[LLVMdev] libstdc++ as bytecode, and compiling C++ to C

[Reid Spencer]

On Tue, 2006-11-21 at 10:57 +1100, Emil Mikulic wrote:
> > 
> > llvmc isn't a completed tool. In particular it doesn't handle native
> > linking properly. However, I'm not sure what it would do in your example
> > above that would cause the program to seg fault. Can you get a stack
> > trace?
> 
> Actually, it looks to me like it generates bad code.

Well, like I said, it isn't complete.

> 
> $ llvm-dis < out.bc | grep -- -3
> 	%tmp4.i.i = getelementptr int (...)** %tmp.i.i, int -3
> [and a bunch more, one of which is in main()...]

I don't see what's so bad about this ?

> $ llc -march=c -o=cbe.c out.bc
> WARNING: this target does not support the llvm.stacksave intrinsic.
> $ gcc -g cbe.c
> [n.b.: compiles successfully!]
> $ gdb a.out
> (gdb) run
> Starting program: a.out 
> 
> Program received signal SIGSEGV, Segmentation fault.
> 0x08048958 in main () at cbe.c:1154
> 1154      ltmp_173_15 = *((int *)(&ltmp_172_54[-3]));
> (gdb) print ltmp_172_54
> $1 = (int (**)()) 0x0

Yeah, I was asking for a stack trace in the program generated by llvmc.
The CBE isn't going to help much here, at least not without the cbe
source.

> 
> Can I send you bytecode files or any further diagnostics to chase this
> up?

Sure.

> 
> > > Oops, no go.  Try a different way:
> > > 
> > > $ llvm-g++ -emit-llvm -c x.cpp
> > > $ llvm-link -o=linked.o x.o std/*.o sup/*.o
> > > $ lli linked.o
> > > hello world
> > > $
> > > 
> > > So far, so good!  Compile to C:
> > 
> > llvm-link should work fine because its just dealing with bytecode. BTW,
> > You could put all the std/*.o and sup/*.o files into a bc archive using
> > llvm-ar and then just link with that archive.
> 
> I thought so, and I even tried playing with llvm-ar, but couldn't figure
> it out.
> 
> $ llvm-ar rs mylib.a std/*.o sup/*.o
> llvm-ar: creating mylib.a

That's correct.

> $ llvm-link -o=linked.o x.o mylib.a 
> llvm-link: error loading file 'mylib.a'

llvm-link only understands bytecode files. It can't read archives. Try
llvm-ld or gccld instead. If gccld, use the -link-as-library option.

> > > $ llc -o=cbe.c -march=c linked.o
> > > WARNING: this target does not support the llvm.stacksave intrinsic.
> > > $ gcc cbe.c
> > > /var/tmp//ccVAM4W2.o(.text+0x329a): In function `operator new(unsigned int)':
> > > : undefined reference to `__cxa_throw'
> > > [...and more errors]
> > > $
> > > 
> > > But __cxa_throw is right there, in sup/eh_throw.o, and in linked.o, it
> > > just isn't being emitted as C code.  (at this point, I suspect the
> > > problem might be that I don't quite understand how some of the llvm
> > > tools are intended to work - e.g. llvm-link vs llvmc)
> > 
> > CBE doesn't support exceptions ?
> 
> Fair enough.  It's just that if I use llc to compile sup/eh_throw.o to a
> C file, it contains a __cxa_throw() function.  (whether it does the
> right thing or not, I don't know - I haven't gotten that far)

Probably not. Again, the CBE doesn't support exceptions and it would
have to understand that __cxa_throw is part of the exception handling. 

> But, after the big llvm-link step, cbe.c doesn't contain a __cxa_throw()
> function even though it's called from the code.

I'm not surprised. 

> 
> > > Strangely enough:
> > > 
> > > $ llvm-g++ -emit-llvm -c x.cpp
> > > $ llvm-link -o=linked.o x.o std/*.o sup/*.o
> > > $ llvmc -o=out linked.o
> > > $ llc -o=cbe.c -march=c out.bc
> > > WARNING: this target does not support the llvm.stacksave intrinsic.
> > 
> > CBE doesn't support llvm.stacksave
> 
> Fair enough.  =)
> 
> > > $ gcc cbe.c
> > > $ ./a.out
> > > hello world
> > > $
> > > 
> > > Works!
> > 
> > However it doesn't seem to matter for this program.
> 
> The program is very simple, and doesn't use any EH itself, but I think
> pieces of libstdc++ do...

Okay.

> 
> --Emil
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