Fix SLP downward group access classification (PR92420)

This PR was caused by the SLP handling in get_group_load_store_type
returning VMAT_CONTIGUOUS rather than VMAT_CONTIGUOUS_REVERSE for
downward groups.

A more elaborate fix would be to try to combine the reverse permutation
into SLP_TREE_LOAD_PERMUTATION for loads, but that's really a follow-on
optimisation and not backport material.  It might also not necessarily
be a win, if the target supports (say) reversing and odd/even swaps
as independent permutes but doesn't recognise the combined form.

2019-11-11  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/92420
	* tree-vect-stmts.c (get_negative_load_store_type): Move further
	up file.
	(get_group_load_store_type): Use it for reversed SLP accesses.

gcc/testsuite/
	* gcc.dg/vect/pr92420.c: New test.

From-SVN: r278064

4 files changed, 118 insertions, 51 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 72d1e3d..329d107 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,10 @@
+2019-11-11  Richard Sandiford  <richard.sandiford@arm.com>
+
+	PR tree-optimization/92420
+	* tree-vect-stmts.c (get_negative_load_store_type): Move further
+	up file.
+	(get_group_load_store_type): Use it for reversed SLP accesses.
+
 2019-11-11  Jan Hubicka  <hubcika@ucw.cz>
 
 	* ipa-prop.c (ipa_propagate_indirect_call_infos): Remove ipcp
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index e734954..88088fa 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,7 @@
+2019-11-11  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* gcc.dg/vect/pr92420.c: New test.
+
 2019-11-11  Claudiu Zissulescu <claziss@gmail.com>
 
 	* gcc.target/arc/delay-slot-limm.c: New test.
diff --git a/gcc/testsuite/gcc.dg/vect/pr92420.c b/gcc/testsuite/gcc.dg/vect/pr92420.c
new file mode 100644
index 0000000..e43539f
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr92420.c
@@ -0,0 +1,48 @@
+/* { dg-additional-options "-mavx2" { target avx_runtime } } */
+
+#include "tree-vect.h"
+
+#define N 16
+struct C { int r, i; };
+struct C a[N], b[N], c[N], d[N], e[N];
+
+__attribute__((noipa)) static void
+foo (struct C *__restrict x, struct C *__restrict y, struct C *__restrict z, int w)
+{
+  int i;
+  for (int i = 0; i < w; i++)
+    {
+      z[i].r = x[i].r * y[-1 - i].r - x[i].i * y[-1 - i].i;
+      z[i].i = x[i].i * y[-1 - i].r + x[i].r * y[-1 - i].i;
+    }
+}
+
+__attribute__((noipa)) static void
+bar (struct C *__restrict x, struct C *__restrict y, struct C *__restrict z, int w)
+{
+  int i;
+  for (int i = 0; i < w; i++)
+    {
+      z[i].r = x[i].r * y[i].r - x[i].i * y[i].i;
+      z[i].i = x[i].i * y[i].r + x[i].r * y[i].i;
+    }
+}
+
+int
+main ()
+{
+  check_vect ();
+  int i;
+  for (i = 0; i < N; ++i)
+    {
+      a[i].r = N - i; a[i].i = i - N;
+      b[i].r = i - N; b[i].i = i + N;
+      c[i].r = -1 - i; c[i].i = 2 * N - 1 - i;
+    }
+  foo (a, b + N, d, N);
+  bar (a, c, e, N);
+  for (i = 0; i < N; ++i)
+    if (d[i].r != e[i].r || d[i].i != e[i].i)
+      __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-vect-stmts.c b/gcc/tree-vect-stmts.c
index eaca014..1da949a 100644
--- a/gcc/tree-vect-stmts.c
+++ b/gcc/tree-vect-stmts.c
@@ -2147,6 +2147,56 @@ perm_mask_for_reverse (tree vectype)
   return vect_gen_perm_mask_checked (vectype, indices);
 }
 
