binary compatibility ABI (was: Re: [boehm-gc] Import 6.3 alpha 1)

Thu Aug 14 16:27:00 GMT 2003

Jeff Sturm writes:
 > On 2003年8月14日, Andrew Haley wrote:
 > > Jeff Sturm writes:
 > > >
 > > > One of my sore points is the performance gap between static and
 > > > position-independent code with the current gcj compiler. For example:
 > > > reading a static field requires one memory load without -fPIC, but two
 > > > loads with -fPIC (to obtain the memory address from the GOT offset).
 > >
 > > Short of overwriting the instruction that does the fetch, how would you
 > > do that?
 > 
 > That wasn't really such a good example.
 > 
 > Consider:
 > 
 > static int a, b;
 > ...
 > return a + b;
 > 
 > Does this require 2, 3 or 4 memory loads?
 > 
 > (I believe 2 is the answer for non-PIC, 4 for PIC. If we take advantage
 > of the fact that static fields are at a fixed offset from the class
 > record, it can be done in three, while preserving position independence.)
Unless a and b are in the exact same class as the 'return a + b'
statment, the positions of a and b relative to the class record may
change. I guess this isn't true if the whole application is compiled
and we give up on binary compatibility.
Andrew.