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.

diff --git a/gcc/gimple-fold.c b/gcc/gimple-fold.c
index 41b84ba3bb310326d331faba87bd18f5a6f7cee4..dfda6db5e9670166fadd3426c69ab4cce9fb7823 100644 (file)
--- a/gcc/gimple-fold.c
+++ b/gcc/gimple-fold.c
@@ -6875,10 +6875,17 @@ 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);
     }

diff --git a/gcc/testsuite/gcc.dg/torture/pr96133.c b/gcc/testsuite/gcc.dg/torture/pr96133.c
new file mode 100644 (file)
index 0000000..8d051ce
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr96133.c
@@ -0,0 +1,16 @@
+/* { 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;
+}