Re: [Xen-devel] [PATCH] x86/time: Adjust init-time handling of pit0_ticks

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.12.16 at 17:58, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 19/12/16 16:51, Jan Beulich wrote:
>>>>> On 19.12.16 at 17:38, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> There is no need for the volatile cast in the timer interrupt.  pit0_ticks 
>>> has
>>> external linkage, preventing the compiler from eliding the update.  This
>>> reduces the generated assembly from a read, local modify, write to a single
>>> add instruction.
>> I don't think external linkage is the reason here, considering the
>> effects of whole-program-optimization.
> 
> In the case of whole-program-optimisation, the compiler would observe
> that one function wrote to the variable, and one function read from it. 
> I presume that is also sufficient to prevent the eliding?

I would think so, yes (albeit the end result of that process may
be that everything which isn't recursive and doesn't serve as
independent entry point ends up as a few huge functions); I
merely wanted to point out that linkage isn't really relevant here.

Jan


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