[LLVMdev] llvm interpreter cannot execute llvm-gcc generated bitcode

[Nick Lewycky]

2010/2/1 Török Edwin <edwintorok at gmail.com>

> On 02/01/2010 01:13 PM, Kristaps Straupe wrote:
> > Hello again!
> >
> > We have fetched the latest llvm sources from repository and the
> > original problem has went away. Though now we are facing a new problem
> > with interpreter on the following c code:
> >
> > --------------
> > #include <stdarg.h>
> > #include <stdio.h>
> >
> > void doTheThing(int dummy, ...)
> > {
> >   va_list ap;
> >   int z;
> >
> >   va_start(ap, dummy);
> >   while( (z = va_arg(ap, int))!=0)
> >   {
> >     printf("== %i ==\n", z);
> >   }
> >   va_end(ap);
> > }
> >
> > int main()
> > {
> >   doTheThing(-1, 1, 2, 3, 0);
> > }
> >
>
> I think this doesn't work because libffi doesn't support vararg functions:
> "There is no support for calling varargs functions.  This may work on
> some platforms, depending on how the ABI is defined, but it is not
> reliable."
>

I don't think that's the problem. The call to doTheThing shouldn't involve
libffi since it's not an external call, and the call to printf doesn't
involve libffi since it's a hard-coded known-varargs function. See the table
at the end of lib/ExecutionEngine/Interpreter/ExternalFunctions.cpp for a
list of functions we trap instead of sending to libffi.

I have no idea why it's failing yet.

Nick


> Since you are on a platform where the JIT doesn't work properly, I guess
> libffi's varargs
> don't work either. What platform is it?
>
> This only refers to external varargs functions, not ones you define
> yourself.
> It works just fine with the JIT though.
>
> I think it would be easier if you'd debug JIT problems this way (to
> avoid running into libffi
> limitations in the interpreter):
>  1. compile your code with -O0 to LLVM BC
>  2. compile the BC to native code (using llc + assembler + linker), test
> that it works (output is correct)
>  3. use the JIT to run the BC, test that it works properly (output is
> correct)
>  4. Repeat steps 1-3 with -O1, -O2, until it fails
>
> Once you identified at which stage the problem occurs, you can try the
> instructions here to further narrow down the problem:
> http://llvm.org/docs/HowToSubmitABug.html#codegen
>
> Best regards,
> --Edwin
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