[Gd-hackers] Fixing up the C backend

[Rayiner Hashem]

I spent some time this week fixing up OD's C backend. It was in  
surprisingly good shape, all told.

Attached is a preliminary patch against fundev trunk to fix and  
enable the C backend. It allows you to run through a three-stage  
bootstrap, ie: OD/C compiled with itself can compile itself cleanly.

Below is a summary of the changes:

1) Bring c-run-time up to date, fixing compilation issues, and giving  
it a Makefile.in modeled on the one in lib/run-time/pentium-linux.

2) Make sure c-linker doesn't give the same variable both static and  
extern attributes.

3) Fix illegal behavior in unix-interface.dylan that OD/HARP accepts.

4) Update the configure and jam scripts appropriately.

A few things are worth considering:

1) How do you do backend selection? Currently, I have all backends  
compiled-in, but selectable only via setting default-back-end().

2) The Jam changes are pretty ugly. I really don't know enough Jam to  
do this elegantly.

3) c-run-time should probably be moved out of the dfmc directory.

4) My fix to c-emit-object.dylan is a hack. The basic problem is that  
when the C and HARP back-ends encounter two lexical variables with  
the same name, they rename them by appending the frame-offset of the  
variable. However, implicit closure parameters created as a result of  
lambda lifting appear with the same frame-offset (this happens  
compiling harp/pentium-harp/bits.dylan).

I'll have to think a bit to fix the last issue. Whether or not its  
illegal for two lexical variables in the same function to have the  
same frame-offset, the C backend doesn't otherwise care about frame- 
offsets anyway, so it shouldn't be using them for variable renaming.

5) I'm using the dummy threads implementation in the C runtime. I'll  
fix the pthreads implementation as soon as other things are in better  
shape.

I think that covers it. I'd appreciate any feedback you guys have on  
the patch. Oh, and if you get compilation errors in the C code saying  
that funny symbols are undefined, make sure the linker isn't  
accidentally seeing OD/HARP-compiled libraries, because those are  
incompatible.

Sincerely,
     Rayiner Hashem