[Bug debug/31230] New: debug information depends on gc parameters

["jsm28 at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

The following problem can cause bootstrap comparison failures in some
circumstances (and was originally observed when bootstrapping with CFLAGS=-g
BOOT_CFLAGS=-g).

Consider the code from tree.c:

/* See if the data pointed to by the type hash table is marked.  We consider
   it marked if the type is marked or if a debug type number or symbol
   table entry has been made for the type.  This reduces the amount of
   debugging output and eliminates that dependency of the debug output on
   the number of garbage collections.  */

static int
type_hash_marked_p (const void *p)
{
  tree type = ((struct type_hash *) p)->type;

  return ggc_marked_p (type) || TYPE_SYMTAB_POINTER (type);
}

(TYPE_SYMTAB_POINTER, TYPE_SYMTAB_ADDRESS and TYPE_SYMTAB_DIE all name 
fields in the same union; the one applicable depends on the debug format 
in use.)

The array type char[100] is generated, and handled for debug info 
purposes.  Some time later, char[100] arises again.

If there was no garbage collection inbetween, the same type is used and 
the debug info is shared.  If however there was a garbage collection, the 
index type (the special integer type with range 0 to 99 used for 
TYPE_DOMAIN of the array type) gets garbage collected - while the array 
type is no doubt marked for debug info, the index type (which doesn't 
appear directly in debug info) isn't.  Thus for the new char[100] type a 
new index type is generated, and so a new array type, and so extra debug 
info.

I think there may also be other conditions required for the types to be garbage
collected and to trigger this bug.  I'll attach a .i file that shows it on
i686-pc-linux-gnu if you compare the results of compiling with -g --param
ggc-min-expand=0 --param ggc-min-heapsize=0 and -g --param ggc-min-expand=100
--param ggc-min-heapsize=131072, but it seems quite fragile.


-- 
           Summary: debug information depends on gc parameters
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jsm28 at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31230