Re: [PATCH] ARM: Convert BUG() to use unreachable()

From: Richard Guenther
Date: Thu Dec 17 2009 - 12:17:19 EST

On Thu, Dec 17, 2009 at 6:09 PM, David Daney <ddaney@xxxxxxxxxxxxxxxxxx> wrote:
> Jamie Lokier wrote:
>> Uwe Kleine-König wrote:
>>> Use the new unreachable() macro instead of for(;;);
>>>        *(int *)0 = 0;
>>>          /* Avoid "noreturn function does return" */
>>> -       for (;;);
>>> +       unreachable();
>> Will GCC-4.5 remove ("optimise away") the *(int *)0 = 0 because it
>> knows the branch of the code leading to unreachable can never be reached?
> I don't know the definitive answer, so I am sending to gcc@...
> FYI: #define unreachable() __builtin_unreachable()

It shouldn't as *(int *)0 = 0; might trap. But if you want to be sure
__builtin_trap ();
instead for the whole sequence (the unreachable is implied then).
GCC choses a size-optimal trap representation for your target then.


>> If GCC-4.5 does not, are you sure a future version of GCC will never
>> remove it?  In other words, is __builtin_unreachable() _defined_ in
>> such a way that it cannot remove the previous assignment?
>> We have seen problems with GCC optimising away important tests for
>> NULL pointers in the kernel, due to similar propagation of "impossible
>> to occur" conditions, so it's worth checking with GCC people what the
>> effect of this one would be.
>> In C, there is a general theoretical problem with back-propagation of
>> optimisations from code with undefined behaviour.  In the case of
>> __builtin_unreachable(), it would depend on all sorts of unclearly
>> defined semantics whether it can remove a preceding *(int *)0 = 0.
>> I'd strongly suggest asking on the GCC list.  (I'd have mentioned this
>> earlier, if I'd known about the patch for other architectures).
>> The documentation for __builtin_unreachable() only says the program is
>> undefined if control flow reaches it.  In other words, it does not say
>> what effect it can have on previous instructions, and I think it's
>> quite likely that it has not been analysed in a case like this.
>> One thing that would give me a lot more confidence, because the GCC
>> documentation does mention asm(), is this:
>>>      *(int *)0 = 0;
>>>      /* Ensure unreachableness optimisations cannot propagate back. *I/
>>>      __asm__ volatile("");
>>>      /* Avoid "noreturn function does return" */
>>>      unreachable();
>> -- Jamie
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