Re: GSOC 2018 - Textual LTO dump tool project

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Mar 6, 2018 at 2:30 PM, Hrishikesh Kulkarni
<hrishikeshparag@gmail.com> wrote:
> Hi,
>
> Thank you Richard and Honza for the suggestions. If I understand correctly,
> the issue is that LTO file format keeps changing per compiler versions, so
> we need a more “stable” representation and the first step for that would be
> to “stabilize” representations for lto-cgraph and symbol table ?

Yes.  Note the issue is that the current format is a 1:1 representation of
the internal representation -- which means it is the internal representation
that changes frequently across releases.  I'm not sure how Honza wants
to deal with those changes in the context of a "stable" IL format.  Given
we haven't been able to provide a stable API to plugins I think it's much
harder to provide a stable streaming format for all the IL details....

> Could you
> please elaborate on what initial steps need to be taken in this regard, and
> if it’s feasible within GSoC timeframe ?

I don't think it is feasible in the GSoC timeframe (nor do I think it's feasible
at all ...)

> Thanks!
>
>
> I am trying to break down the project into milestones for the proposal. So
> far, I have identified the following objectives:
>
> 1] Creating a separate driver, that can read LTO object files. Following
> Richard’s estimate, I’d leave around first half of the period for this task.
>
> Would that be OK ?

Yes.

> Coming to 2nd half:
>
> 2] Dumping pass summaries.
>
> 3] Stabilizing lto-cgraph and symbol table.

So I'd instead do

 3] Enhance the user-interface of the driver

like providing a way to list all function bodies, a way to dump
the IL of a single function body, a way to create a dot graph file
for the cgraph in the file, etc.

Basically while there's a lot of dumping infrastructure in GCC
it may not always fit the needs of a LTO IL dumping tool 1:1
and may need refactoring enhancement.

Richard.

>
> Thanks,
>
> Hrishikesh
>
>
>
> On Fri, Mar 2, 2018 at 6:31 PM, Jan Hubicka <hubicka@ucw.cz> wrote:
>>
>> Hello,
>> > On Fri, Mar 2, 2018 at 10:24 AM, Hrishikesh Kulkarni
>> > <hrishikeshparag@gmail.com> wrote:
>> > > Hello everyone,
>> > >
>> > >
>> > > Thanks for your suggestions and engaging response.
>> > >
>> > > Based on the feedback I think that the scope of this project comprises
>> > > of
>> > > following three indicative actions:
>> > >
>> > >
>> > > 1. Creating separate driver i.e. separate dump tool that uses lto
>> > > object API
>> > > for reading the lto file.
>> >
>> > Yes.  I expect this will take the whole first half of the project,
>> > after this you
>> > should be somewhat familiar with the infrastructure as well.  With the
>> > existing dumping infrastructure it should be possible to dump the
>> > callgraph and individual function bodies.
>> >
>> > >
>> > > 2. Extending LTO dump infrastructure:
>> > >
>> > > GCC already seems to have dump infrastructure for pretty-printing tree
>> > > nodes, gimple statements etc. However I suppose we’d need to extend
>> > > that for
>> > > dumping pass summaries ? For instance, should we add a new hook say
>> > > “dump”
>> > > to ipa_opt_pass_d that’d dump the pass
>> > > summary ?
>> >
>> > That sounds like a good idea indeed.  I'm not sure if this is the most
>> > interesting
>> > missing part - I guess we'll find out once a dump tool is available.
>>
>> Concering the LTO file format my longer term aim is to make the symbol
>> table sections (symtab used by lto-plugin as well as the callgraph
>> section)
>> and hopefully also the Gimple streams) documented and well behaving
>> without changing the format in every revision.
>>
>> On the other hand the summaries used by individual passes are intended to
>> be
>> pass specific and envolving as individula passes become stronger/new
>> passes
>> are added.
>>
>> It is quite a lot of work to stabilize gimple representation to this
>> extend,
>> For callgraph&symbol table this is however more realistic. That would mean
>> to
>> move some of existing random stuff streamed there into summaries and
>> additionaly
>> cleaning up/rewriting lto-cgraph so the on disk format actually makes
>> sense.
>>
>> I will be happy to help with any steps in this direction as well.
>>
>> Honza
>
>