[llvm-dev] Problem of getting two unused registers in eliminateFrameIndex()

[Krzysztof Parzyszek via llvm-dev]

There isn't really much else that you can do here. If you anticipate 
that two extra registers will be needed at some particular point in the 
function, reserve two spill slots.

Creating virtual registers in replaceFrameIndex should work, but make 
sure that your target implements "requiresRegisterScavenging", 
"requiresFrameIndexScavenging", 
"requiresFrameIndexReplacementScavenging" (all from TargetRegisterInfo) 
as appropriate.

-Krzysztof

On 8/14/2017 8:39 PM, Ehsan Ali via llvm-dev wrote:
> Hello all,
> 
> For my custom processor backend I am trying add some instruction using
> BuildMI() inside eliminateFrameIndex().
> 
> I tried RegScavenger like this:
> 
> unsigned RegUnused0 = RS->FindUnusedReg(&LASER::GNPRegsRegClass);
> if (!RegUnused0)
>    RegUnused0 = RS->scavengeRegister(&LASER::GNPRegsRegClass, II, SPAdj);
> assert(RegUnused0 && "Register scavenger failed");
> RS->setRegUsed(RegUnused0);
> 
> It works but there are two issues:
> 
> 1) I need to registers and RegScavenger only returns one.
> 2) I cannot unset the used register and I get spill slot error message
> when I ran out of the available registers.
> 
> So I gave up and tried createVirtualRegister():
> 
> MachineRegisterInfo &RegInfo = MBB.getParent()->getRegInfo();
> const TargetRegisterClass *RC = &LASER::GNPRegsRegClass;
> unsigned Reg = RegInfo.createVirtualRegister(RC);
> 
> But then I get the following error:
> 
> MachineCopyPropagation.cpp:267: void
> anonymous}::MachineCopyPropagation::CopyPropagateBlock(llvm::MachineBasicBlock&):
> Assertion `!TargetRegisterInfo::isVirtualRegister(Reg) &&
> "MachineCopyPropagation should be run after register allocation!"'
> failed.
> 
> How can I solve my problem of getting two free registers in
> eliminateFrameIndex() ?
> 
> Thank you,
> Ehsan Ali.
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> llvm-dev at lists.llvm.org
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> 

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