linux-kernel - Re: [PATCH 0/5] RFC: strip 15,000 lines from a typical autoconf.h

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Date:	Wed, 11 Apr 2012 18:19:58 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, davem@...emloft.net,
	tony@...eyournoodle.com, mmarek@...e.cz, lacombar@...il.com
Subject: Re: [PATCH 0/5] RFC: strip 15,000 lines from a typical autoconf.h

On Wed, Apr 11, 2012 at 4:58 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I think we can easily drop IS_ENABLED() if it has been causing
> problems. And it clearly is.
>
> And I had never even looked at that generated file. It's disgusting.
> We need to stop doing that. It must be driving compile-times up too -
> just parsing that monster is RealWork(tm).

Oh, but I have a sick and twisted mind. And I'm incredibly smart and
photogenic too.

Here's an implementation of IS_ENABLED() that works in C. Well,
"gcc-C". It depends on the optimizer DTRT, but gcc really does seem to
do it.

Ready? You *will* go blind - blinded by the pure beauty and intellect
in this thing:

    #define IS_DEFINED(x) (__stringify(CONFIG_##x)[0]=='1')

That really is a piece of art. I'm expecting that the Guggenheim will
contact me any moment now about it.

Of course, the above does *not* work for a pre-processor, but hey, you
can't have everything. It literally is *only* valid in C code. You
can't have everything. In the pre-processor, you can just use

   #if CONFIG_OPTION

like you always could, or use another helper macro for the "defined(x)" magic.

                      Linus
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