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

[Denys Vlasenko]

On 08/17/2016 07:30 PM, Christian König 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.