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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 20/12/2016 07:25, Jan Beulich wrote:
>>>> 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.

What about this?

There is no need for the volatile cast in the timer interrupt; the compiler
may not elide the update.  This reduces the generated assembly from a read,
local modify, write to a single add instruction.

~Andrew

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