Re: compiling C++ to C, was writing a compiler

Marco van de Voort wrote:
> On 2008-11-03, Alex Colvin <alexc@TheWorld.com> wrote:
>>> Tony said, "From what I've read, it seems that CFront couldn't implement
>>> the whole language."
>>> I said I thought it could, at least theoretically, although modern
>>> compilers which implement the current standard all compile to machine code.
>> Runtime and linker features such as exceptions and template instantiation
>> would be very difficult to generate in C.
>
> Templates are no problem, the C++ compiler instantiates them, and writes
> them out in C code.
>
> Exceptions are harder. Sure, it is doable in C, but would the resulting,
> stack unwinding C be portable to a different compiler (IOW free of (ab)use of
> specific ABI knowledge )?
>


The cfront wikipedia entry has this to say about exceptions:


"Cfront 4.0 was abandoned after a failed attempt to add exception
support, and it appears it is no longer commercially available. The C++
language had grown beyond its capabilities, however a compiler with
similar approach became available later, namely Comeau C/C++, which is
considered highly standard compliant."


And on the Comeau website,


http://www.comeaucomputing.com/faqs/genfaq.html>compiler


says this:


"The C compiler is used merely and only for the sake of obtaining native
code generation. This means that Comeau C++ is tailored for use with
specific C compilers on each respective platform. Please note that it is
a requirement that tailoring must be done by Comeau. Otherwise, the
generated C code is meaningless as it is tied to a specific platform
(where platform includes at least the CPU, OS, and C compiler) and
furthermore, the generated C code is not standalone. Therefore, it
cannot be used by itself (note that this is both a technical and legal
requirement when using Comeau C++), and this is why there is not
normally an option to see the generated C code: it's almost always
unhelpful and the compile process, including its generation, should be
considered as internal phases of translation."


So it sounds like Tony and Marco are basically right: cfront as we knew
it can't do exceptions.


Templates are a challenge for any compiler, cfront or not. Back in 1994
and 1995, I was making a living porting C++ code to DEC C++ (which at
the time didn't support automatic template instantiation) and to Sun C++
(whose old cfront compiler did it well but whose brand new native
compiler did it badly). I eventually wrote something using nm and perl
which kept track of which templates were needed and compiled them (with
C++), iterating until there were no more uninstantiated templates left.


I think I still have a copy of that code somewhere.


Louis

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