Re: [PATCH] lib/string: shrink lib/string.i via IWYU

From: Nick Desaulniers <ndesaulniers@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: tanzirh@google.com, Kees Cook <keescook@chromium.org>,
	 Andy Shevchenko <andy@kernel.org>,
	linux-hardening@vger.kernel.org,  linux-kernel@vger.kernel.org,
	Nick DeSaulniers <nnn@google.com>,
	llvm@lists.linux.dev
Subject: Re: [PATCH] lib/string: shrink lib/string.i via IWYU
Date: Tue, 5 Dec 2023 13:14:16 -0800	[thread overview]
Message-ID: <CAKwvOdn+VTM+LY6ALcgaZTL57JpiKt5rBPMSPNXsgS3MCENhDQ@mail.gmail.com> (raw)
In-Reply-To: <20231205130449.8e330a26ecbed1f7b5ad5d7a@linux-foundation.org>

On Tue, Dec 5, 2023 at 1:04 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Tue, 05 Dec 2023 20:58:53 +0000 tanzirh@google.com wrote:
>
> > This diff uses an open source tool include-what-you-use (IWYU) to modify
> > the include list changing indirect includes to direct includes.
> > IWYU is implemented using the IWYUScripts github repository which is a tool that is
> > currently undergoing development. These changes seek to improve build times.
> >
> > This change to lib/string.c resulted in a preprocessed size of
> > lib/string.i from 26371 lines to 5232 lines (-80%).
> >
> > If there are any problems with the output of the tool please raise an
> > issue on the github.
> >
> > Link: https://github.com/ClangBuiltLinux/IWYUScripts
>
> Issue:
>
> > --- a/lib/string.c
> > +++ b/lib/string.c
> > @@ -16,16 +16,16 @@
> >
> >  #define __NO_FORTIFY
> >  #include <linux/types.h>
> > +#include <asm/bitsperlong.h>

Thanks for the quick feedback.  For additional context, Tanzir is my
intern working on getting IWYU working on the kernel.  Welcome Tanzir
to LKML!

>
> The preferred way to import bit-fiddling stuff is to include
> <linux/bits.h>.  Under the hood this may include asm/bitsperlong.h.  Or
> it may not, depending on Kconfig settings (particularly architecture).
>

Just triple checking my understanding; it looks like
include/linux/bits.h unconditionally includes asm/bitsperlong.h (which
is implemented per arch) most of which seem to include
asm-generic/bitsperlong.h.

include/linux/bits.h also defines a few macros (BIT_MASK, BIT_WORD,
BITS_PER_BYTE, GENMASK, etc).  If lib/string.c is not using any of
those, why can't we go straight to #including asm/bitsperlong.h?  That
should resolve to the arch specific impl which may include
asm-generic/bitsperlong.h?
-- 
Thanks,
~Nick Desaulniers