Mark constant-sized objects as addressable if they have poly-int accesses

[Richard Sandiford]

If SVE code is written for a specific vector length, it might load from
or store to fixed-sized objects.  This needs to work even without
-msve-vector-bits=N (which should never be needed for correctness).

There's no way of handling a direct poly-int sized reference to a
fixed-size register; it would have to go via memory.  And in that
case it's more efficient to mark the fixed-size object as
addressable from the outset, like we do for array references
with non-constant indices.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?

Richard


2019-11-08  Richard Sandiford  <richard.sandif...@arm.com>

gcc/
        * cfgexpand.c (discover_nonconstant_array_refs_r): If an access
        with POLY_INT_CST size is made to a fixed-size object, force the
        object to live in memory.

gcc/testsuite/
        * gcc.target/aarch64/sve/acle/general/deref_1.c: New test.

Index: gcc/cfgexpand.c
===================================================================
--- gcc/cfgexpand.c     2019-10-01 09:55:35.062089236 +0100
+++ gcc/cfgexpand.c     2019-11-08 09:39:13.105130902 +0000
@@ -6106,6 +6106,20 @@ discover_nonconstant_array_refs_r (tree
 {
   tree t = *tp;
 
+  /* References of size POLY_INT_CST to a fixed-size object must go
+     through memory.  It's more efficient to force that here than
+     to create temporary slots on the fly.  */
+  if (TYPE_SIZE (TREE_TYPE (t))
+      && POLY_INT_CST_P (TYPE_SIZE (TREE_TYPE (t))))
+    {
+      t = get_base_address (t);
+      if (t
+         && DECL_P (t)
+         && DECL_MODE (t) != BLKmode
+         && GET_MODE_BITSIZE (DECL_MODE (t)).is_constant ())
+       TREE_ADDRESSABLE (t) = 1;
+    }
+
   if (IS_TYPE_OR_DECL_P (t))
     *walk_subtrees = 0;
   else if (TREE_CODE (t) == ARRAY_REF || TREE_CODE (t) == ARRAY_RANGE_REF)
Index: gcc/testsuite/gcc.target/aarch64/sve/acle/general/deref_1.c
===================================================================
--- /dev/null   2019-09-17 11:41:18.176664108 +0100
+++ gcc/testsuite/gcc.target/aarch64/sve/acle/general/deref_1.c 2019-11-08 
09:39:13.105130902 +0000
@@ -0,0 +1,13 @@
+/* { dg-options "-O2" } */
+
+#include <arm_sve.h>
+
+uint64_t
+f (int32_t *x, int32_t *y)
+{
+  union { uint64_t x; char c[8]; } u;
+  svbool_t pg = svptrue_b32 ();
+  *(svbool_t *)&u.c[0] = svcmpeq (pg, svld1 (pg, x), 0);
+  *(svbool_t *)&u.c[4] = svcmpeq (pg, svld1 (pg, y), 1);
+  return u.x;
+}