[LLVMdev] C back-end differences

Mohd-Hanafiah Abdullah napi at axiomsol.com
Fri May 11 19:47:03 PDT 2007

On Tue, 2007-05-08 at 22:45 -0700, Bill Wendling wrote:
> On May 8, 2007, at 10:05 PM, Mohd-Hanafiah Abdullah wrote:
> > On Tue, 2007-05-08 at 11:58 -0700, Bill wrote:
> >> On 5/8/07, Mohd-Hanafiah Abdullah <napi at axiomsol.com> wrote:
> >>> How does the C back-end of LLVM differ from the one in gcc2c  
> >>> developed
> >>> by SUN several years ago?
> >>>
> >> Hi Napi,
> >>
> >> For one, it converts LLVM's bytecode to C instead of GCC's RTL. It's
> >> also under a different license.
> >
> > Hi Bill:
> >
> > Would it be easier to convert from LLVM's bytecode to C as opposed to
> > from RTL?  What about the readability of the produced C code.  I would
> > think since RTL maintains a structure that is directly inherited from
> > the high-level language structure it would be less difficult to  
> > turn it
> > into some C code that is relatively more readable, as compared to
> > synthesizing C code from the low-level bytecode.  I stand corrected.
> >
> Hi Napi,
> Considering that LLVM already has a C backend, then it's easier to  
> generate it from the LLVM bytecode. :-) LLVM's intermediate  
> representation of the program (the bytecode) is also derived directly  
> from the high-level language. When it gets to the C backend part, it  
> simply has had transformations done on it.
> I don't believe that the C backend was made for readability. In fact,  
> because LLVM is in SSA form, it uses many temporary variables and  
> loses a lot of the information about what the original variable's  
> name was.
> The best way to get a feel for how it is is to write a program and  
> generate the C code for it with the LLVM backend. (See the llc tool  
> for information on this.)
> Though, this begs the question of why you want to use something which  
> converts it to C instead of just using one of the target backends to  
> generate native code.

Hi Bill:

I've been looking for a C++ to C translator for quite some time.
The purpose is to support C++ for the compiler I developed targeting the
JVM.  But the compiler I wrote only supports ANSI C (1989).  So the C++
to C translator is needed, and among the options are as follows:

a) EDG (rather expensive, around USD 100K with source code)
b) Comeau (around USD 25K for customization service and you don't get 
   the source code)
c) LLVM (tried, but the resulting C code is quite hairy, pardon the  
d) gcc2c (trying now, don't know how useful yet)

For more info on the said C to JVM compiler you can check out our
website: http://www.axiomsol.com



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