Re: [PATCH GCC8][29/33]New register pressure estimation

["Bin.Cheng" <amker.cheng@gmail.com>]

On Thu, May 18, 2017 at 11:41 AM, Richard Sandiford
<richard.sandiford@linaro.org> wrote:
> "Bin.Cheng" <amker.cheng@gmail.com> writes:
>> On Wed, May 17, 2017 at 1:37 PM, Richard Sandiford
>> <richard.sandiford@linaro.org> wrote:
>>> "Bin.Cheng" <amker.cheng@gmail.com> writes:
>>>> -/* Calculates cost for having N_REGS registers.  This number includes
>>>> -   induction variables, invariant variables and invariant expressions.  */
>>>> +/* Estimate register pressure for loop having N_INVS invariants and N_CANDS
>>>> +   induction variables.  Note N_INVS includes both invariant variables and
>>>> +   invariant expressions.  */
>>>>
>>>>  static unsigned
>>>> -ivopts_global_cost_for_size (struct ivopts_data *data, unsigned n_regs)
>>>> +ivopts_estimate_reg_pressure (struct ivopts_data *data, unsigned n_invs,
>>>> +                           unsigned n_cands)
>>>>  {
>>>> -  unsigned cost = estimate_reg_pressure_cost (n_regs,
>>>> -                                           data->regs_used, data->speed,
>>>> -                                           data->body_includes_call);
>>>> - /* Add n_regs to the cost, so that we prefer eliminating ivs if
>>>> possible.  */
>>>> -  return n_regs + cost;
>>>> +  unsigned cost;
>>>> +  unsigned n_old = data->regs_used, n_new = n_invs + n_cands;
>>>> +  unsigned regs_needed = n_new + n_old, available_regs = target_avail_regs;
>>>> +  bool speed = data->speed;
>>>> +
>>>> +  /* If there is a call in the loop body, the call-clobbered registers
>>>> +     are not available for loop invariants.  */
>>>> +  if (data->body_includes_call)
>>>> +    available_regs = available_regs - target_clobbered_regs;
>>>> +
>>>> +  /* If we have enough registers.  */
>>>> +  if (regs_needed + target_res_regs < available_regs)
>>>> +    cost = n_new;
>>>> +  /* If close to running out of registers, try to preserve them.  */
>>>> +  else if (regs_needed <= available_regs)
>>>> +    cost = target_reg_cost [speed] * regs_needed;
>>>> +  /* If we run out of available registers but the number of candidates
>>>> +     does not, we penalize extra registers using target_spill_cost.  */
>>>> +  else if (n_cands <= available_regs)
>>>> +    cost = target_reg_cost [speed] * available_regs
>>>> +        + target_spill_cost [speed] * (regs_needed - available_regs);
>>>> +  /* If the number of candidates runs out available registers, we penalize
>>>> +     extra candidate registers using target_spill_cost * 2.  Because it is
>>>> +     more expensive to spill induction variable than invariant.  */
>>>> +  else
>>>> +    cost = target_reg_cost [speed] * available_regs
>>>> +        + target_spill_cost [speed] * (n_cands - available_regs) * 2
>>>> +        + target_spill_cost [speed] * (regs_needed - n_cands);
>>>> +
>>>> +  /* Finally, add the number of candidates, so that we prefer eliminating
>>>> +     induction variables if possible.  */
>>>> +  return cost + n_cands;
>>>
>>> It looks like the return is mixing units.  Would it work to return
>>> a <cost, n_cands> pair instead, and use lexicographical ordering?
>> Hi Richard,
>> It just penalizes the cost by the number of candidates, rather than
>> returns n_cands to caller.  Actually that information is available all
>> the time in ivopts_data structure.
>
> Yeah, but what I meant was: "cost" seems to be measured in abstract
> instruction units, while "n_cands" counts a number of variables.
> It doesn't seem to make sense to add them together.
Yes, unfortunately GCC adds up different costs and compares it together.
>
> If the idea is to use n_cands as a tie-breaker when the instruction
We don't have such cost model which different costs could be
applied/compared separately, also we don't have obvious break
conditions.  Sometimes the "instruction cost" is lower but results in
worse code because register cost is higher; but sometimes the opposite
holds.  LLVM has such separated cost models I guess?

Thanks,
bin
> costs are the same, then lexicographical ordering of pairs would give
> that without mixing the units.
>
> Thanks,
> Richard