Re: [Qemu-devel] [RFC PATCH] tcg: Optimize fence instructions

[Pranith Kumar <bobby.prani@gmail.com>]

Alex Bennée writes:

> Pranith Kumar <bobby.prani@gmail.com> writes:
>
>> This patch applies on top of the fence generation patch series.
>>
>> This commit optimizes fence instructions. Two optimizations are
>> currently implemented. These are:
>>
>> 1. Unnecessary duplicate fence instructions
>>
>>    If the same fence instruction is detected consecutively, we remove
>>    one instance of it.
>>
>>    ex: mb; mb => mb, strl; strl => strl
>>
>> 2. Merging weaker fence with subsequent/previous stronger fence
>>
>>    load-acquire/store-release fence can be combined with a full fence
>>    without relaxing the ordering constraint.
>>
>>    ex: a) ld; ldaq; mb => ld; mb
>>        b) mb; strl; st => mb; st
>
> What test cases do you have for this?
>
> Currently the litmus tests don't fire (as they have exactly what they
> need). Most programs don't seem to trigger multiple barriers.
>

Indeed, these cases are not so commonly seen in the wild. To test it I wrote
small test programs using C11 builtins to generate appropriate
instructions. Then verified it by running 'qemu-aarch64 -d in_asm,out_asm'.

_Atomic int val;

int main()
{
  val = __atomic_load_n(&val, __ATOMIC_ACQUIRE);
  __atomic_store_n(&val, val, __ATOMIC_RELEASE);
  barrier();
  barrier();
}

-- 
Pranith