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Re: [RFC] lto partitioning of varpool_nodes for section anchors


On Wed, Jul 6, 2016 at 5:00 AM, Prathamesh Kulkarni
<prathamesh.kulkarni@linaro.org> wrote:
> On 4 July 2016 at 13:51, Andrew Pinski <pinskia@gmail.com> wrote:
>> On Mon, Jul 4, 2016 at 12:58 AM, Prathamesh Kulkarni
>> <prathamesh.kulkarni@linaro.org> wrote:
>>> Hi,
>>> I have attached a "quick and dirty" prototype patch (var-partition-1.diff),
>>> that attempts to partition variables to reduce number of
>>> external references and to increase usage of section-anchors
>>> to CSE address computation of global variables.
>>>
>>> We could put a variable in a partition that has max references for it,
>>> however it doesn't lend itself directly to section anchor optimization.
>>> For instance if a partition has max references for variables 'a' and 'b',
>>> but no function in that partition references both 'a', and 'b' then AFAIU
>>> it doesn't make any difference from section anchors perspective to have them
>>> in same partition.
>>>
>>> The patch tries to assign a set of variables (>= 2)
>>> to a partition whose functions have maximum references for that set.
>>> Functions within the partition that reference the variables
>>> in the set can take advantage of section-anchors. Functions
>>> referencing the variables in the set outside the partition
>>> would need to load them as external references (using movw/movt),
>>> however since we are placing the set in partition that has maximal
>>> references for it, number of external references should be overall
>>> reduced.
>>>
>>> Partitioning is gated by -flto-var-partition and enabled
>>> only for arm and aarch64.
>>
>> Why only for arm and aarch64?  Shouldn't it be enabled for all section
>> anchor targets?
> AFAIK the only targets supporting section anchors are arm, aarch64 and powerpc.
> I didn't enable it for ppc64 because I am not sure how much profitable
> it is for that target.
> Honza mentioned to me some time back that effect of partitioning on
> powerpc was nearly zero.


No MIPS has section anchors enabled too.  Plus MIPS will benefit the
same way as AARCH64 and ARM.  PowerPC32 would too.

I don't think it is correct to enable it only for arm and aarch64.

Thanks,
Andrew Pinski

>
> Thanks,
> Prathamesh
>>
>> Thanks,
>> Andrew
>>
>>> As per previous discussion [1], I haven't
>>> touched function partitioning. Does this approach look ok
>>> especially regarding correctness ?
>>> So far, I have cross-tested patch on arm*-*-*, aarch64*-*-*.
>>>
>>> I haven't yet managed to benchmark the patch.
>>> As a cheap measurement, I tried to measure number of external
>>> references with and without patch by writing a small ipa pass
>>> which is run during ltrans and simply walks over varpool nodes
>>> and counts number of varpool_nodes for which DECL_EXTERNAL (vnode->decl) is true
>>> and vnode->definition is 0. Is that sufficient condition to determine
>>> if variable is externally defined ? I have attached the pass
>>> (count-external-refs.diff)
>>> and the comparison done with it for for SPEC2000 [2]. The entries
>>> in "before" and "after" column contain summation of number of
>>> external refs (total_count) across all partitions before and after applying
>>> the patch. Does the comparison hold any merit ?
>>> I was wondering if we could we use a better way for
>>> measuring statically the effects of variable partitioning ?
>>> I hope also to get done with benchmarking soon.
>>>
>>> I have not yet figured out how to integrate it with existing cost metrics for
>>> balanced partitioning, I am looking into that.
>>> I would be grateful for suggestions on the patch.
>>>
>>> [1] https://gcc.gnu.org/ml/gcc/2016-04/msg00090.html
>>>
>>> [2] SPEC2000 comparison:
>>> https://docs.google.com/spreadsheets/d/1xnszyw04ksoyBspmCVYesq6KARLw-PA2n3T4aoaKdYw/edit?usp=sharing


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