llvm dev - May 2007 - [LLVMdev] C back-end differences

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.

Thanks.

Napi

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.

-bw

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  
   expression)
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

Cheers.

Napi