aliasing warnings during libgcj build

Tue Dec 13 16:53:00 GMT 2005

Richard Guenther writes:
 > The following code from java::lang::Class causes an aliasing warning:
 > 
 > class java::lang::Class : public java::lang::Object
 > { 
 > public:
 > ...
 > inline jclass getComponentType (void)
 > {
 > return isArray () ? (* (jclass *) &methods) : 0;
 > }
 > ...
 > // Methods. If this is an array class, then this field holds a
 > // pointer to the element type.
 > _Jv_Method *methods;
 > ...
 > };
 > 
 > 
 > First of all, taking the address of methods before casting looks
 > fishy here and from the comments I would have written
 > 
 > return isArray () ? (* (jclass *) methods) : 0;
 > 
 > here (so at least the comment needs clarification). The proper
 > fix here is to use a union (matching the code above):
 > 
 > union {
 > _Jv_Method *m;
 > jclass c; // or jclass *c, if the comment is really correct
 > } methods;
 > 
 > 
 > The code as current may cause miscompilations, especially because
 > getComponentType is inline.
The fix for this I've attached is almost unbelievably trivial, but
there is at least the possibility that this might not be a
binary-compatible change. That is to say,
 union {
 _Jv_Method *methods;
 jclass element_type;
 };
might actually have different alignment/padding from
 _Jv_Method *methods;
To avoid this possibility, it might actually be best to declare
methods as type void*.
Andrew.
Index: java/lang/natClassLoader.cc
===================================================================
*** java/lang/natClassLoader.cc	(revision 108423)
--- java/lang/natClassLoader.cc	(working copy)
*************** _Jv_NewArrayClass (jclass element, java:
*** 435,441 ****
 = java::lang::Object::class$.vtable_method_count;

 // Stash the pointer to the element type.
! array_class->methods = (_Jv_Method *) element;

 // Register our interfaces.
 static jclass interfaces[] =
--- 435,441 ----
 = java::lang::Object::class$.vtable_method_count;

 // Stash the pointer to the element type.
! array_class->element_type = element;

 // Register our interfaces.
 static jclass interfaces[] =
Index: java/lang/Class.h
===================================================================
*** java/lang/Class.h	(revision 108423)
--- java/lang/Class.h	(working copy)
*************** public:
*** 336,342 ****

 inline jclass getComponentType (void)
 {
! return isArray () ? (* (jclass *) &methods) : 0;
 }

 jboolean isAssignableFrom (jclass cls);
--- 336,342 ----

 inline jclass getComponentType (void)
 {
! return isArray () ? element_type : 0;
 }

 jboolean isAssignableFrom (jclass cls);
*************** private: 
*** 514,520 ****
 _Jv_Constants constants;
 // Methods. If this is an array class, then this field holds a
 // pointer to the element type.
! _Jv_Method *methods;
 // Number of methods. If this class is primitive, this holds the
 // character used to represent this type in a signature.
 jshort method_count;
--- 514,523 ----
 _Jv_Constants constants;
 // Methods. If this is an array class, then this field holds a
 // pointer to the element type.
! union {
! _Jv_Method *methods;
! jclass element_type;
! };
 // Number of methods. If this class is primitive, this holds the
 // character used to represent this type in a signature.
 jshort method_count;