Created attachment 28758 [details]
C++ source file

In this code:

struct s
{
    int i1, i2;
};

const s c1 = { 1, 2 };
const s ca1[] = { { 1, 2} };
const s c2 = c1;
const s ca2[] = { c1 };

int main(void)
{
    return 0;
}


gdb sees all variables as const except for ca2:

(gdb) ptype ca1
type = const struct s {
    int i1;
    int i2;
} [1]
(gdb) ptype ca2
type = struct s {
    int i1;
    int i2;
} [1]

The problem seems to be in split_nonconstant_init() in cp/typeck2.c;  when TREE_READONLY (dest) is set to zero, the information that it was ever read-only is lost before the DWARF record is written.

The following patch seems to fix the problem.  I would not be surprised if there were a more elegant way of doing this.

(You might be wondering:  How did I find this, and why do I care?  I've been working on something to read object files and then flag variables that raise thread-safety issues because they are (1) global or static and (2) not const.  Reading DWARF records works really well, except for this particular problem.)

Index: gcc/cp/typeck2.c
===================================================================
--- gcc/cp/typeck2.c	(revision 193640)
+++ gcc/cp/typeck2.c	(working copy)
@@ -633,6 +633,7 @@
 	init = NULL_TREE;
       code = pop_stmt_list (code);
       DECL_INITIAL (dest) = init;
+      TREE_WASREADONLY (dest) = TREE_READONLY (dest);
       TREE_READONLY (dest) = 0;
     }
   else
Index: gcc/dwarf2out.c
===================================================================
--- gcc/dwarf2out.c	(revision 193640)
+++ gcc/dwarf2out.c	(working copy)
@@ -18031,7 +18031,9 @@
       if (decl_by_reference_p (decl_or_origin))
 	add_type_attribute (var_die, TREE_TYPE (type), 0, 0, context_die);
       else
-	add_type_attribute (var_die, type, TREE_READONLY (decl_or_origin),
+	add_type_attribute (var_die, type,
+                        TREE_READONLY (decl_or_origin) ||
+                        TREE_WASREADONLY (decl_or_origin),
 			    TREE_THIS_VOLATILE (decl_or_origin), context_die);
     }
 
Index: gcc/tree.h
===================================================================
--- gcc/tree.h	(revision 193640)
+++ gcc/tree.h	(working copy)
@@ -464,8 +464,9 @@
   unsigned packed_flag : 1;
   unsigned user_align : 1;
   unsigned nameless_flag : 1;
+  unsigned wasreadonly_flag : 1;
 
-  unsigned spare : 12;
+  unsigned spare : 11;
 
   /* This field is only used with type nodes; the only reason it is present
      in tree_base instead of tree_type is to save space.  The size of the
@@ -1344,6 +1345,7 @@
    Nonzero in a FUNCTION_DECL means this function should be treated
    as "const" function (can only read its arguments).  */
 #define TREE_READONLY(NODE) (NON_TYPE_CHECK (NODE)->base.readonly_flag)
+#define TREE_WASREADONLY(NODE) (NON_TYPE_CHECK (NODE)->base.wasreadonly_flag)
 
 /* Value of expression is constant.  Always on in all ..._CST nodes.  May
    also appear in an expression or decl where the value is constant.  */