llvm dev - Jul 2016 - Can we get interprocedural register allocation work across module boundary?

Hello Mentors,

I have a very naive idea to get IPRA woking at link time and thus extending
its scope to intermodule. I seek some help if it seems to be a feasible
idea.

So idea is to take advantage of LTO's capability to work with bit code
files. LTO can optimize number of bitcode files and combine them into a big
module. LTO then generates native code for the big module.

So does it seem feasible to have clang generates bit code files for the
source code and then combine then with LTO and LTOCodeGenerater will use
current IPRA infrastructure, so IPRA will be improved as now it can
propagate actual regmask for procedures define in other modules (not
system/external library calls).  If this is possible then one problem I can
sense is requirement of huge memory to hold a big module for large
projects.

If this idea seems feasible please help me to shape a concrete work plan on
this.

Sincerely,
Vivek
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> On Jul 5, 2016, at 9:28 AM, vivek pandya <vivekvpandya at gmail.com>
wrote:
> 
> Hello Mentors,
> 
> I have a very naive idea to get IPRA woking at link time and thus extending
its scope to intermodule. I seek some help if it seems to be a feasible idea.
> 
> So idea is to take advantage of LTO's capability to work with bit code
files. LTO can optimize number of bitcode files and combine them into a big
module. LTO then generates native code for the big module.
Yes, that’s how LTO works.
> 
> So does it seem feasible to have clang generates bit code files for the
source code and then combine then with LTO and LTOCodeGenerater will use current
IPRA infrastructure, so IPRA will be improved as now it can propagate actual
regmask for procedures define in other modules (not system/external library
calls).  If this is possible then one problem I can sense is requirement of huge
memory to hold a big module for large projects.
I don’t understand what you describe, or how it changes from LTO how it works
right now.

IPRA, how we implemented it, is a simple “codegen option” that can be enabled
during LTO.

I think at some point at the beginning of the project, you mentioned some paper
about IPRA during link-time and we told you that it was not relevant for LLVM
because of the way LTO works: IPRA can be enabled transparently during LTO.

— 
Mehdi

> 
> If this idea seems feasible please help me to shape a concrete work plan on
this.
> 
> Sincerely,
> Vivek

On Tue, Jul 5, 2016 at 11:54 PM, Mehdi Amini <mehdi.amini at apple.com>
wrote:
>
> > On Jul 5, 2016, at 9:28 AM, vivek pandya <vivekvpandya at
gmail.com> wrote:
> >
> > Hello Mentors,
> >
> > I have a very naive idea to get IPRA woking at link time and thus
> extending its scope to intermodule. I seek some help if it seems to be a
> feasible idea.
> >
> > So idea is to take advantage of LTO's capability to work with bit
code
> files. LTO can optimize number of bitcode files and combine them into a big
> module. LTO then generates native code for the big module.
>
> Yes, that’s how LTO works.
>
> >
> > So does it seem feasible to have clang generates bit code files for
the
> source code and then combine then with LTO and LTOCodeGenerater will use
> current IPRA infrastructure, so IPRA will be improved as now it can
> propagate actual regmask for procedures define in other modules (not
> system/external library calls).  If this is possible then one problem I can
> sense is requirement of huge memory to hold a big module for large
projects.
>
> I don’t understand what you describe, or how it changes from LTO how it
> works right now.
>
> IPRA, how we implemented it, is a simple “codegen option” that can be
> enabled during LTO.
>
Ok I got your point. So using IPRA during LTO is a way to get most benefit
out of IPRA but h\this will incur some overheads due to LTO. I think it
would be better to try this experiment with a small application to provide
a good example of this.
-Vivek
> I think at some point at the beginning of the project, you mentioned some
> paper about IPRA during link-time and we told you that it was not relevant
> for LLVM because of the way LTO works: IPRA can be enabled transparently
> during LTO.
>
> —
> Mehdi
>
>
> >
> > If this idea seems feasible please help me to shape a concrete work
plan
> on this.
> >
> > Sincerely,
> > Vivek
>
>
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