Re: [rfc patch] i386: don't save eflags on task switch

H. Peter Anvin
2006-11-05T20:11:30Z

Zachary Amsden wrote:
> Benjamin LaHaise wrote:
>> On Sat, Nov 04, 2006 at 11:09:42AM -0800, Zachary Amsden wrote:
>>
>>> Every processor I've ever measured it on, popf is slower. On P4, for
>>> example, pushf is 6 cycles, and popf is 54. On Opteron, it is 2 /
>>> 12. On Xeon, it is 7 / 91.
>>
>> pushf has to wait until all flag dependancies can be resolved. On the
>> P4 with >100 instructions in flight, that can take a long time. Popf
>> on the other hand has no dependancies on outstanding instructions as
>> it resets the machine state.
>
> Yes, but as Linus points out popf is most likely microcoded, thus much
> slower. Flag dependency is not unique to pushf, many much more common
> instructions (adc, jcc, sbc, cmovcc, movs, stos, ...) have flag
> dependencies, which can still be pipeline forwarded. I think the raw
> cycle counts speak for themselves, despite the fact that I only measured
> instruction latency, not throughput. Using a branch to eliminate a
> pushf is thus probably not a win in most cases.
>

The "sane" decomposition of popf into uops something like this:

- Memory read
- Mask bits that are immutable in the current mode
- Trap to microcode on changing any bit that alters the pipeline state

The "trap to microcode" can obviously be arbitrarily expensive. So when
timing popf, it's also important to know *which* bits change.

The simplest case, obviously, is when no flags change. I still *fully*
expect that to be more painful than pushf ever is.

-hpa