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SubjectRe: [RFC PATCH v3 03/10] PM: Introduce an Energy Model management framework
On Thu, Jun 07, 2018 at 06:04:19PM +0200, Juri Lelli wrote:
> On 07/06/18 16:19, Quentin Perret wrote:
> > Hi Juri,
> >
> > On Thursday 07 Jun 2018 at 16:44:09 (+0200), Juri Lelli wrote:
> > > On 21/05/18 15:24, Quentin Perret wrote:
>
> [...]
>
> > > > +static void fd_update_cs_table(struct em_cs_table *cs_table, int cpu)
> > > > +{
> > > > + unsigned long cmax = arch_scale_cpu_capacity(NULL, cpu);
> > > > + int max_cap_state = cs_table->nr_cap_states - 1;
> > > ^
> > > You don't need this on the stack, right?
> >
> > Oh, why not ?
> >
>
> Because you use it only once here below? Anyway, more a (debatable)
> nitpick than anything.

The compiler optimizes that for you because it knows that it is used
only once. It doesn't put it in the stack, it uses a register. As it
is, it's more readable so I'd rather keep it.

For reference, this is the code gcc 7.3 generates for arm64 for
fd_update_cstable() (which is inlined in em_rescale_cpu_capacity():

x27 holds the address to cs_table

1ac: b9400b63 ldr w3, [x27, #8] ; w3 = cs_table->nr_cap_states
1b0: b9406fa4 ldr w4, [x29, #108] ; w4 = 0x18 (sizeof(struct em_cap_state))
1b4: f9400362 ldr x2, [x27] ; x2 = &cs_table[state]
1b8: 51000461 sub w1, w3, #0x1 ; w1 max_cap_state = cs_table->nr_cap_states - 1
[...]
1cc: 9b240821 smaddl x1, w1, w4, x2 ; x1 = &cs_table->state[max_cap_state]
[...]
1d4: f9400427 ldr x7, [x1, #8] ; x7 fmax = cs_table->state[max_cap_state].frequency
[...] ; calculates cmax * cs_table->state[i].frequency in x0
200: 9ac70800 udiv x0, x0, x7 ; x0 = x0 / fmax
; x0 is then stored to cs_table->state[i].capacity

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