On Wed, Apr 25, 2012 at 6:41 PM, Richard Guenther
<richard.guent...@gmail.com> wrote:
> On Wed, Apr 25, 2012 at 4:32 PM, Igor Zamyatin <izamya...@gmail.com> wrote:
>> On Wed, Apr 25, 2012 at 1:14 PM, Richard Guenther
>> <richard.guent...@gmail.com> wrote:
>>> On Wed, Apr 25, 2012 at 10:56 AM, Igor Zamyatin <izamya...@gmail.com> wrote:
>>>> Hi all!
>>>>
>>>> I'd like to post for review the patch which makes some costs adjusting
>>>> in get_computation_cost_at routine in ivopts part.
>>>> As mentioned in the PR changes also fix the bwaves regression from PR
>>>> 52272.
>>>> Also changes introduce no degradations on spec2000/2006 and
>>>> EEMBC1.1/2.0(this was measured on Atom) on x86
>>>>
>>>>
>>>> Bootstrapped and regtested on x86. Ok to commit?
>>>
>>> I can't make sense of the patch and the comment does not help.
>>>
>>> + diff_cost = cost.cost;
>>> cost.cost /= avg_loop_niter (data->current_loop);
>>> + add_cost_val = add_cost (TYPE_MODE (ctype), data->speed);
>>> + /* Do cost correction if address cost is small enough
>>> + and difference cost is high enough. */
>>> + if (address_p && diff_cost > add_cost_val
>>> + && get_address_cost (symbol_present, var_present,
>>> + offset, ratio, cstepi,
>>> + TYPE_MODE (TREE_TYPE (utype)),
>>> + TYPE_ADDR_SPACE (TREE_TYPE (utype)),
>>> + speed, stmt_is_after_inc,
>>> + can_autoinc).cost <= add_cost_val)
>>> + cost.cost += add_cost_val;
>>>
>>> Please explain more thoroughly. It also would seem to be better to add
>>> an extra case, as later code does
>>
>> For example for such code
>>
>> for (j=0; j<M;j++) {
>> for (i=0; i<N; i++)
>> sum += ptr->a[j][i] * ptr->c[k][i];
>> }
>> we currently have following gimple on x86 target (I provided a piece
>> of all phase output):
>>
>> # ivtmp.13_30 = PHI <ivtmp.13_31(3), ivtmp.13_33(7)>
>> D.1748_34 = (void *) ivtmp.13_30;
>> D.1722_7 = MEM[base: D.1748_34, offset: 0B];
>> D.1750_36 = ivtmp.27_28;
>> D.1751_37 = D.1750_36 + ivtmp.13_30; <-- we got
>> non-invariant add which is not taken into account currently in cost
>> model
>> D.1752_38 = (void *) D.1751_37;
>> D.1753_39 = (sizetype) k_8(D);
>> D.1754_40 = D.1753_39 * 800;
>> D.1723_9 = MEM[base: D.1752_38, index: D.1754_40, offset: 16000B];
>> ...
>>
>> With proposed fix we produce:
>>
>> # ivtmp.14_30 = PHI <ivtmp.14_31(3), 0(7)>
>> D.1749_34 = (struct S *) ivtmp.25_28;
>> D.1722_7 = MEM[base: D.1749_34, index: ivtmp.14_30, offset: 0B];
>> D.1750_35 = (sizetype) k_8(D);
>> D.1751_36 = D.1750_35 * 800;
>> D.1752_37 = ptr_6(D) + D.1751_36;
>> D.1723_9 = MEM[base: D.1752_37, index: ivtmp.14_30, offset:
>> 16000B];
>>
>> which is more effective on platforms where address cost is cheaper
>> than cost of addition operation. That's basically what this adjustment
>> is for.
>
> If we generally miss to account for the add then why is the adjustment
> conditional on diff_cost > add_cost and address_cost <= add_cost?
>
> Is this a new heuristic or a fix for not accurately computing the cost for the
> stmts we generate?
I'd say this is closer to heuristic since diff_cost > add_cost is an
attempt to catch the case with non-invariant add produced by pointer
difference and address_cost <=add_cost leaves the cases with cheap
address operations
>
> Richard.
>
>> So comment in the source code now looks as follows
>>
>> /* Do cost correction when address difference produces
>> additional non-invariant add operation which is less
>> profitable if address cost is cheaper than cost of add. */
>>
>>>
>>> /* Now the computation is in shape symbol + var1 + const + ratio * var2.
>>> (symbol/var1/const parts may be omitted). If we are looking for an
>>> address, find the cost of addressing this. */
>>> if (address_p)
>>> return add_costs (cost,
>>> get_address_cost (symbol_present, var_present,
>>> offset, ratio, cstepi,
>>> TYPE_MODE (TREE_TYPE (utype)),
>>> TYPE_ADDR_SPACE (TREE_TYPE (utype)),
>>> speed, stmt_is_after_inc,
>>> can_autoinc));
>>>
>>> thus refactoring the code a bit would make it possible to CSE the
>>> get_address_cost
>>> call and eventually make it clearer what the code does.
>>
>> 'offset' could be changed beetween two calls of get_address_cost so
>> such refactoring looks useless.
>>
>> New patch (only the comment was changed) attached. Changelog was
>> changed as well.
>>
>>>
>>> Richard.
>>>
>>
>> Changelog:
>>
>> 2012-04-26 Yuri Rumyantsev <yuri.rumyant...@intel.com>
>>
>> * tree-ssa-loop-ivopts.c (get_computation_cost_at): Adjust
>> cost model when address difference produces additional
>> non-invariant add operation which is less profitable if
>> address cost is cheaper than cost of add.
>>
>> Thanks,
>> Igor
