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

[Prathamesh Kulkarni <prathamesh dot kulkarni at linaro dot org>]

On 6 July 2016 at 22:25, Andrew Pinski <pinskia@gmail.com> wrote:
> 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, I updated the patch to remove -flto-var-partition
and gated the partition currently on target_supports_section_anchors_p()
(although it doesn't test if -fsection-anchors is passed).
Um I am not able to see where mips has section anchors enabled ?
mips.c does not seem to override min_anchor_offset and max_anchor_offset
hooks. Both these hooks have values 0 for default, and
target_supports_section_anchors_p()
returns false if both these hooks have value 0.

Thanks,
Prathamesh
>
> 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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