Re: [PATCH] x86: do not allow to optimize flag_is_changeable_p()

[Jeremy Fitzhardinge <jeremy@goop.org>]

Yinghai Lu wrote:
> On Mon, Sep 29, 2008 at 11:14 PM, Jeremy Fitzhardinge <jeremy@goop.org> wrote:
>   
>> Krzysztof Helt wrote:
>>     
>>> From: Krzysztof Helt <krzysztof.h1@wp.pl>
>>>
>>> The flag_is_changeable_p() is used by
>>> has_cpuid_p() which can return different results
>>> in the code sequence below:
>>>
>>>  if (!have_cpuid_p())
>>>       identify_cpu_without_cpuid(c);
>>>
>>>   /* cyrix could have cpuid enabled via c_identify()*/
>>>   if (!have_cpuid_p())
>>>       return;
>>>
>>> Otherwise, the gcc 3.4.6 optimizes these two calls
>>> into one which make the code not working correctly.
>>> Cyrix cpus have the CPUID instruction enabled but
>>> it is not detected due to the gcc optimization.
>>> Thus the ARR registers (mtrr like) are not detected
>>> on such a cpu.
>>>
>>>       
>> If "asm volatile" changes the code and fixes the bug, it seems like
>> you're making use of an undocumented - or at least non-portable - behaviour.
>>
>> Does adding a "memory" clobber also fix the problem?  That would have
>> better defined characteristics.
>>
>>     
>
> how about
>
>         if (!have_cpuid_p()) {
>                 identify_cpu_without_cpuid(c);
>
>                 /* cyrix could have cpuid enabled via c_identify()*/
>                 if (!have_cpuid_p())
>                         return;
>         }
>   

That doesn't help, does it?  If gcc thinks it can get away with
evaluating have_cpuid_p() once, then that's the same as:

	if (!have_cpuid_p()) {
		identify_cpu_without_cpuid(c);

		return;
	}

even though identify_cpu_without_cpuid() can cause the cpu to suddenly
start supporting cpuid.

The trouble is that flag_is_changeable_p() doesn't have any obvious
global dependencies; it just takes a constant argument and returns a
result.   The asm() needs to be updated to have a "memory" constraint as
a stand-in for the specific constraint of "cpu has switched into
cpuid-supporting state".

    J