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

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Date:	Mon, 25 Jul 2011 17:41:02 -0700
From:	Ian Lance Taylor <iant@...gle.com>
To:	Arnaud Lacombe <lacombar@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, 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

Arnaud Lacombe <lacombar@...il.com> writes:

> gcc will only emits the warning at -Os. It seems to me that the
> resulting code clearly ends-up testing an uninitialized value, ie.
> assuming the following test-case:
>
> extern void *e(void);
> extern void *f(void);
> extern void g(void);
>
> void fn(void)
> {
>         void *b, *a;
>
>         a = e();
>         if (a != 0)
>                 b = f();
>         if (a != 0 && b != 0)
>                 g();
> }
>
> ...
>
> It seems gcc transforms the conditional from:
>
> if (a != NULL && b != NULL) ...
>
> to
>
> if (b != NULL && a != NULL) ...
>
> In which case the warning is fully valid. I'm not sure what's the C
> standard guarantee in term of conditional test order. gcc 4.7.0 has
> the same behavior.

Not quite.  C guarantees that && is executed in order.  In this case gcc
is generating

  a = e();
  if (a != NULL)
    b = f();
  if (a != NULL & b != NULL)
    g();

Note the change from && to & in the last conditional.  This
transformation is safe, in that it does not change the meaning of the
program.  However, it does cause a read of an uninitialized memory
location, and this is causing a later gcc pass to generate a false
positive warning.

Please consider filing a bug report about this false positive.  Thanks.

Ian
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