Re: [Qemu-devel] [PATCH 2/3] atomics: Use __atomic_*_n() variant primitives

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

Paolo Bonzini writes:

> On 24/08/2016 22:44, Pranith Kumar wrote:
>> Use the __atomic_*_n() primitives which take the value as argument. It
>> is not necessary to store the value locally before calling the
>> primitive, hence saving us a stack store and load.
>
> If you do this, you might as well do it for __atomic_load and
> __atomic_compare_exchange.  However, you'd still need typeof_strip_qual
> for __atomic_compare_exchange's "_old" local variable.

OK, I will update the patch doing the same for load and compare exchange.

>
> The question is, does this actually cause a change in generated code?  I
> would be very surprised if it did, but then it's perfectly possible to
> have a bug in GCC.

It looks like it does. I tested atomic_load() with previous and the
atomic_load_n() version and it generates the following code:

current:
  mov    0x200c4a(%rip),%eax        # 601030 <value>
  mov    %eax,-0x4(%rsp)
  mov    -0x4(%rsp),%eax

atomic_load_n():
  mov    0x200c4a(%rip),%eax        # 601030 <value>

So looks like it's worth it.

Thanks,
-- 
Pranith