Messages in this thread | | | Date | Wed, 20 Dec 2017 09:57:47 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 2/4] sched/fair: add util_est on top of PELT |
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On Fri, Dec 15, 2017 at 03:41:40PM +0000, Patrick Bellasi wrote: > Close enough, the actual code is: > > util_est = p->util_est.ewma; > 5218: f9403ba3 ldr x3, [x29,#112] > 521c: f9418462 ldr x2, [x3,#776] > if (abs(util_est - util_last) <= (SCHED_CAPACITY_SCALE / 100)) > 5220: eb010040 subs x0, x2, x1 > 5224: da805400 cneg x0, x0, mi > 5228: f100281f cmp x0, #0xa > 522c: 54fff9cd b.le 5164 <dequeue_task_fair+0xa04>
Ah, that cneg instruction is cute; on x86 we end up with something like:
bool abs_test(long s) { return abs(s) < 32; }
cmpl $-31, %eax jl .L107 movq -8(%rbp), %rax cmpl $31, %eax jg .L107 movl $1, %eax jmp .L108 .L107: movl $0, %eax .L108:
But I figured you can actually do:
abs(x) < y := (unsigned)(x + y - 1) < (2 * y - 1)
Which, if y is a constant, should result in nicer code, and it does for x86:
addq $31, %rax cmpq $62, %rax setbe %al movzbl %al, %eax
Just not measurably faster, I suppose because of all the dependencies :/
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