fix constant folding from array CTORs

This fixes the case where we try to fold a read from an
array initalizer and happen to cross the boundary of
multiple CTORs which isn't really supported.  For the
interesting cases like the testcase we actually handle
the folding by encoding the whole initializer.

2020-07-10  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/96133
	* gimple-fold.c (fold_array_ctor_reference): Do not
	recurse to folding a CTOR that does not fully cover the
	asked for object.

	* gcc.dg/torture/pr96133.c: New testcase.

(cherry picked from commit 6e41c27bf549d957eb399d39d7d0c213f8733351)

gcc/gimple-fold.c

@@ -6811,10 +6811,17 @@ fold_array_ctor_reference (tree type, tree ctor,
 	     SIZE to the size of the accessed element.  */
 	  inner_offset = 0;
 	  type = TREE_TYPE (val);
-	  size = elt_size.to_uhwi () * BITS_PER_UNIT;
+	  size = elt_sz * BITS_PER_UNIT;
 	}
+      else if (size && access_index < CONSTRUCTOR_NELTS (ctor) - 1
+	       && TREE_CODE (val) == CONSTRUCTOR
+	       && (elt_sz * BITS_PER_UNIT - inner_offset) < size)
+	/* If this isn't the last element in the CTOR and a CTOR itself
+	   and it does not cover the whole object we are requesting give up
+	   since we're not set up for combining from multiple CTORs.  */
+	return NULL_TREE;
 
-      *suboff += (access_index * elt_size * BITS_PER_UNIT).to_uhwi ();
+      *suboff += access_index.to_uhwi () * elt_sz * BITS_PER_UNIT;
       return fold_ctor_reference (type, val, inner_offset, size, from_decl,
 				  suboff);
     }

gcc/testsuite/gcc.dg/torture/pr96133.c

+/* { dg-do run } */
+
+typedef int T;
+static const T a[2][3] = { { 1, 2, 3 }, { 4, 5, 6 } };
+typedef T v2 __attribute__((vector_size(2*sizeof(T))));
+
+int
+main()
+{
+  const T *p = &a[0][2];
+  v2 x = *(const v2 *)p;
+  T z = x[1];
+  if (z != 4)
+    __builtin_abort ();
+  return 0;
+}