[llvm-dev] Allowing virtual registers after register allocation

[Hal Finkel via llvm-dev]

----- Original Message -----
> From: "Derek Schuff via llvm-dev" <llvm-dev at lists.llvm.org>
> To: llvm-dev at lists.llvm.org
> Sent: Wednesday, December 9, 2015 4:31:31 PM
> Subject: [llvm-dev] Allowing virtual registers after register allocation
> 
> 
> Hi all,
> Virtual ISAs such as WebAssembly and NVPTX use infinite virtual
> register sets instead of traditional phsyical registers.
> PrologEpilogInserter is run after register allocation and asserts
> that all virtuals have been allocated but doesn't otherwise depend
> on this if scavenging is not needed. We'd like to use the
> target-independent PEI code for WebAssembly, so we're proposing a
> TargetRegisterInfo hook for targets to indicate that they use
> virtual registers in this way (currently called
> usesVirtualRegstersAfterRegAlloc(), other suggestions welcome). The
> code is at http://reviews.llvm.org/D15394 and an example of the
> intended use for WebAssembly is at http://reviews.llvm.org/D15344 .
> 

I think this makes sense, and generally speaking, I think it will be good for us to have better support for VM targets without fixed-sized register sets.

Bikeshedding: usesVirtualRegstersAfterRegAlloc() - Are you actually "allocating" virtual registers, or just using the ones that the infrastructure already provides? Is the answer the same for the NVPTX backend? Maybe something like: targetLacksPhysicalRegissters() would be better?

> 
> The actual change to PrologEpilogInserter itself is quite minimal,
> but we thought we'd ask a wider audience for feedback since it's a
> target-independent change. For WebAssembly we would implement
> prolog/epilog insertion and FrameIndex elimination but most of the
> rest of the PEI code (dealing with callee-saved registers,
> scavenging) does nothing.
> 
> For other reference the NVPTX backend currently disables the
> PrologEpilogInserter pass but has its own pass which is just a copy
> (inevitably slightly out-of-date) of PEI with the irrelevant bits
> just deleted; it could probably be updated to use this mechanism
> too.

That sounds good improvement.

 -Hal

> 
> 
> Any comments?
> Thanks,
> -Derek
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory