Hi Richard,
On 7/27/20 2:36 PM, Richard Biener wrote:
> On Fri, Jul 24, 2020 at 5:43 PM Erick Ochoa
> <[email protected]> wrote:
>>
>> This patchset brings back struct reorg to GCC.
>>
>> We’ve been working on improving cache utilization recently and would
>> like to share our current implementation to receive some feedback on it.
>>
>> Essentially, we’ve implemented the following components:
>>
>> Type-based escape analysis to determine if we can reorganize a type
>> at link-time
>>
>> Dead-field elimination to remove unused fields of a struct at
>> link-time
>>
>> The type-based escape analysis provides a list of types, that are not
>> visible outside of the current linking unit (e.g. parameter types of
>> external functions).
>>
>> The dead-field elimination pass analyses non-escaping structs for fields
>> that are not used in the linking unit and thus can be removed. The
>> resulting struct has a smaller memory footprint, which allows for a
>> higher cache utilization.
>>
>> As a side-effect a couple of new infrastructure code has been written
>> (e.g. a type walker, which we were really missing in GCC), which can be
>> of course reused for other passes as well.
>>
>> We’ve prepared a patchset in the following branch:
>>
>> refs/vendors/ARM/heads/arm-struct-reorg-wip
>
> Just had some time to peek into this. Ugh. The code doesn't look like
> GCC code looks :/ It doesn't help to have one set of files per C++ class
> (25!).
Any suggestions how to best structure these?
Are there some coding guidelines in the GCC project,
which can help us to match the expectation?
> The code itself is undocumented - it's hard to understand what the purpose
> of all the Walker stuff is.
>
> You also didn't seem to know walk_tree () nor walk_gimple* ().
True, we were not aware of that code.
Thanks for pointing to that code.
We will have a look.
> Take as example - I figured to look for IPA pass entries, then I see
>
> +
> +static void
> +collect_types ()
> +{
> + GimpleTypeCollector collector;
> + collector.walk ();
> + collector.print_collected ();
> + ptrset_t types = collector.get_pointer_set ();
> + GimpleCaster caster (types);
> + caster.walk ();
> + if (flag_print_cast_analysis)
> + caster.print_reasons ();
> + ptrset_t casting = caster.get_sets ();
> + fix_escaping_types_in_set (casting);
> + GimpleAccesser accesser;
> + accesser.walk ();
> + if (flag_print_access_analysis)
> + accesser.print_accesses ();
> + record_field_map_t record_field_map = accesser.get_map ();
> + TypeIncompleteEquality equality;
> + bool has_fields_that_can_be_deleted = false;
> + typedef std::set<unsigned> field_offsets_t;
>
> there's no comments (not even file-level) that explains how type escape
> is computed.
>
> Sorry, but this isn't even close to be coarsely reviewable.
Sad to hear.
We'll work on the input that you provided and provide a new version.
Thanks,
Christoph
>
>> We’ve also added a subsection in the GCC internals document to allow
>> other compiler devs to better understand our design and implementation.
>> A generated PDF can be found here:
>>
>> https://cloud.theobroma-systems.com/s/aWwxPiDJ3nCgc7F
>> https://cloud.theobroma-systems.com/s/aWwxPiDJ3nCgc7F/download
>>
>> page: 719
>>
>> We’ve been testing the pass against a range of in-tree tests and
>> real-life applications (e.g. all SPEC CPU2017 C benchmarks). For
>> testing, please see testing subsection in the gcc internals we prepared.
>>
>> Currently we see the following limitations:
>>
>> * It is not a "true" ipa pass yes. That is, we can only succeed with
>> -flto-partition=none.
>> * Currently it is not safe to use -fipa-sra.
>> * Brace constructors not supported now. We handle this gracefully.
>> * Only C as of now.
>> * Results of sizeof() and offsetof() are generated in the compiler
>> frontend and thus can’t be changed later at link time. There are a
>> couple of ideas to resolve this, but that’s currently unimplemented.
>> * At this point we’d like to thank the GCC community for their patient
>> help so far on the mailing list and in other channels. And we ask for
>> your support in terms of feedback, comments and testing.
>>
>> Thanks!
