[llvm-dev] IPRA, interprocedural register allocation, question

Lawrence, Peter via llvm-dev llvm-dev at lists.llvm.org
Tue Jul 12 12:55:04 PDT 2016

            In my mind at least, “whole program” means no dynamic libraries, so the only
external functions are simple runtime support, do you have a suggested term for that ?


From: mehdi.amini at apple.com [mailto:mehdi.amini at apple.com]
Sent: Tuesday, July 12, 2016 12:31 PM
To: Lawrence, Peter <c_plawre at qca.qualcomm.com>
Cc: vivek pandya <vivekvpandya at gmail.com>; llvm-dev <llvm-dev at lists.llvm.org>; llvm-dev-request at lists.llvm.org; Hal Finkel <hfinkel at anl.gov>
Subject: Re: [llvm-dev] IPRA, interprocedural register allocation, question

On Jul 12, 2016, at 12:20 PM, Lawrence, Peter <c_plawre at qca.qualcomm.com<mailto:c_plawre at qca.qualcomm.com>> wrote:

             I am looking for an understanding of   1) IPRA in general,   2) IPRA in LLVM.
Whether I want to use LTO or not is a separate issue.

1)  I currently believe it is a true statement that:
                If all external functions are known to not call back into the “whole-program”
                Being compiled, then IPRA is free to do anything at all to the functions being
                Compiled, not limited to only “upgrades” calling convention changes, but
                Also allowing “downgrades” calling convention changes as well.

Do you think my current belief #1 is correct ?

Yes, with some extra assumptions (you don’t use dlsym for instance, and you won’t link to another file with a global initializer that can call any of these).

I expressed this earlier (which include the other issues I mentioned just before) as “we can turn the linkage of every function into local” (or private, or static, whatever denomination you prefer).

2) it seems that LLVM currently limits itself to “upgrades” calling convention changes,
The reason being so that not all call sites are required to be changed,
therefore calls through function pointers can use the default calling convention
If for example there is insufficient analysis to know for sure what functions can be
called from that site.

Is my understanding #2 of IPRA in LLVM correct ?

I don’t believe this is correct, currently IPRA will limit itself to this for function that can be called from another module.
I will freely change the calling convention, including downgrades, when it knows that it can see all call sites (+ extra conditions, like no recursion being involved I think).

“whole-program” here is a misnomer since there are external functions, but I don’t
Have a better term for this.

I believe you can talk about “main module”, i.e. the module defines the entry point for the program.
Note LLVM can’t make assumption about the lack of dlsym() or global initializer in other module for example, so the linkage type of functions is what tells us about the possibility to call back or not.


“upgrades” means some scratch regs are converted to save
(the callee either doesn’t touch them at all, or does do save/restore)
“downgrades” means some save regs are converted to scratch
                (the callee no longer does save/restore to some registers, and does clobber them)

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