Re: [Tinycc-devel] modern c++ compiler written in C (food for thought)

[Christian Jullien]

> : in fact, Comeau C/C++ compiles to C, and until recently was the ONLY
major compiler to support some C++ features.

Sure, C backend as interim code generator works well provided you rely on C
optimizer to polish the code. I do the same with my OpenLisp compiler (see
how it works here https://en.wikipedia.org/wiki/OpenLisp#Compiler).
The idea is to first optimize as much as you can and then target a so simple
C code that it's a piece of cake for C optimizer to finish the job.
My compiler beats native "Lisp->bare metal assembler" compilers because it
uses all tricks -O3 is able to do. It achieves good performances at the
price of a very long optimization analysis (sometimes, a single lisp file
generates half megabyte non commented C file!!).

NOW

What is "funny" with tinycc is that, due to the lack of optimization,
compiled code is twice as SLOW as interpreted code and remember that my lisp
compiler already does branch tension, dead code elimination, constant
folding, redundant store elimination and few other optimization before C
code generation.

I bet it would be even worse with the code Cfront generates.

-----Original Message-----
From: address@hidden
[mailto:address@hidden On Behalf Of Jared
Maddox
Sent: samedi 31 octobre 2015 18:39
To: address@hidden
Subject: Re: [Tinycc-devel] modern c++ compiler written in C (food for
thought)

> Date: Wed, 28 Oct 2015 07:11:47 +0100
> From: Basile Starynkevitch <address@hidden>
> To: address@hidden
> Subject: Re: [Tinycc-devel] modern c++ compiler written in C (food for
>         thought)
> Message-ID: <address@hidden>
> Content-Type: text/plain; charset=windows-1252; format=flowed
>
> On 10/28/2015 05:14 AM, Sergey Korshunoff wrote:
>> Hi Basile
>>
>>> A C++ compiler needs to be optimizing. The C++ language 
>>> specification sort-of requires an > > optimizing compiler
>> Did you say that a usual C compiler (gcc, clang) can not optimize as a
C++ does?
>> A tcc compiler is a compiler for a development speedup. A final stage 
>> can be build by the usual compiler.
>
> No, I was just saying the obvious point that practically speaking the
> C++11 standard is expecting any compiler implementing that standard to
> optimize significantly.

I wouldn't write a true C++ compiler because the language is a mess, but if
someone else wants to, then lack of optimizations should not precisely be
considered a stopping-point.


> A C++ compiler should inline lots of functions (including most member 
> functions declared inside a class) and do a lot of optimizations (in 
> particular constant folding, some loop unrolling or if (true) 
> optimizations, dead code elimination, etc...). I giuess that some 
> template expansion cannot happen without optimizations.
>

The only one of those optimizations that I understand to be needed is dead
code elimination, and that's really just because some codebases have started
using if( 0 ) as a replacement for #if( 0 ), thus producing linking
complications. It could probably be justified for ordinary TCC as a
constants-optimization (some of which TCC does actually have), if someone
was interested in implementing it. The rest of those optimizations may be
intended, but proper function inlining is intended with C99, yet TCC
compiles C99 code okay without it.
Optimizations are only a sticking point if their absence interferes with
parsing or code generation.

Template expansion, meanwhile, probably calls for a virtual machine, as I
understand that it was one of the major stumbling-points for CFront, and I
believe their full power was only discovered after it was abandoned.
Exceptions are more of a problem, since you need
setjump()/longjmp() to play nicely: in some cases you'll need to write your
own version, and some platforms might not make this as easy as
x86 does.

At any rate, whether revolving around TCC or not, this is not precisely a
TCC project, since it's inclusion in TCC can't be justified due to the
danger of running afoul of patent licensing rules. This is really a CFront
project, and CFront already has been a
C++ compiler, so it's rather nonsensical to claim that it can't in any
way be brought up to a semi-modern standard: in fact, Comeau C/C++ compiles
to C, and until recently was the ONLY major compiler to support some C++
features.

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