linux-kernel - Re: [Patch v2] mm/vmscan: fix unsequenced modification and access warning

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Message-ID: <20170526044343.autu63rpfigbzhyi@lostoracle.net>
Date:   Thu, 25 May 2017 21:43:43 -0700
From:   Nick Desaulniers <nick.desaulniers@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     akpm@...ux-foundation.org, hannes@...xchg.org,
        mgorman@...hsingularity.net, vbabka@...e.cz, minchan@...nel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [Patch v2] mm/vmscan: fix unsequenced modification and access
 warning

On Tue, May 16, 2017 at 10:27:46AM +0200, Michal Hocko wrote:
> I guess it is worth reporting this to clang bugzilla. Could you take
> care of that Nick?

>From https://bugs.llvm.org//show_bug.cgi?id=33065#c5
it seems that this is indeed a sequence bug in the previous version of
this code and not a compiler bug.  You can read that response for the
properly-cited wording but my TL;DR/understanding is for the given code:

struct foo bar = {
  .a = (c = 0),
  .b = c,
};

That the compiler is allowed to reorder the initializations of bar.a and
bar.b, so what the value of c here might not be what you expect.