linux-kernel - Re: [rfc] built-in native compiler for Linux?

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Date:	Mon, 27 Apr 2009 11:41:10 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Christoph Lameter <cl@...ux.com>,
	David Miller <davem@...emloft.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [rfc] built-in native compiler for Linux?

Al Viro <viro@...IV.linux.org.uk> writes:

> On Mon, Apr 27, 2009 at 09:52:46AM -0400, Christoph Lameter wrote:
>> On Sat, 25 Apr 2009, Eric W. Biederman wrote:
>> 
>> > If this was about teaching sparse to run lockdep at compile time, or
>> > generally about making the kernel compilation much faster and able to
>> > catch many more bugs there might be a point where the effort is worth
>> > the investment.
>> 
>> The preprocessor and its interaction with regular C code is quite
>> nasty. If sparse could get rid of the complexities and idiosyncrasies at
>> that level then it may be useful as a "pre" compiler.
>
> Explain, please.  BTW, at the risk of being called an elitist bastard, could
> I ask the participants of that thread to read C99 standard?  It's not hard
> to find (http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf, for one
> thing - that's C99 + errata) and at least chapter 5 and 6.10 are really
> must-read if we are talking about that stuff.
>
> In particular, C preprocessor does *NOT* work on text-to-text level and
> hasn't since way back.  It works on token stream.
>
> That's actually one of the areas where C99 is a huge improvement over
> earlier language - instead of more or less nasty kludges still trying
> to pretend that preprocessor was a filter with text output piped into
> compiler, it gives reasonably clear semantics approximating what the
> earlier variants had in common.

Well it came in C89, not C99.  Which could well be the reason the
specification is clear and reasonable.

> I'd done fairly complete rewrite of macro expansion (and a bunch of other
> places in preprocessor) in sparse, and places where it used to deviate from
> standard were by far the worst in terms of convoluted logics and corner
> cases.  Switching to what C99 asked had simplified the things a *lot*;
> it's surprisingly well thought through in that area (unlike e.g. the unholy
> mess around 'restrict' qualifier semantics).

Thank you.  I can't even imagine someone writing a C Preprocessor with
K&R semantics at this late date.

Eric
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