wolfgang.thaller at gmx.net
Sun Jun 27 19:18:54 EDT 2004
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
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
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
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
Well, that's a status report of just a few hours' hacking... I guess
I've only just now arrived at the hard part :-).
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