linux-kernel - Re: [PATCH] TRACING: Fix a copmile warning

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Message-ID: <CACqU3MW2bXNQeTDqy2NcUBzzeZ7CLNGtb3et=UHq4DRxhsFEDQ@mail.gmail.com>
Date:	Mon, 25 Jul 2011 19:58:24 -0400
From:	Arnaud Lacombe <lacombar@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>, gcc-help@....gnu.org
Cc:	stufever@...il.com, linux-kernel@...r.kernel.org,
	Wang Shaoyan <wangshaoyan.pt@...bao.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] TRACING: Fix a copmile warning

Hi,

On Mon, Jul 25, 2011 at 7:50 PM, Arnaud Lacombe <lacombar@...il.com> wrote:
> Hi,
>
> [adding gcc-help@ to the Cc: list]
>
> On Mon, Jul 25, 2011 at 6:38 PM, Arnaud Lacombe <lacombar@...il.com> wrote:
>> Hi,
>>
>> On Mon, Jul 25, 2011 at 4:19 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>>> On Mon, 2011-07-25 at 15:43 -0400, Arnaud Lacombe wrote:
>>>
>>>> Actually, we have a special uninitialized_var(x) macro to handle such
>>>> false positive. From include/linux/compiler-gcc.h:
>>>>
>>>> /*
>>>>  * A trick to suppress uninitialized variable warning without generating any
>>>>  * code
>>>>  */
>>>> #define uninitialized_var(x) x = x
>>>
>>> I'm aware of that too, but I think that is inappropriate as well. As I
>>> said, some versions of gcc report it, others don't. Seems that gcc 4.6.0
>>> says this is an error where 4.5.1 does not (I just tried both).
>>>
> [...]
> 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 :(

 - Arnaud
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