Patch: FYI: GC descriptors for interpreted classes

[Tom Tromey <tromey at redhat dot com>]

I'm checking this in on the trunk.

With the patch the runtime will compute the GC mark descriptor for
interpreted classes.

Tom

Index: ChangeLog
from  Tom Tromey  <tromey@redhat.com>

	* boehm.cc (_Jv_BuildGCDescr): Wrote.
	Include limits.h.

Index: boehm.cc
===================================================================
RCS file: /cvs/gcc/gcc/libjava/boehm.cc,v
retrieving revision 1.37
diff -u -r1.37 boehm.cc
--- boehm.cc 3 Jan 2003 16:34:37 -0000 1.37
+++ boehm.cc 23 Jul 2003 01:41:05 -0000
@@ -1,6 +1,6 @@
 // boehm.cc - interface between libjava and Boehm GC.
 
-/* Copyright (C) 1998, 1999, 2000, 2001, 2002  Free Software Foundation
+/* Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003  Free Software Foundation
 
    This file is part of libgcj.
 
@@ -11,6 +11,7 @@
 #include <config.h>
 
 #include <stdio.h>
+#include <limits.h>
 
 #include <jvm.h>
 #include <gcj/cni.h>
@@ -325,11 +326,48 @@
 // since another one could be registered first.  But the compiler also
 // knows this, so in that case everything else will break, too.
 #define GCJ_DEFAULT_DESCR GC_MAKE_PROC(GC_GCJ_RESERVED_MARK_PROC_INDEX,0)
+
 void *
-_Jv_BuildGCDescr(jclass)
+_Jv_BuildGCDescr(jclass self)
 {
-  /* FIXME: We should really look at the class and build the descriptor. */
-  return (void *)(GCJ_DEFAULT_DESCR);
+  jlong desc = 0;
+
+  // Note: for now we only consider a bitmap mark descriptor.  We
+  // could also handle the case where the first N fields of a type are
+  // references.  However, this is not very likely to be used by many
+  // classes, and it is easier to compute things this way.
+
+  for (jclass klass = self; klass != NULL; klass = klass->getSuperclass())
+    {
+      jfieldID field = JvGetFirstInstanceField(klass);
+      int count = JvNumInstanceFields(klass);
+
+      for (int i = 0; i < count; ++i)
+	{
+	  if (field->isRef())
+	    {
+	      unsigned int off = field->getOffset();
+	      // If we run into a weird situation, we bail.
+	      if (off % sizeof (void *) != 0)
+		return (void *) (GCJ_DEFAULT_DESCR);
+	      off /= sizeof (void *);
+	      // Bottom 2 bits are reserved.
+	      off += 2;
+	      // If we find a field outside the range of our bitmap,
+	      // fall back to procedure marker.
+	      if (off > CHAR_BIT * sizeof (void *))
+		return (void *) (GCJ_DEFAULT_DESCR);
+	      desc |= 1 << off;
+	    }
+
+	  field = field->getNextField();
+	}
+    }
+
+  // For bitmap mark type, bottom bits are 01.
+  desc |= 1;
+  // Bogus warning avoidance (on many platforms).
+  return (void *) (unsigned long) desc;
 }
 
 // Allocate some space that is known to be pointer-free.