Strange, elusive bug in boolean return values with latest fetch from cvs

Thu Jun 10 09:20:00 GMT 2004

Tom Tromey writes:
 > >>>>> "Andrew" == Andrew Haley <aph@redhat.com> writes:
 > 
 > Andrew> The problem seems to be due to the fact that the type system of the
 > Andrew> Java Virtual Machine is subtly different from the type system of the
 > Andrew> Java Programming Language. In particular, booleans and ints are
 > Andrew> incompatible in the source language but are both treated as integer
 > Andrew> types in the VM.
 > 
 > Yeah. And booleans are treated as bytes for the purposes of array
 > operations.
 > 
 > Andrew> + 	 if (TREE_TYPE (node) != TREE_TYPE (decl))
 > Andrew> + 	 decl = convert (TREE_TYPE (node), decl);
 > Andrew> 	 java_add_stmt (build (MODIFY_EXPR, TREE_TYPE (node), decl, node));
 > 
 > This looks like an lvalue cast to me... sort of weird.
Yes, you're right. It's a mistake.
 > Andrew> type = promote_type (type);
 > Andrew> + /* Although Java local variables may be of boolean type, the Java VM
 > Andrew> + treats them as though they're integers. */
 > Andrew> + if (type == promoted_boolean_type_node)
 > Andrew> + type = int_type_node;
 > 
 > This is curious since I would have expected promote_type() to do
 > something along these lines. But I find I don't really understand
 > promote_type() at all -- when can BOOLEAN_TYPE and boolean_type_node
 > differ?
There are two nodes of boolean type: boolean itself, and
promoted_boolean. One is wider than the other. But both are
BOOLEAN_TYPE. The thing is that most of this stuff is correct for the
Java language, but wrong for the VM.
 > Anyway, for the stack and local variables, the VM does not distinguish
 > between boolean, byte, char, short, or int. It represents them all as
 > int. The conversion operators are basically about masking and
 > sign-extension, and during array and field stores you simply mask off
 > the extra bits. I don't understand all the code here that well, but I
 > would assume something similar would be needed for the char and short
 > cases.
Yes. For those cases we have promoted_short and promoted_char.
Andrew.