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

[David Daney <ddaney@caviumnetworks.com>]

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()


> 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