[llvm-dev] Checking when Register Allocation has been performed

Tue Jan 16 23:31:59 PST 2018

Thanks Matthias,

I have both a pre-RA and a post-RA scheduler, and I had thought that I could track “has RA happened?” by setting a flag in my pre-RA scheduler as it completes - my suspicion (which you have confirmed) was that “#vregs == 0” was not a safe assumption.  What I cannot be sure of, is what passes execute after my pre-RA scheduler but before RA, and what passes execute after RA but before my post-RA scheduler since this is largely target-independent.  If any of those passes trigger a custom lowering action (frame setup for example), and it requires additional scratch registers, then my code generation strategy needs to change.  As it happens, I know that frame lowering always occurs post-RA so that is not a real example, but is it possible that another pass between pre-RA and post-RA scheduling could trigger lowering actions?  If not, then setting a flag in my pre-RA scheduler will solve the problem perfectly.

All the best,

            MartinO

From: mbraun at apple.com [mailto:mbraun at apple.com] 
Sent: 16 January 2018 17:31
To: Craig Topper <craig.topper at gmail.com>
Cc: Martin J. O'Riordan <MartinO at theheart.ie>; LLVM Developers <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Checking when Register Allocation has been performed

Please don't rely on this for checking whether regalloc was run: You can have functions without vregs pre-RA[1].

We don't need or should track state such as pre/post-RA as part of the function. Instead it really is a property of where a pass was scheduled, so the pass should know and not the function.

I'd recommend simply creating a pre-RA and a post-RA pass instead of scheduling the same pass twice. (Of course you can share most of the pass implementation if they turn out to be similar).

- Matthias

[1]: While currently we do not set the NoVRrgs flag pre-ra even if there are no vregs used, when loading a .mir file for example the flag is computed from scratch and will be set.

On Jan 15, 2018, at 12:26 PM, Craig Topper via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > wrote:

Maybe  MF.getProperties().hasProperty(MachineFunctionProperties::Property::NoVRegs))?

~Craig

On Mon, Jan 15, 2018 at 12:07 PM, Martin J. O'Riordan via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > wrote:

Hi LLVM Devs,

I have some shared code that performs lowering operations that can occur before or after register allocation.  When it is pre-RA I want to only use virtual registers for intermediate results, but post-RA I have to use only a very restricted set of physical registers.

Code generation using the restricted set is not as efficient as it is when I can use virtual registers.  At the moment I have a clunky implementation for checking whether or not the register allocator has been run, and I am wondering if there is a “correct way” of checking whether the RA pass has been run?  Some part of the register info API that I have missed perhaps?

Thanks,

            MartinO

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