PIC

[Wolfgang Thaller]

Hi!

I've been playing around with the NCG in the new backend lately, trying 
to implement position independent code generation and dynamic linking 
(for Mac OS X first, of course ;-) ).
Right now, I'm aiming for being able to use self-contained Haskell code 
in a dynamic library as a plug-in for some foreign-language program - 
All Haskell code + libraries + RTS in the same dynamic library.

PIC/dynamic code generation for Mac OS X "almost" works now; there are 
sill a few problems left:

*) Info tables have a reference to a SRT, and info tables are in the 
text segment, so that reference can't be relocated, and the linker 
complains.
A solution might be to store the offset from the info table to the SRT 
instead instead of the SRT's address. I can convince the Mac OS X 
assembler to do that, but GCC doesn't like it. It's easy however to 
make the mangler convert ".long XYZ_srt" to ".long XYZ_srt - XYZ_info". 
We could also set the low bit of the offset value to distinguish it 
from an absolute address.

*) The code generator is giving me two different CLabels (with 
different uniques) for the same label (i.e. they pretty-print as the 
same label).
I have to do my (map head . group . sort) on the pretty-printed labels 
instead of the labels themselves, but that's very ugly...

*) Is there any way to find out if a label is defined within the 
current module?

Right now, the PIC generated by the NCG on Mac OS X is about the same 
as what I get when I run GCC-generated Mac OS X PIC through the 
mangler; the code works when run as an executable, and it is position 
independent except for those SRT references noted above.
I've had a look at how PIC code works on Windows and Linux, and I think 
support for that can be integrated into my code. I've put the 
indirections for dynamic linking into cmmToCmm (with some 
platform-specific helper functions for deciding when to use 
indirections); generating position-independent references to labels is 
left entirely to MachCodeGen, as things work too differently between 
platforms.

Well, that's a status report of just a few hours' hacking... I guess 
I've only just now arrived at the hard part :-).

Cheers,

Wolfgang