From: Michal Hocko <mhocko@kernel.org>
To: Nick Desaulniers <nick.desaulniers@gmail.com>
Cc: akpm@linux-foundation.org, hannes@cmpxchg.org,
mgorman@techsingularity.net, vbabka@suse.cz, minchan@kernel.org,
linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [Patch v2] mm/vmscan: fix unsequenced modification and access warning
Date: Tue, 16 May 2017 10:27:46 +0200 [thread overview]
Message-ID: <20170516082746.GA2481@dhcp22.suse.cz> (raw)
In-Reply-To: <20170510083844.GG31466@dhcp22.suse.cz>
I have discussed this with our gcc guys and here is what they say:
On Wed 10-05-17 10:38:44, Michal Hocko wrote:
[...]
> But I
> still do not understand which part of the code is undefined and why. My
> reading and understanding of the C specification is that
> struct A {
> int a;
> int b;
> };
>
> struct A f = { .a = c = foo(c), .b = c};
>
> as long as foo(c) doesn't have any side effects because because .a is
> initialized before b and the assignment ordering will make sure that c
> is initialized before a.
>
> 6.7.8 par 19 (ISO/IEC 9899)
> 19 The initialization shall occur in initializer list order, each
> initializer provided for a particular subobject overriding any
> previously listed initializer for the same subobject; all subobjects
> that are not initialized explicitly shall be initialized implicitly
> the same as objects that have static storage duration.
>
> So is my understanding of the specification wrong or is this a bug in
> -Wunsequenced in Clang?
: This is not the reason why the above is okay. The following part:
: { .a = c = ..., .b = c }
: is okay because there's a sequence point after each full expression, and
: an initializer is a full expression, so there's a sequence point between
: both initializers. The following part:
: { ... c = foo(c) ... }
: is okay as well, because there's a sequence point after evaluating all
: arguments and before the actual call (otherwise the common 'i=next(i)'
: idiom doesn't work). So both constructs that potentially could be sources
: of sequence point violations actually aren't and hence okay. clangs
: warning is invalid.
I guess it is worth reporting this to clang bugzilla. Could you take
care of that Nick?
--
Michal Hocko
SUSE Labs
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2017-05-16 8:27 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-10 6:53 [PATCH] mm/vmscan: fix unsequenced modification and access warning Nick Desaulniers
2017-05-10 7:15 ` Michal Hocko
2017-05-10 8:27 ` [Patch v2] " Nick Desaulniers
2017-05-10 8:38 ` Michal Hocko
2017-05-16 8:27 ` Michal Hocko [this message]
2017-05-17 3:01 ` Nick Desaulniers
2017-05-26 4:43 ` Nick Desaulniers
2017-05-31 15:21 ` Michal Hocko
2017-05-10 8:46 ` [PATCH] " Nick Desaulniers
2017-05-10 9:25 ` Michal Hocko
2017-05-10 15:40 ` [Patch v3] " Nick Desaulniers
2018-03-21 21:37 ` [PATCH] " Nick Desaulniers
2018-03-22 9:50 ` Michal Hocko
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20170516082746.GA2481@dhcp22.suse.cz \
--to=mhocko@kernel.org \
--cc=akpm@linux-foundation.org \
--cc=hannes@cmpxchg.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mgorman@techsingularity.net \
--cc=minchan@kernel.org \
--cc=nick.desaulniers@gmail.com \
--cc=vbabka@suse.cz \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).