[llvm-dev] Can we get interprocedural register allocation work across module boundary?

Mehdi Amini via llvm-dev llvm-dev at lists.llvm.org
Tue Jul 5 11:24:27 PDT 2016

> 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.


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

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