Re: RFC: Petition Intel/AMD to add POPF_IF insn

[Denys Vlasenko <dvlasenk@redhat.com>]

On 08/17/2016 09:32 PM, Linus Torvalds wrote:
> On Wed, Aug 17, 2016 at 12:26 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>>
>> Exactly. And more:
>
> All of which is ENTIRELY IRRELEVANT.
>
> The 2-page pseudo-code is about behavior. It's trivial to
> short-circuit. There are obvious optimizations, which involve just
> making the rare cases go slow and have a trivial "if those bits didn't
> change, don't go through the expense of testing them".
>
> The problem is that IF is almost certainly right now in that rare case
> mask, and so popf is stupidly slow for IF.

I ran the test, see the first email in the thread.

Experimentally, POPF is stupidly slow _always_. 6 cycles
even if none of the "scary" flags are changed.
Either:
*  its microcode has no rare case mask
or
* its microcode is so slow that even fast path is slow,
   and slow path is worse