Re: [boost] Clang: Open-source C/C++ front end under development

From: David Abrahams (dave_at_[hidden])
Date: 2007-09-01 14:01:02

on Sat Sep 01 2007, Douglas Gregor <doug.gregor-AT-gmail.com> wrote:

> On Sat, 2007-09-01 at 10:41 -0400, David Abrahams wrote:
>> on Fri Aug 31 2007, Doug Gregor <dgregor-AT-osl.iu.edu> wrote:
>> 
>> > 		- Video: http://llvm.org/devmtg/2007-05/09-Naroff-CFE.mov
>> 
>> The goal of separating semantic analysis from parsing is a noble one,
>> but it sounds like he may be underestimating the amount of semantic
>> analysis that's required to parse C++.  Inside templates we have the
>> benefit of the typename and template keywords to tell us which are the
>> types, but not inside regular code.  AFAICT that means it has to do
>> template instantiation just to tell whether foo<bar>::x is a type or
>> not.  Am I missing something?
>
> No, you're technically correct. Some semantic analysis is certainly
> required to parse C++, so you can't completely drop semantic analysis
> and still parse. 

Isn't "some" a huge understatement?  I mean, c'mon, you need to do
overload resolution!  Just evaluate
boost::detail::is_incrementable<X>::value for some X, for example.

> However, you can keep the two notions in separate modules, 

Sure.

> and the
> parser will certainly need to call into the semantic analysis module
> to figure out whether a particular name is a type, a value, a
> template, etc... just like a C parser needs to consult a symbol
> table to figure out whether a name is a typedef name or something
> else.

Yeah, only more so.  At one point he said of the parser, "we don't do
constant folding," but clearly you need to do that to decide whether a
name is a type or not.  

  foo<3*5>::x * y;

It seems to me that for C++ with templates, during parsing you have to
all the semantic analysis that isn't code generation -- and that's a
lot.

> What this probably means is that the "minimal" semantic analysis for
> C++ is a whole lot more heavyweight than the minimal semantic
> analysis for C. But you still get some benefit from separating out
> the semantics from the parser, because there are many semantic bits
> that you *can* ignore if you only want an (unchecked) parse tree.

What, other than code generation?

It has often seemed to me that it might make sense to parse C++
nondeterministically, just to avoid some of these issues.  The number
of real instances of ambiguity is probably pretty small.

Anyway, I should probably take this over to the LLVM list...

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com
The Astoria Seminar ==> http://www.astoriaseminar.com