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

[Denys Vlasenko]

On 08/17/2016 07:30 PM, Christian KÃnig wrote:
> > But in my measurements POPF is not fast even in the case where restored
> > flags are not changed at all:
> >
> >                 mov     $200*1000*1000, %eax
> >                 pushf
> >                 pop     %rbx
> >                 .balign 64
> > loop:           push    %rbx
> >                 popf
> >                 dec     %eax
> >                 jnz     loop
> >
> > # perf stat -r20 ./popf_1g
> >      4,929,012,093      cycles                    #    3.412 GHz                      ( +-  0.02% )
> >        835,721,371      instructions              #    0.17  insn per cycle           ( +-  0.02% )
> >        1.446185359 seconds time elapsed                                          ( +-  0.46% )
> >
> > If I replace POPF with a pop into an unused register, I get this:
>
> You are comparing apples and bananas here.

Yes, I know. Pop into a register here is basically free.
I'd also add a STI and measure how much it takes to
enable interrupts, but unfortunately STI throws a #GP
in CPL 3.