[LLVMdev] Using LLVM as cross-compiler to C

[Dave Nadler]

Thanks John and Bill for your feedback.
A bit more detail below...
Thanks again,
Best Regards, Dave

At 09:03 PM 1/21/2009, John Criswell wrote:
>Bill Wendling wrote:
> > On Wed, Jan 21, 2009 at 3:40 PM, Dave Nadler <Dave.Nadler at nadler.com> 
> wrote:
> >
> >> Hi All - Newbie warning... I am developing for a target that does not have
> >> a functioning C++ environment, and need to use C++. It was suggested 
> that I
> >> could use LLVM to compile to C (using llc -march=c), then run the 
> resulting
> >> C-code through the working C-cross-compilation tools. So, some newbie
> >> questions:
> >>
> >> (1) Is this crazy ?
> >>
> >
> > It depends on you definition of "crazy". It will turn it into C code.
> > However, you'll still need to link in the libstdc++ library.
> >
>One additional problem you may have a C header files.  Different
>operating systems and C library implementations will implement the same
>functions differently, so when cross-compiling, you need to make sure
>your compiling against header files that work on your target platform.

Its an bare-metal embedded target, so there is no use of any headers
other than the C and C++ library functions. As long as the versions
with LLVM are standards-compliant, no problem, right ?

> >> (2) What happens to all the C++ RTL ? Is it statically linked as LLVM 
> code,
> >> then translated back into C ?
> >> (3) Where does the LLVM RTL come from - newlib ? glibc ? other ?
> >>
> > LLVM doesn't use RTL. It has its own IR.
> >
>I believe he means the C++ Run-Time Library and not GCC's RTL
>Intermediate Representation.

Sorry I wasn't clear, yes I meant the C++ runtime library.
Its a two-part question:
- the compiler internals library (for example, exception handling), and
- the "standard" library (for example printf)

>Older versions of llvm-gcc compiled libstdc++ to LLVM bitcode and linked
>it statically into the LLVM bitcode for your program.  I am not sure
>what newer versions of llvm-gcc do.
>
>I do know that libc functions and other system functions are not
>compiled into LLVM bitcode.  They are (as Bill described below) either
>converted into LLVM intrinsics (like memcpy) or become calls to external
>functions (like printf).  After generation to native code, the program
>is linked against the system libraries.
>
>-- John T.
>
> >> (4) Is it manageable to create a small number of intrinsics to allow 
> access
> >> to the C runtime functions on the target, or is this difficult ? In the
> >> mailing list archive I saw some unresolved question with warnings about
> >> type conversions for someone trying this...
> >>
> > I'm not sure I understand your question. The program will have a call
> > to the C runtime functions (like strtof), and those are compiled into
> > calls in the assembly language. (Of course, the libc library needs to
> > be linked in to get the executable.) There are a few C runtime calls
> > that LLVM handles specially -- like memset, memcpy, etc. But it's not
> > necessary to have *all* C runtime calls be builtins or intrinsics for
> > LLVM to handle them.
> >
> > -bw

Do I understand correctly from above: LLVM provides a version of
the runtime library, of which some routines are converted to "external"
calls already ? So, there's already a printf binding to an external ?
For the standard-C library, my question is:
- how much of this has already been implemented (mapping to external),
- how hard will it be to add additional bindings ?

newlib makes it relatively easy by using common IO routines that
you replace with platform-specific implementations; is there something
analogous in the LLVM library implementation ? Hope I was more
clear this time...

Thanks again for your advice,
Best Regards, Dave

Dave Nadler, USA East Coast voice (978) 263-0097,  drn at nadler.com, Skype 
Dave.Nadler1