llvm dev - Jan 2018 - Checking when Register Allocation has been performed

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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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> 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
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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Thanks Craig,

 

Is asking if the MF has the NoVRegs property okay?  My clunky hack does
something similar I must admit, currently I use ‘bool isPostRA =
(MF.getRegInfo().getNumVirtRegs() == 0);’ and your suggestion is semantically
very similar, though with a more official stamp :)

 

All the best,

 

            MartinO

 

From: Craig Topper [mailto:craig.topper at gmail.com] 
Sent: 15 January 2018 20:26
To: Martin J. O'Riordan <MartinO at theheart.ie>
Cc: LLVM Developers <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Checking when Register Allocation has been performed

 

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

 


_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> 
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

 

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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
> 
>  
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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