Messages in this thread | | | Date | Sat, 9 Jun 2018 10:13:57 +0200 | From | Javi Merino <> | Subject | Re: [RFC PATCH v3 03/10] PM: Introduce an Energy Model management framework |
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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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