+/* A subroutine of get_load_store_type, with a subset of the same
+   arguments.  Handle the case where STMT_INFO is a load or store that
+   accesses consecutive elements with a negative step.  */
+
+static vect_memory_access_type
+get_negative_load_store_type (stmt_vec_info stmt_info, tree vectype,
+			      vec_load_store_type vls_type,
+			      unsigned int ncopies)
+{
+  dr_vec_info *dr_info = STMT_VINFO_DR_INFO (stmt_info);
+  dr_alignment_support alignment_support_scheme;
+
+  if (ncopies > 1)
+    {
+      if (dump_enabled_p ())
+	dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
+			 "multiple types with negative step.\n");
+      return VMAT_ELEMENTWISE;
+    }
+
+  alignment_support_scheme = vect_supportable_dr_alignment (dr_info, false);
+  if (alignment_support_scheme != dr_aligned
+      && alignment_support_scheme != dr_unaligned_supported)
+    {
+      if (dump_enabled_p ())
+	dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
+			 "negative step but alignment required.\n");
+      return VMAT_ELEMENTWISE;
+    }
+
+  if (vls_type == VLS_STORE_INVARIANT)
+    {
+      if (dump_enabled_p ())
+	dump_printf_loc (MSG_NOTE, vect_location,
+			 "negative step with invariant source;"
+			 " no permute needed.\n");
+      return VMAT_CONTIGUOUS_DOWN;
+    }
+
+  if (!perm_mask_for_reverse (vectype))
+    {
+      if (dump_enabled_p ())
+	dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
+			 "negative step and reversing not supported.\n");
+      return VMAT_ELEMENTWISE;
+    }
+
+  return VMAT_CONTIGUOUS_REVERSE;
+}
+
 /* STMT_INFO is either a masked or unconditional store.  Return the value
    being stored.  */
 
@@ -2273,7 +2323,15 @@ get_group_load_store_type (stmt_vec_info stmt_info, tree vectype, bool slp,
 				 "Peeling for outer loop is not supported\n");
 	      return false;
 	    }
-	  *memory_access_type = VMAT_CONTIGUOUS;
+	  int cmp = compare_step_with_zero (stmt_info);
+	  if (cmp < 0)
+	    *memory_access_type = get_negative_load_store_type
+	      (stmt_info, vectype, vls_type, 1);
+	  else
+	    {
+	      gcc_assert (!loop_vinfo || cmp > 0);
+	      *memory_access_type = VMAT_CONTIGUOUS;
+	    }
 	}
     }
   else
@@ -2375,56 +2433,6 @@ get_group_load_store_type (stmt_vec_info stmt_info, tree vectype, bool slp,
   return true;
 }
 
-/* A subroutine of get_load_store_type, with a subset of the same
-   arguments.  Handle the case where STMT_INFO is a load or store that
-   accesses consecutive elements with a negative step.  */
-
-static vect_memory_access_type
-get_negative_load_store_type (stmt_vec_info stmt_info, tree vectype,
-			      vec_load_store_type vls_type,
-			      unsigned int ncopies)
-{
-  dr_vec_info *dr_info = STMT_VINFO_DR_INFO (stmt_info);
-  dr_alignment_support alignment_support_scheme;
-
-  if (ncopies > 1)
-    {
-      if (dump_enabled_p ())
-	dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
-			 "multiple types with negative step.\n");
-      return VMAT_ELEMENTWISE;
-    }
-
-  alignment_support_scheme = vect_supportable_dr_alignment (dr_info, false);
-  if (alignment_support_scheme != dr_aligned
-      && alignment_support_scheme != dr_unaligned_supported)
-    {
-      if (dump_enabled_p ())
-	dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
-			 "negative step but alignment required.\n");
-      return VMAT_ELEMENTWISE;
-    }
-
-  if (vls_type == VLS_STORE_INVARIANT)
-    {
-      if (dump_enabled_p ())
-	dump_printf_loc (MSG_NOTE, vect_location,
-			 "negative step with invariant source;"
-			 " no permute needed.\n");
-      return VMAT_CONTIGUOUS_DOWN;
-    }
-
-  if (!perm_mask_for_reverse (vectype))
-    {
-      if (dump_enabled_p ())
-	dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
-			 "negative step and reversing not supported.\n");
-      return VMAT_ELEMENTWISE;
-    }
-
-  return VMAT_CONTIGUOUS_REVERSE;
-}
-
 /* Analyze load or store statement STMT_INFO of type VLS_TYPE.  Return true
    if there is a memory access type that the vectorized form can use,
    storing it in *MEMORY_ACCESS_TYPE if so.  If we decide to use gathers