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

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Date:	Mon, 25 Jul 2011 21:08:04 -0400
From:	Arnaud Lacombe <lacombar@...il.com>
To:	Ian Lance Taylor <iant@...gle.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

Hi,

On Mon, Jul 25, 2011 at 8:41 PM, Ian Lance Taylor <iant@...gle.com> wrote:
> 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.
I've got some trouble linking the final optimized tree with the code
generated. That is, the final tree (generated by -fdump-tree-all) is:

foo ()
{
  void * b;
  void * a;
  _Bool D.1993;
  _Bool D.1992;
  _Bool D.1991;

<bb 2>:
  a_2 = e ();
  if (a_2 != 0B)
    goto <bb 3>;
  else
    goto <bb 4>;

<bb 3>:
  b_4 = f ();

<bb 4>:
  # b_1 = PHI <b_3(D)(2), b_4(3)>
  D.1991_5 = a_2 != 0B;
  D.1992_6 = b_1 != 0B;
  D.1993_7 = D.1992_6 & D.1991_5;
  if (D.1993_7 != 0)
    goto <bb 5>;
  else
    goto <bb 6>;

<bb 5>:
  g (); [tail call]

<bb 6>:
  return;

}

but the code generated seem to test %esi (`b', potentially
uninitialized) before %ebx (`a'). Am I still missing something ?

Thanks,
 - Arnaud
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