[cfe-dev] HandWritten Compiler FrontEnd

Shang Yu yusunn at gmail.com
Sun Jul 22 19:45:06 PDT 2012

and the generated parser code is ugly :(

2012/7/23 Sean Silva <silvas at purdue.edu>:
> A couple reasons off the top of my head:
> 1. Parsing C++ (C/ObjC to a lesser extent) is extremely difficult with
> a generated parser since the languages are context-dependent. Also,
> with C++ things get really nasty because there are some syntactic
> constructs that have to be disambiguated by the compiler in rather
> elaborate ways; see lib/Parse/ParseTentative.cpp for the most
> significant ones (consider the disambiguation that has to happen to
> resolve the ambiguity described in the comment in
> <http://clang.llvm.org/doxygen/ParseTentative_8cpp_source.html#l00440>).
> 2. It's really hard (if not impossible) to give good diagnostics with
> a compiler compiler. Good diagnostics are extremely important for
> Clang.
> 3. Clang's parser simultaneously handles C/C++/ObjC/ObjC++ and a huge
> variety of different language options for each one, all of which need
> to be properly recognized or diagnosed if valid/invalid given the
> current language and settings. This would be difficult to do with a
> compiler compiler.
> 4. You can hack on it without knowing any fancy compiler compiler's
> domain-specific language for expressing parsing and the theory behind
> it (LR, LALR, LL(k), etc.). All you need to know is C++. This is not
> to say that hacking on it is "easy" (for a variety of reasons, one
> being that it is poorly documented), but you can at least look at the
> code and understand what it is doing.
> 5. This is similar to the last one, but worth stating separately: it
> is easy to reason about performance, and it's easy to optimize using
> "normal" techniques for optimizing C++. Clang is very fast.
> --Sean Silva
> On Sun, Jul 22, 2012 at 7:53 AM, Xin Tong <xerox.time.tech at gmail.com> wrote:
>> I have little experience with compiler frontends. I am curious why is
>> clang hand written, instead of automatically generated by compiler
>> compilers such as lex and yacc. what are some advantages of a hand
>> written compiler frontend.
>> Thanks
>> Xin
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