[llvm-dev] Allowing virtual registers after register allocation

Derek Schuff via llvm-dev llvm-dev at lists.llvm.org
Wed Dec 9 16:04:47 PST 2015

On Wed, Dec 9, 2015 at 3:02 PM Hal Finkel <hfinkel at anl.gov> wrote:

> ----- 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?

Not exactly; the actual register allocation does nothing (i.e.
WebAssemblyPassConfig::createTargetRegisterAllocator() returns nullptr) and
we just use the regular infrastructure virtual registers. However we do run
a custom register coloring pass which reduces the total number of virtual
registers used.

> Is the answer the same for the NVPTX backend?

Yes (at least, they have a null TargetRegisterAllocator too).

> Maybe something like: targetLacksPhysicalRegissters() would be better?

Maybe. We actually do have "physical" registers called SP and FP (returned
by TargetRegisterInfo::getFrameRegister() and used by some default ISel
lowerings and by FrameIndex elimination) but of course they aren't really
physical registers either.
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