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

[Matthias Braun via llvm-dev]

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> 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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