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

[vivek pandya via llvm-dev]

On Wed, Jul 6, 2016 at 8:58 AM, vivek pandya <vivekvpandya at gmail.com> wrote:

>
>
> 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 tried to use IPRA with -flto option like  -flto  -mllvm -enable-ipra
-mllvm -debug-only=ip-regalloc but it seems that while lto is enabled code
generator does not enable IPRA. because I don't get any debug information.
Or Am I missing any options related to LTO here ?

-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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