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

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Message-ID: <1311638934.3526.43.camel@gandalf.stny.rr.com>
Date:	Mon, 25 Jul 2011 20:08:54 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Arnaud Lacombe <lacombar@...il.com>
Cc:	gcc-help@....gnu.org, 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

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


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