[ltt-dev] [PATCH] fix the "unknown" case

[Paolo Bonzini]

On 06/13/2010 11:28 PM, Paul E. McKenney wrote:
> On Sun, Jun 13, 2010 at 05:20:34PM -0400, Mathieu Desnoyers wrote:
>> * Paul E. McKenney (paulmck at linux.vnet.ibm.com) wrote:
>>> On Thu, Jun 10, 2010 at 09:46:31PM -0400, Mathieu Desnoyers wrote:
>>
>> [...]
>>
>> (will reply to the rest in the individual patches)
>>
>>>> Can we trust __sync_lock_test_and_set/__sync_add_and_fetch given that
>>>> __sync_synchronize is broken ?
>>>
>>> I don't know yet.  If it turns out that we cannot, then I will use some
>>> form of global locking.  But the __sync_lock_test_and_set() do at least
>>> generate instructions, unlike __sync_synchronize().  ;-)
>>
>> I'm concerned about the fact that their synchronization primitives might have
>> the assembly all with, except for the memory barriers.

The default implementation of these __sync_* builtins is based on 
cmpxchg, and will cause a link error unless cmpxchg is also available 
(either in libgcc or with a compiler-provided inline implementation).

Instead, the default implementation of __sync_synchronize is to just do 
a compiler barrier.  ARM implements __sync_synchronize only for Linux, 
so at least there it is not needed.  Strange that Paul needs it too.

Anyway, a simple configure test is to compile this with -fdump-rtl-expand:

	int
	f()
	{
	  __sync_synchronize();
	}

If the assembly output includes "__sync_synchronize", or the dump file 
includes the text "unspec:BLK", it should be fine.  In particular, ia64, 
mips, and Alpha are ok.  Else you can use the pthreads trick.  I can try 
to make a patch if you're interested.  Or, more simply, it's possible to 
hardcode the above three platforms since it's unlikely that others will 
be added soon.

Paolo