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From: Nick Desaulniers <ndesaulniers@google.com>
To: sfr@canb.auug.org.au
Cc: axboe@kernel.dk, josef@toxicpanda.com, libaokun1@huawei.com,
	 linux-kernel@vger.kernel.org, linux-next@vger.kernel.org,
	 Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	llvm@lists.linux.dev,  Arnd Bergmann <arnd@kernel.org>,
	Nathan Chancellor <nathan@kernel.org>
Subject: Re: linux-next: build failure while building Linus' tree
Date: Fri, 10 Sep 2021 15:02:14 -0700	[thread overview]
Message-ID: <CAKwvOd=hoPv2GMZws6UsnMHop6+662wer=Hfo2Tw2=1XXCY2JA@mail.gmail.com> (raw)
In-Reply-To: <20210909225035.3990728-1-ndesaulniers@google.com>

On Thu, Sep 9, 2021 at 3:50 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
>
> + Rasmus
>
> This was introduced in
> commit f0907827a8a91 ("compiler.h: enable builtin overflow checkers and add
> fallback code")
> which added division using the `/` operator, which is problematic when checking
> for overflows of 64b operands on 32b targets.
>
> We'll probably need helpers from linux/math64.h and some combination of
> __builtin_choose_expr/__builtin_types_compatible_p.
>
> That will help us fix another compiler bug for older clang releases, too.
> https://github.com/ClangBuiltLinux/linux/issues/1438.

Ok, I have something hacked up that I think will work:
https://gist.github.com/nickdesaulniers/2479818f4983bbf2d688cebbab435863
This incomplete diff is a little hacked up to reproduce the issue with
a known-bad revision of clang that demonstrates a similar issue to GCC
4.9.  You can ignore the movement of check_mul_overflow and friends in
include/linux/overflow.h.

I think I'm going to break that up into 2 or 3 patches:
1. move is_signed_type from include/linux/overflow.h to perhaps
include/linux/typecheck.h.
2. add div64_x64, div_x64, and div_64 to include/linux/math64.h, use
them in include/linux/overflow.h to fix GCC 4.9
3. move multiply fallbacks out of
COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW for clang < 14.
-- 
Thanks,
~Nick Desaulniers

  reply	other threads:[~2021-09-10 22:02 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-09  8:25 Stephen Rothwell
2021-09-09 22:50 ` Nick Desaulniers
2021-09-09 22:50   ` Nick Desaulniers
2021-09-10 22:02   ` Nick Desaulniers [this message]
2021-09-10 22:02     ` Nick Desaulniers
2021-09-10 22:17     ` Nick Desaulniers
2021-09-10 22:17       ` Nick Desaulniers
2021-09-10 22:26       ` Nick Desaulniers
2021-09-10 22:26         ` Nick Desaulniers
2021-09-10 22:34         ` Nick Desaulniers
2021-09-10 22:34           ` Nick Desaulniers

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