linux-kernel - RE: mainline build failure due to 281d0c962752 ("fortify: Add Clang support")

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Message-ID: <b5982766c36049b595e7a5e8168ba612@AcuMS.aculab.com>
Date:   Thu, 23 Jun 2022 10:12:15 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Nick Desaulniers' <ndesaulniers@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        clang-built-linux <llvm@...ts.linux.dev>
Subject: RE: mainline build failure due to 281d0c962752 ("fortify: Add Clang
 support")

From: Nick Desaulniers
> Sent: 22 June 2022 23:40
....
> > We don't actually take full advantage of that, because we do end up
> > doing a real "build" of an empty file, so "cc1" does actually get
> > executed, but even then it's done fairly efficiently with 'vfork()'.
> > That "cc-option" part of the kernel build is actually noticeable
> > during configuration etc, and clang is *much* slower because of how it
> > is built.
> >
> > See
> >
> >     time clang -Werror -c -x c /dev/null
> >
> > and compare it with gcc. And if you want to see a really *big*
> > difference, pick a command line option that causes an error because it
> > doesn't exist..
> 
> Looking at a profile, there's a lot of stupid stuff we're doing.  We
> can probably get faster "at doing nothing." See
> https://gist.github.com/nickdesaulniers/81a87ffa784c13d0bf60f60b1d54651b
> for the profile and my commentary/initial thoughts.
> 
> >
> > I really wish clang wasn't so much noticeably slower. It's limiting
> > what I do with it, and I've had other developers say the same.
> 
> We can do better.  I'll keep pushing on this up my chain of command.
> That statement stands in stark contrast to the below:

The slow startup must also make a big difference to anything
that uses autoconf.
That tends to run a lot of compiles of trivial code just to
find out that the system is 'normal'.

	David

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