On Wed, Dec 20, 2017 at 09:57:47AM +0100, Peter Zijlstra wrote:
> 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 :/
Ah no, it actually is, I'm an idiot and used 'long' for return value. If
I use bool we loose that last movzbl and we go from around 4.0 cycles
down to 3.4 cycles.
