From: Steven Rostedt <rostedt@goodmis.org>
To: Arnaud Lacombe <lacombar@gmail.com>
Cc: gcc-help@gcc.gnu.org, stufever@gmail.com,
linux-kernel@vger.kernel.org,
Wang Shaoyan <wangshaoyan.pt@taobao.com>,
Frederic Weisbecker <fweisbec@gmail.com>,
Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH] TRACING: Fix a copmile warning
Date: Mon, 25 Jul 2011 20:08:54 -0400 [thread overview]
Message-ID: <1311638934.3526.43.camel@gandalf.stny.rr.com> (raw)
In-Reply-To: <CACqU3MW2bXNQeTDqy2NcUBzzeZ7CLNGtb3et=UHq4DRxhsFEDQ@mail.gmail.com>
On Mon, 2011-07-25 at 19:58 -0400, Arnaud Lacombe wrote:
> Hi,
> > [...]
> > In which case the warning is fully valid. I'm not sure what's the C
> > standard guarantee in term of conditional test order.
> I'd assume that the following apply:
>
> 6.5 Expressions
>
> 3 The grouping of operators and operands is indicated by the
> syntax.72) Except as specified
> later (for the function-call (), &&, ||, ?:, and comma operators),
> the order of evaluation
> of subexpressions and the order in which side effects take place are
> both unspecified.
>
>
> in which case gcc is free to do whatever it wants :(
No it does not! Read what you wrote: "Except as specified later (for the
function-call(), &&, ||, ?:...)"
&& and || must be short cuts. That is, it must evaluate the earlier
statements before the later, and exit when it can. We use that all over
the kernel (and in all C code):
if (ptr && ptr->field)
If it were to switch that to:
if (ptr->field && ptr)
we would have segfaults everywhere.
This looks like a serious gcc bug.
-- Steve
next prev parent reply other threads:[~2011-07-26 0:09 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-07-18 9:40 stufever
2011-07-25 18:32 ` Steven Rostedt
2011-07-25 19:43 ` Arnaud Lacombe
2011-07-25 20:19 ` Steven Rostedt
2011-07-25 20:28 ` Arnaud Lacombe
2011-07-25 22:38 ` Arnaud Lacombe
2011-07-25 23:49 ` Steven Rostedt
2011-07-25 23:52 ` Arnaud Lacombe
2011-07-26 0:14 ` Steven Rostedt
2011-07-25 23:50 ` Arnaud Lacombe
2011-07-25 23:58 ` Arnaud Lacombe
2011-07-26 0:08 ` Steven Rostedt [this message]
2011-07-26 0:35 ` Steven Rostedt
2011-07-26 0:44 ` Ian Lance Taylor
2011-07-26 0:41 ` Ian Lance Taylor
2011-07-26 1:08 ` Arnaud Lacombe
2011-07-26 1:12 ` Steven Rostedt
2011-07-26 1:19 ` Arnaud Lacombe
2011-07-26 20:43 ` Jeff Law
2011-07-26 1:10 ` Steven Rostedt
2011-07-26 5:55 ` Ian Lance Taylor
2011-07-26 12:00 ` Paulo Marques
2011-07-26 13:18 ` Jesper Juhl
2011-07-26 13:32 ` Paulo Marques
2011-07-26 13:55 ` Steven Rostedt
-- strict thread matches above, loose matches on Subject: below --
2011-07-18 9:35 stufever
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