[llvm-dev] Undef physical registers?

[Krzysztof Parzyszek via llvm-dev]

The "undef" flag on a use means that the use doesn't care about the 
value in that register. In terms of data-flow, it means that that use is 
not associated with any definitions. Liveness information simply ignores 
such uses.

The problem with "undef" uses is that they can be reordered with respect 
to other uses/definitions of that register. If the push instruction was 
also marked as clobbering the register, it could probably work, but it 
could still be optimized away. That could be prevented by marking the 
push instruction (you could create a separate pseudo-instruction for 
this) as having side-effects.

The best way to implement this would be to have a helper routine that 
inserts push/pop instructions only when the register is actually live in 
that range. Whether it's live or not can be checked using LivePhysRegs, 
so it wouldn't require a lot of new code.

-Krzysztof


On 2/13/2018 3:11 AM, Jesper Antonsson via llvm-dev wrote:
> Hi,
> 
> I'm a bit unsure of the semantics of undef physical registers. The explanations I've seen in the code and in the langref seems to
> pertain more to constant values and virtual registers.
> 
> What I really want to achieve is a push-pop of a register to have a temporary to work with, without having to check if this
> register is defined or not. However, whenever the reg is not defined before the push, the MachineVerifier complains about that.
> 
> If I add RegState::Undef to the push, the verifier complaint goes away, but I worry that this might be unsafe. I would like
> "Undef" for physical registers to mean "we don't care if this register has been defined or not", but perhaps it means "this
> register can now have any bits set", so it'd be allowed to optimize the push away?
> 
> So, is the latter interpretation true, and if so, is there some other idiom that would allow me to push-pop a possibly undefined
> physical register?
> 
> Thanks, Jesper
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