linux-kernel - Re: [RFC PATCH] compiler.h: give up __compiletime_assert

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Message-ID: <6b25ce92-7ffa-9fdb-c114-e7aa8f987b06@pobox.com>
Date:   Tue, 21 Aug 2018 03:11:18 -0500
From:   Daniel Santos <daniel.santos@...ox.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        james.hogan@...tec.com, joe@....org,
        Rusty Russell <rusty@...tcorp.com.au>,
        Arnd Bergmann <arnd@...db.de>, sparse@...isli.org,
        linux-sparse@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        George Burgess <gbiv@...gle.com>,
        James Y Knight <jyknight@...gle.com>
Subject: Re: [RFC PATCH] compiler.h: give up __compiletime_assert_fallback()

On 08/19/2018 03:25 PM, Nick Desaulniers wrote:
> + gbiv who wrote this cool paste (showing alternatives to
> _Static_assert, which is supported by both compilers in -std=gnu89,
> but not until gcc 4.6): https://godbolt.org/g/DuLsxu
>
> I can't help but think that BUILD_BUG_ON_MSG should use
> _Static_assert, then have fallbacks for gcc < 4.6.

Unfortunately _Static_assert is a woefully inadequate replacement
because it requires a C constant expression.  Example:

    int a = 1;
    _Static_assert(a == 1, "a != 1");

results in "error: expression in static assertion is not constant." 
Language standards tend to shy away from defining implementation details
like optimizations, but we need to have completed a good data flow
analysis and constant propagation in order to do BUILD_BUG_ON_MSG, et.
al.; this is why they only work when optimizations are enabled.  As the
optimizer improves, new expressions can be used with BUILD_BUG_ON*.  I
did an analysis of this back in 2012 of how various types of variables
could be resolved to constants at compile-time and how that evolved from
gcc 3.4 to 4.7:

https://drive.google.com/open?id=1cQRAAOzjFy6Aw7CDc4QauHvd_spVkd5a

This changed again when -findirect-inline was added -- i.e.,
BUILD_BUG_ON could be used on parameters of inline functions even when
called by pointer, although the caller needed __flatten in some cases --
a bit messy.

Daniel