On 08/18/2016 07:24 PM, Linus Torvalds wrote:
That said, your numbers really aren't very convincing. If popf really is just 10 cycles on modern Intel hardware, it's already fast enough that I really don't think it matters.
It's 20 cycles. I was wrong in my email, I forgot that the insn count also counts "push %ebx" insns. Since I already made a mistake, let me double-check. 200 million iterations of this loop execute under 17 seconds: 400100: b8 00 c2 eb 0b mov $0xbebc200,%eax # 1000*1000*1000 400105: 9c pushfq 400106: 5b pop %rbx 400107: 90 nop .... 0000000000400140 <loop>: 400140: 53 push %rbx 400141: 9d popfq 400142: 53 push %rbx 400143: 9d popfq 400144: 53 push %rbx 400145: 9d popfq 400146: 53 push %rbx 400147: 9d popfq 400148: 53 push %rbx 400149: 9d popfq 40014a: 53 push %rbx 40014b: 9d popfq 40014c: 53 push %rbx 40014d: 9d popfq 40014e: 53 push %rbx 40014f: 9d popfq 400150: 53 push %rbx 400151: 9d popfq 400152: 53 push %rbx 400153: 9d popfq 400154: 53 push %rbx 400155: 9d popfq 400156: 53 push %rbx 400157: 9d popfq 400158: 53 push %rbx 400159: 9d popfq 40015a: 53 push %rbx 40015b: 9d popfq 40015c: ff c8 dec %eax 40015e: 75 e0 jne 400140 <loop> The loop is exactly 32 bytes, aligned. There are 14 POPFs. Other insns are very fast. No perf, just "time taskset 1 ./test". My CPU frequency hovers around 3500 MHz when loaded. 17 seconds is 17*3500 million cycles. 17*3500 million cycles / 200*14 million cycles = 21.25 Thus, one POPF in CPL3 is ~20 cycles on Skylake.
