d<10; d++) {
+    info->m1[d] = 0;
+    info->m2[d][0] = 1;
+    for (i=1; i<8; i++)
+      info->m2[d][i] = 2;
+  }
+}
diff --git a/gcc/tree-vect-stmts.c b/gcc/tree-vect-stmts.c
index a4f2d71..819a981 100644
--- a/gcc/tree-vect-stmts.c
+++ b/gcc/tree-vect-stmts.c
@@ -1849,10 +1849,16 @@ get_group_load_store_type (gimple *stmt, tree vectype, bool slp,
 	  && (can_overrun_p || !would_overrun_p)
 	  && compare_step_with_zero (stmt) > 0)
 	{
-	  /* First try using LOAD/STORE_LANES.  */
-	  if (vls_type == VLS_LOAD
-	      ? vect_load_lanes_supported (vectype, group_size)
-	      : vect_store_lanes_supported (vectype, group_size))
+	  /* First cope with the degenerate case of a single-element
+	     vector.  */
+	  if (known_eq (TYPE_VECTOR_SUBPARTS (vectype), 1U))
+	    *memory_access_type = VMAT_CONTIGUOUS;
+
+	  /* Otherwise try using LOAD/STORE_LANES.  */
+	  if (*memory_access_type == VMAT_ELEMENTWISE
+	      && (vls_type == VLS_LOAD
+		  ? vect_load_lanes_supported (vectype, group_size)
+		  : vect_store_lanes_supported (vectype, group_size)))
 	    {
 	      *memory_access_type = VMAT_LOAD_STORE_LANES;
 	      overrun_p = would_overrun_p;