Hello Richard:
On 11/07/24 2:21 pm, Richard Biener wrote:
> On Thu, Jul 11, 2024 at 10:30 AM Ajit Agarwal <aagar...@linux.ibm.com> wrote:
>>
>> Hello All:
>>
>> Unroll factor is determined with max distance across loop iterations.
>> The logic for determining the loop unroll factor is based on
>> data dependency across loop iterations.
>>
>> The max distance across loop iterations is the unrolling factor
>> that helps in predictive commoning.
>
> The old comment in the code says
>
>> - /* The best unroll factor for this chain is equal to the number of
>> - temporary variables that we create for it. */
>
> why is that wrong and why is the max dependence distance more correct?
>
> Do you have a testcase that shows how this makes a (positive) difference?
>
There is nothing wrong in the existing implementation of unroll
factor for predictive commoning.
But with max dependence distance we get performance improvement
with spec 2017 benchmarks (INT) of 0.01% (Geomean) with and without
changes. Improvement in benchmarks with max dependence distance
changes.
I have used the following flags:
-O2 -funroll-loops --param max-unroll-times=8 -fpredictive-commoning
-fno-tree-pre
With above flags I ran with and without changes.
There is no degradation with spec 2017 (FP benchmarks).
Because in predictive commoning we reuse values computed in
earlier iterations of a loop in the later ones, max distance is the
better choice.
> Richard.
>
Thanks & Regards
Ajit
>> Bootstrapped and regtested on powerpc64-linux-gnu.
>>
>> Thanks & Regards
>> Ajit
>>
>> tree-optimization, predcom: Improve unroll factor for predictive commoning
>>
>> Unroll factor is determined with max distance across loop iterations.
>> The logic for determining the loop unroll factor is based on
>> data dependency across loop iterations.
>>
>> The max distance across loop iterations is the unrolling factor
>> that helps in predictive commoning.
>>
>> 2024-07-11 Ajit Kumar Agarwal <aagar...@linux.ibm.com>
>>
>> gcc/ChangeLog:
>>
>> * tree-predcom.cc: Change in determining unroll factor with
>> data dependence across loop iterations.
>> ---
>> gcc/tree-predcom.cc | 51 ++++++++++++++++++++++++++++++++++-----------
>> 1 file changed, 39 insertions(+), 12 deletions(-)
>>
>> diff --git a/gcc/tree-predcom.cc b/gcc/tree-predcom.cc
>> index 9844fee1e97..029b02f5990 100644
>> --- a/gcc/tree-predcom.cc
>> +++ b/gcc/tree-predcom.cc
>> @@ -409,6 +409,7 @@ public:
>> /* Perform the predictive commoning optimization for chains, make this
>> public for being called in callback execute_pred_commoning_cbck. */
>> void execute_pred_commoning (bitmap tmp_vars);
>> + unsigned determine_unroll_factor (const vec<chain_p> &chains);
>>
>> private:
>> /* The pointer to the given loop. */
>> @@ -2400,13 +2401,46 @@ pcom_worker::execute_pred_commoning_chain (chain_p
>> chain,
>> copies as possible. CHAINS is the list of chains that will be
>> optimized. */
>>
>> -static unsigned
>> -determine_unroll_factor (const vec<chain_p> &chains)
>> +unsigned
>> +pcom_worker::determine_unroll_factor (const vec<chain_p> &chains)
>> {
>> chain_p chain;
>> - unsigned factor = 1, af, nfactor, i;
>> + unsigned factor = 1, i;
>> unsigned max = param_max_unroll_times;
>> + struct data_dependence_relation *ddr;
>> + unsigned nfactor = 0;
>> + int nzfactor = 0;
>> +
>> + /* Best unroll factor is the maximum distance across loop
>> + iterations. */
>> + FOR_EACH_VEC_ELT (m_dependences, i, ddr)
>> + {
>> + for (unsigned j = 0; j < DDR_NUM_DIST_VECTS (ddr); j++)
>> + {
>> + lambda_vector vec = DDR_DIST_VECT (ddr, j);
>> + widest_int distance = vec[j];
>> + unsigned offset = distance.to_uhwi ();
>> + if (offset == 0)
>> + continue;
>> +
>> + int dist = offset - nzfactor;
>> + if (dist == 0)
>> + continue;
>>
>> + if (nfactor == 0)
>> + {
>> + nfactor = offset;
>> + nzfactor = offset;
>> + }
>> + else if (dist <= nzfactor)
>> + nfactor = offset;
>> +
>> + if (nfactor > 0 && nfactor <= max)
>> + factor = nfactor;
>> + }
>> + }
>> +
>> + int max_use = 0;
>> FOR_EACH_VEC_ELT (chains, i, chain)
>> {
>> if (chain->type == CT_INVARIANT)
>> @@ -2427,17 +2461,10 @@ determine_unroll_factor (const vec<chain_p> &chains)
>> continue;
>> }
>>
>> - /* The best unroll factor for this chain is equal to the number of
>> - temporary variables that we create for it. */
>> - af = chain->length;
>> if (chain->has_max_use_after)
>> - af++;
>> -
>> - nfactor = factor * af / gcd (factor, af);
>> - if (nfactor <= max)
>> - factor = nfactor;
>> + max_use++;
>> }
>> -
>> + factor += max_use;
>> return factor;
>> }
>>
>> --
>> 2.43.5
>>
