[gcc r8-10884] c++: Fix zero initialization of flexible array members [PR99033]

[gcc r8-10884] c++: Fix zero initialization of flexible array members [PR99033]

[Jakub Jelinek]

https://gcc.gnu.org/g:5e677eeab4d37ae9c19e24a56a899f1f05d01280

commit r8-10884-g5e677eeab4d37ae9c19e24a56a899f1f05d01280
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Feb 11 17:24:17 2021 +0100

    c++: Fix zero initialization of flexible array members [PR99033]
    
    array_type_nelts returns error_mark_node for type of flexible array members
    and build_zero_init_1 was placing an error_mark_node into the CONSTRUCTOR,
    on which e.g. varasm ICEs.  I think there is nothing erroneous on zero
    initialization of flexible array members though, such arrays should simply
    get no elements, like they do if such classes are constructed (everything
    except when some larger initializer comes from an explicit initializer).
    
    So, this patch handles [] arrays in zero initialization like [0] arrays
    and fixes handling of the [0] arrays - the
    tree_int_cst_equal (max_index, integer_minus_one_node) check
    didn't do what it thought it would do, max_index is typically unsigned
    integer (sizetype) and so it is never equal to a -1.
    
    What the patch doesn't do and maybe would be desirable is if it returns
    error_mark_node for other reasons let the recursive callers not stick that
    into CONSTRUCTOR but return error_mark_node instead.  But I don't have a
    testcase where that would be needed right now.
    
    2021-02-11  Jakub Jelinek  <jakub@redhat.com>
    
            PR c++/99033
            * init.c (build_zero_init_1): Handle zero initialiation of
            flexible array members like initialization of [0] arrays.
            Use integer_minus_onep instead of comparison to integer_minus_one_node
            and integer_zerop instead of comparison against size_zero_node.
            Formatting fixes.
    
            * g++.dg/ext/flexary38.C: New test.
    
    (cherry picked from commit ea535f59b19f65e5b313c990ee6c194a7b055bd7)

Diff:
---
 gcc/cp/init.c                        | 20 +++++++++++---------
 gcc/testsuite/g++.dg/ext/flexary38.C | 18 ++++++++++++++++++
 2 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/gcc/cp/init.c b/gcc/cp/init.c
index 5671d860a39..26f5c3a0461 100644
--- a/gcc/cp/init.c
+++ b/gcc/cp/init.c
@@ -245,9 +245,12 @@ build_zero_init_1 (tree type, tree nelts, bool static_storage_p,
 
       /* Iterate over the array elements, building initializations.  */
       if (nelts)
-	max_index = fold_build2_loc (input_location,
-				 MINUS_EXPR, TREE_TYPE (nelts),
-				 nelts, integer_one_node);
+	max_index = fold_build2_loc (input_location, MINUS_EXPR,
+				     TREE_TYPE (nelts), nelts,
+				     build_one_cst (TREE_TYPE (nelts)));
+      /* Treat flexible array members like [0] arrays.  */
+      else if (TYPE_DOMAIN (type) == NULL_TREE)
+	max_index = build_minus_one_cst (sizetype);
       else
 	max_index = array_type_nelts (type);
 
@@ -259,20 +262,19 @@ build_zero_init_1 (tree type, tree nelts, bool static_storage_p,
 
       /* A zero-sized array, which is accepted as an extension, will
 	 have an upper bound of -1.  */
-      if (!tree_int_cst_equal (max_index, integer_minus_one_node))
+      if (!integer_minus_onep (max_index))
 	{
 	  constructor_elt ce;
 
 	  /* If this is a one element array, we just use a regular init.  */
-	  if (tree_int_cst_equal (size_zero_node, max_index))
+	  if (integer_zerop (max_index))
 	    ce.index = size_zero_node;
 	  else
 	    ce.index = build2 (RANGE_EXPR, sizetype, size_zero_node,
-				max_index);
+			       max_index);
 
-	  ce.value = build_zero_init_1 (TREE_TYPE (type),
-					 /*nelts=*/NULL_TREE,
-					 static_storage_p, NULL_TREE);
+	  ce.value = build_zero_init_1 (TREE_TYPE (type), /*nelts=*/NULL_TREE,
+					static_storage_p, NULL_TREE);
 	  if (ce.value)
 	    {
 	      vec_alloc (v, 1);
diff --git a/gcc/testsuite/g++.dg/ext/flexary38.C b/gcc/testsuite/g++.dg/ext/flexary38.C
new file mode 100644
index 00000000000..4fa987bcb41
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/flexary38.C
@@ -0,0 +1,18 @@
+// PR c++/99033
+// { dg-do compile }
+// { dg-options "" }
+
+struct T { int t; };
+struct S { char c; int T::*b[]; } a;
+struct U { char c; int T::*b[0]; } b;
+struct V { char c; int T::*b[1]; } c;
+struct W { char c; int T::*b[2]; } d;
+
+void
+foo ()
+{
+  a.c = 1;
+  b.c = 2;
+  c.c = 3;
+  d.c = 4;
+}