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 :/
