RFC: CACAO PR157 ARM memory barrier patch

[Xerxes Rånby]

On 2011-03-11 16:27, Andrew Haley wrote:
> On 03/11/2011 03:12 PM, Xerxes Rånby wrote:
>> On 2011-03-11 16:06, Matthias Klose wrote:
>>> On 11.03.2011 14:59, Xerxes Rånby wrote:
>>>> Hi
>>>>
>>>> The attached patch makes CACAO SMP safe on ARM by fixing
>>>> CACAO PR157: ARM SMP Assertion thinlock failed.
>>>> http://server.complang.tuwien.ac.at/cgi-bin/bugzilla/show_bug.cgi?id=157
>>>>
>>>> Ok to push to IcedTea6 HEAD and 1.10 branch?
>>> please could you make this check depending on the compiler version used? As it
>>> is not in 4.5.x fsf, the safe version to check would be 4.6.0.
>>>
>>>   Matthias
>> No, i dont think making this dependent on the compiler version would work.
>> For what i know
>>
>> __asm__ __volatile__ ("" : : : "memory");
>>
>> do not generate any code and do not work as a memory barrier on ARM for any GCC version.
> The gcc barrier is called __sync_synchronize().
>
> Andrew.

I will look into if I can make a follow up optimization patch for IcedTea6 HEAD
that uses the GCC __sync_synchronize() memory barrier on GCC 4.5 and later in combination with ARMV7.
The GCC __sync_synchronize() do emit an armv7+ dmb instruction directly
that are faster than using the kernel helper on the latest ARM socs but only on GCC 4.5+ for what i have tested.

Cheers
Xerxes