[Bug c/17468] New: Java garbage collector miscompiled at -O1 and higher

["Hans dot Boehm at hp dot com" <gcc-bugzilla at gcc dot gnu dot org>]

The (ancient) GC_descr_obj_sz() function in boehm-gc/typd_mlc.c is miscompiled 
by gcc4.0 20040913.  This was also true for the version from two weeks before 
that.  This usually causes "make check" in boehm-gc to fail.  However this 
function is not normally used by the gcj runtime.

I distilled this down to the self-contained example which is appended.  Some 
quick attempts to simplify this further by removing switch cases failed.

It is fairly easy to trace through the assembly code for the LEAF_TAG (=1) 
case, and see that there is a problem.  For example, the multiplication never 
happens.

I haven't had a chence to check this on other platforms.  It may be a generic 
issue.

-fno-strict-alias has no effect, and I don't see how it could.

Hans

/* Array descriptors.  GC_array_mark_proc understands these.	*/
/* We may eventually need to add provisions for headers and	*/
/* trailers.  Hence we provide for tree structured descriptors, */
/* though we don't really use them currently.			*/
typedef union ComplexDescriptor {
    struct LeafDescriptor {	/* Describes simple array	*/
        unsigned long ld_tag;
#	define LEAF_TAG 1
	unsigned long ld_size;		/* bytes per element	*/
				/* multiple of ALIGNMENT	*/
	unsigned long ld_nelements;	/* Number of elements.	*/
	unsigned long ld_descriptor; /* A simple length, bitmap,	*/
				/* or procedure descriptor.	*/
    } ld;
    struct ComplexArrayDescriptor {
        unsigned long ad_tag;
#	define ARRAY_TAG 2
	unsigned long ad_nelements;
	union ComplexDescriptor * ad_element_descr;
    } ad;
    struct SequenceDescriptor {
        unsigned long sd_tag;
#	define SEQUENCE_TAG 3
	union ComplexDescriptor * sd_first;
	union ComplexDescriptor * sd_second;
    } sd;
} complex_descriptor;
#define TAG ld.ld_tag

/* Return the size of the object described by d.  It would be faster to	*/
/* store this directly, or to compute it as part of			*/
/* GC_push_complex_descriptor, but hopefully it doesn't matter.		*/
unsigned long GC_descr_obj_size(d)
register complex_descriptor *d;
{
    switch(d -> TAG) {
      case LEAF_TAG:
      	return(d -> ld.ld_nelements * d -> ld.ld_size);
      case ARRAY_TAG:
        return(d -> ad.ad_nelements
               * GC_descr_obj_size(d -> ad.ad_element_descr));
      case SEQUENCE_TAG:
        return(GC_descr_obj_size(d -> sd.sd_first)
               + GC_descr_obj_size(d -> sd.sd_second));
      default:
        return 17;
        /*NOTREACHED*/ return 0; /*NOTREACHED*/
    }
}

int main()
{
   complex_descriptor d;

   d.ld.ld_tag = 1;
   d.ld.ld_size = 2;
   d.ld.ld_nelements = 3;
   if (GC_descr_obj_size(&d) != 6) write(2, "wrong answer\n", 13);
   return 0;
}

-- 
           Summary: Java garbage collector miscompiled at -O1 and higher
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: Hans dot Boehm at hp dot com
                CC: gcc-bugs at gcc dot gnu dot org,rth at redhat dot com
 GCC build triplet: ia64-unknown-linux
  GCC host triplet: ia64-unknown-linux
GCC target triplet: ia64-unknown-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17468