[committed] build a zero element array type that renders as T[0] in diagnostics

[Martin Sebor via Gcc-patches]

One of the recently added -Warray-bounds tests fails in ILP32
because the warning code calls build_array_type_nelts(type, zero)
and naively expects it to create an array type that renders as
type[0] in warnings.  While that works in LP64, it apparently
isn't enough to produce such a type in other configurations.
I have committed the patch below following the C front-end
approach of creating a zero-element array that does render as
expected regardless of data models.
Martin

commit 67aeddb785ddcc8688ee5736ecab3c81de34a214 (HEAD -> master, 
origin/master, origin/HEAD)
Author: Martin Sebor <mse...@redhat.com>
Date:   Wed Sep 23 15:19:13 2020 -0600

    Build a zero element array type that reliably renders as T[0] in 
diagnostcs.

    gcc/ChangeLog:

            * gimple-array-bounds.cc (build_zero_elt_array_type): New 
function.
            (array_bounds_checker::check_mem_ref): Call it.

diff --git a/gcc/gimple-array-bounds.cc b/gcc/gimple-array-bounds.cc
index b93ef7a7b74..ab638ba249f 100644
--- a/gcc/gimple-array-bounds.cc
+++ b/gcc/gimple-array-bounds.cc
@@ -372,6 +372,20 @@ array_bounds_checker::check_array_ref (location_t 
location, tree ref,
   return warned;
 }

+/* Hack around the internal representation constraints and build a zero
+   element array type that actually renders as T[0] in diagnostcs.  */
+
+static tree
+build_zero_elt_array_type (tree eltype)
+{
+  tree idxtype = build_range_type (sizetype, size_zero_node, NULL_TREE);
+  tree arrtype = build_array_type (eltype, idxtype);
+  arrtype = build_distinct_type_copy (TYPE_MAIN_VARIANT (arrtype));
+  TYPE_SIZE (arrtype) = bitsize_zero_node;
+  TYPE_SIZE_UNIT (arrtype) = size_zero_node;
+  return arrtype;
+}
+
 /* Checks one MEM_REF in REF, located at LOCATION, for out-of-bounds
    references to string constants.  If VRP can determine that the array
    subscript is a constant, check if it is outside valid range.
@@ -547,7 +561,10 @@ array_bounds_checker::check_mem_ref (location_t 
location, tree ref,
        return false;

       offset_int nelts = arrbounds[1] / eltsize;
-      reftype = build_array_type_nelts (reftype, nelts.to_uhwi ());
+      if (nelts == 0)
+       reftype = build_zero_elt_array_type (reftype);
+      else
+       reftype = build_array_type_nelts (reftype, nelts.to_uhwi ());
     }
   else if (TREE_CODE (arg) == ADDR_EXPR)
     {