[PATCH] AArch64: Fold immediate into the immediate field of logical instructions

[Tim Northover]

Hi Akira,

> The reason I only do sign-extension is that it seemed to be the cheapest way to get the most gain without hurting compile time or making the code overly complicated. I looked at the instructions llvm emits, and I saw many places where logical instructions were followed by truncating stores.

I think the kind of store being done is largely orthogonal to the
calculation most likely to get you a valid immediate. It tells you
what bits can be ignored, but nothing about the best way to fill them.

So I think it's reasonable to start with just assuming that
DemandedBits has some number of low bits set and the rest ignored
based on your observations. I think it's harder to justify coming up
with a single NewImm value by sign extending the existing immediate
and giving up if that fails.

Cheers.

Tim.