java exception

Fri Jun 24 19:31:00 GMT 2005

Yang Chang wrote:
> I am trying to understand the GCJ implementation of Java exception 
> mechanism. Currently, I am aware of that the GCJ compiler uses the 
> return value of a "gen_reg_rtx(ptr_mode);" (in function 
> "get_exception_pointer") to represent something which holds a 
> reference to an _Unwind_Exception struct and such a reference is 
> passed to function _Unwind_RaiseException in libjava. However, I 
> couldn't figure out how the two things map each other. I mean, how the 
> _Unwind_RaiseException or some other lower level functions assign the 
> aforementioned reference to what that rtx represents? and what that 
> rtx really represents?

The code you are looking at is used when generating an exception 
_handler_. When the unwind mechanism gives control to the handler, the 
exception object that was thrown needs to be passed to it somehow. 
get_exception_pointer yields an RTL expression (in this case a REG, ie a 
register) where a pointer to the _Unwind_Exception will be found. The 
_Unwind_Exception will be put in that register by the unwinder before it 
jumps into the handler. You'll want to look in gcc/unwind* and 
libjava/exception.cc for the code for the unwinder itself.
Bryce