c++: Tweak VLA representation.

If we put the SAVE_EXPR for a VLA size inside the MINUS_EXPR rather than
outside, it will work better with constant folding.

The equivalent change was made in the C front-end in 2004, in commit
r0-64535-g8b0b9aefd29dfe6398857bcf5628662e2f0e21f6

gcc/cp/ChangeLog
2020-05-11  Jason Merrill  <jason@redhat.com>

	* decl.c (compute_array_index_type_loc): Stabilize before building
	the MINUS_EXPR.

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index ab5e6abb8668cc2fd8ee8a4c40eb32fd34bf9390..9831c25366841ff1a4299324a6a90beaabb14572 100644 (file)
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,8 @@
+2020-05-11  Jason Merrill  <jason@redhat.com>
+
+       * decl.c (compute_array_index_type_loc): Stabilize before building
+       the MINUS_EXPR.
+
 2020-05-11  Jason Merrill  <jason@redhat.com>
 
        * decl.c (grokdeclarator): Adjust deprecated_state here.

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index adf9465842081430008c0287a616508e4f6c70e8..dea1ba07c0e4fc3467911d7481e7c95f399bb465 100644 (file)
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -10449,6 +10449,15 @@ compute_array_index_type_loc (location_t name_loc, tree name, tree size,
     itype = build_min (MINUS_EXPR, sizetype, size, integer_one_node);
   else
     {
+      if (!TREE_CONSTANT (size))
+       {
+         /* A variable sized array.  Arrange for the SAVE_EXPR on the inside
+            of the MINUS_EXPR, which allows the -1 to get folded with the +1
+            that happens when building TYPE_SIZE.  */
+         size = variable_size (size);
+         stabilize_vla_size (size);
+       }
+
       /* Compute the index of the largest element in the array.  It is
         one less than the number of elements in the array.  We save
         and restore PROCESSING_TEMPLATE_DECL so that computations in
@@ -10466,11 +10475,6 @@ compute_array_index_type_loc (location_t name_loc, tree name, tree size,
 
       if (!TREE_CONSTANT (itype))
        {
-         /* A variable sized array.  */
-         itype = variable_size (itype);
-
-         stabilize_vla_size (itype);
-
          if (sanitize_flags_p (SANITIZE_VLA)
              && current_function_decl != NULL_TREE)
            {