[llvm-dev] Canonical way to handle zero registers?

[Sean Silva via llvm-dev]

I looked around the codebase and didn't see anything that obviously looked
like the natural place to turn constant zero immediates into zero-registers
(i.e. registers that always return zero when read). Right now we are
expanding them in ISelLowering::LowerOperation but that seems too early.

The specific issue I'm hitting is that we have a register that reads as -1
and so when we replace -1 too early with this register, the standard "not"
pattern (xor x, -1) will fail to match to "not".

Thanks,
Sean Silva
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