[llvm-dev] New to LLVM. Need help getting available register

Josh Sharp via llvm-dev llvm-dev at lists.llvm.org
Sat Feb 16 18:38:53 PST 2019

Is it possible to get a virtual register and then use that to create a real register? I've seen it done in unittests/CodeGen/MachineInstrTest.cpp like this:

unsigned VirtualDef1 = -42;
                    MachineOperand::CreateReg(VirtualDef1, /*isDef*/ true));

But when I do that in my code I get an assertion so I wasn't sure if it's legal or not.

From: David Greene <dag at cray.com>
Sent: Wednesday, January 2, 2019 9:23 AM
To: m m
Cc: via llvm-dev
Subject: Re: [llvm-dev] New to LLVM. Need help getting available register

m m via llvm-dev <llvm-dev at lists.llvm.org> writes:

> I'm new to LLVM. I'd like to know if there is a method I can call
> whenever I need any available CPU register.

Take a look at RegisterScavenging.  Of course, this only works after
register allocation.  Before register allocation you'd just create a new
virtual register.

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