Dispatches from the compiler front [LWN.net]

Dispatches from the compiler front

Posted Apr 15, 2010 17:09 UTC (Thu) by da4089 (subscriber, #1195)
In reply to: Dispatches from the compiler front by Cyberax
Parent article: Dispatches from the compiler front

Doesn't the Linux kernel disprove your assertion that "it's impossible to write anything complex in C reliably"?

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Dispatches from the compiler front

Posted Apr 15, 2010 20:10 UTC (Thu) by Cyberax (✭ supporter ✭, #52523) [Link] (4 responses)

That was an exaggeration on my part, certainly.

However, it has a kernel of truth. You need to write quite complex algorithms in compilers which have to traverse trees, annotating its nodes with complex structures, etc. It all looks quite clumsy in C (C++ in LLVM is much better).

In comparison, Linux kernel doesn't really has that kind of complex algorithms. The closest thing in complexity is the scheduler, which still is a frequent source of problems in Linux.

Personally I prefer OCaml for that kind of things. Pattern matching is the killer feature for compiler writers :)

For example: http://llvm.org/docs/tutorial/index.html

Dispatches from the compiler front

Posted Apr 18, 2010 20:08 UTC (Sun) by eparis123 (guest, #59739) [Link] (1 responses)

The closest thing in complexity is the scheduler, which still is a frequent source of problems in Linux.

I like your argument, and I understand it for algorithms-centric userspace code like compilers. But, sorry, citing trouble in the Linux scheduler 'cause it's written in C' is pure unsupported hallucinations.

Dispatches from the compiler front

Posted Apr 19, 2010 6:47 UTC (Mon) by nix (subscriber, #2304) [Link]

Quite. The scheduler keeps changing because it has a nearly impossible job to do (one which could be done perfectly only in the presence of perfect knowledge of the future).

Dispatches from the compiler front

Posted Apr 19, 2010 15:03 UTC (Mon) by daglwn (guest, #65432) [Link] (1 responses)

> However, it has a kernel of truth. You need to write quite complex
> algorithms in compilers which have to traverse trees, annotating its
> nodes with complex structures, etc. It all looks quite clumsy in C (C++
> in LLVM is much better).

Well. The folks at Edison Design Group would disagree with you. Their C++ frontend (a non-trivial project by anyone's definition) is all pure C. It is organized beautifully, commented well and doesn't resort to the "object-oriented C" tricks that end up making C code complex and obfuscated. The printed documentation is the best I've ever seen for any piece of any compiler.

I'm a C++ nut, no question. But I appreciate good design in any language and the Edison frontend comes about as close to perfect for a C project that I've ever seen. It works within the spirit of the language and presents a very clean API.

Dispatches from the compiler front

Posted Apr 27, 2010 20:47 UTC (Tue) by wahern (subscriber, #37304) [Link]

I'm not surprised.

Good design means good encapsulation**. Objected-oriented _syntax_ is most useful when at the outset you're unsure how to encapsulate the data and segregate the logic--both strongly related. In that case, you use a set of generic patterns which will, presumably, get you close to the mark until the solution makes itself clear. Once its clear you probably won't bother re-writing it.

But where something is heavily centered on an abstract algorithm, how to encapsulate the data and segregate the logic is usually self-evident. In this case object-oriented syntax doesn't buy you much if anything. Data structures don't need to be generic and protected with getters and setters. You don't need dynamic methods. One could even argue that C is preferable, being a much simpler language; things like inheritance become more obsfuscation than explication.

So, let's look at LLVM: not only is it a compiler with an easily discernible purpose with suggestive algorithms and data structures, but its centered on an even larger, more comprehensive meta-abstraction: manipulation and transform of bytecodes. This suggests that the choice of C++ has far less to do with the code quality than other choices.

Add to the mix the fact that GCC has been gutted and rebuilt several times over, and that it's 15+ years older than LLVM, I don't think it's reasonable to draw any conclusions whatsoever about C v. C++ in this comparison.

** "object-oriented tricks" sounds a little ambiguous. Any well-encapsulated design is bound to be "object-oriented", at least if juggling lots of data and performing lots of complex transformations internally. Contrast that with well-encapsulated design of unix shell utilities. You wouldn't say grep or sed is object-oriented internally; nor if you put them together. But that sort of segregation of work doesn't suite compiler design, so inevitably a well-written compiler will be object-oriented, no matter the language. The issue is whether C++ makes it easier to accomplish object-orientation. And I'll argue only to the degree the overall design is indeterminate. This is similar to choosing a scripting language over a compiled one. Scripting languages are more attractive the more ill-defined the problem.