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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Thu, 29 Oct 2020 23:13:04 +0100
Message-ID: <CAMj1kXHxX+u5-cN0v3SLdqZTSiKsWsFOvc2SC5=-ScaUZOu8Ng@mail.gmail.com>
Subject: Re: [PATCH v2 1/2] bpf: don't rely on GCC __attribute__((optimize))
 to disable GCSE
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Arvind Sankar <nivedita@alum.mit.edu>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:BPF JIT for MIPS (32-BIT AND 64-BIT)" 
        <netdev@vger.kernel.org>,
        "open list:BPF JIT for MIPS (32-BIT AND 64-BIT)" 
        <bpf@vger.kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Kees Cook <keescook@chromium.org>,
        linux-toolchains@vger.kernel.org
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On Thu, 29 Oct 2020 at 21:35, Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Wed, Oct 28, 2020 at 10:57:45PM -0400, Arvind Sankar wrote:
> > On Wed, Oct 28, 2020 at 04:20:01PM -0700, Alexei Starovoitov wrote:
> > > All compilers have bugs. Kernel has bugs. What can go wrong?
>
> Heh.
>
> > +linux-toolchains. GCC updated the documentation in 7.x to discourage
> > people from using the optimize attribute.
> >
> > https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=893100c3fa9b3049ce84dcc0c1a839ddc7a21387
>
> https://patchwork.ozlabs.org/project/gcc/patch/20151213081911.GA320@x4/
> has all the discussion around that GCC patch.
>

For everyone's convenience, let me reproduce here how the GCC
developers describe this attribute on their wiki [0]:

"""
Currently (2015), this attribute is known to have several critical
bugs (PR37565, PR63401, PR60580, PR50782). Using it may produce not
effect at all or lead to wrong-code.

Quoting one GCC maintainer: "I consider the optimize attribute code
seriously broken and unmaintained (but sometimes useful for debugging
- and only that)." source

Unfortunately, the people who added it are either not working on GCC
anymore or not interested in fixing it. Do not try to guess how it is
supposed to work by trial-and-error. There is not a list of options
that are safe to use or known to be broken. Bug reports about the
optimize attribute being broken will probably be closed as WONTFIX
(PR59262), thus it is not worth to open new ones. If it works for you
for a given version of GCC, it doesn't mean it will work on a
different machine or a different version.

The only realistic choices are to not use it, to use it and accept its
brokenness (current or future one, since it is unmaintained), or join
GCC and fix it (perhaps motivating other people along the way to join
your effort).
"""

[0] https://gcc.gnu.org/wiki/FAQ#optimize_attribute_broken