RyuJIT: Implement induction variable analysis

[BruceForstall]

This analysis would be used for IV widening (#7312) as well as to help implement better arm64 post-increment addressing modes.

Strength reduction: Re-express "secondary" IVs involving multiplication into addition. Pre-requisite for general post-increment. May be an alternative to IV widening if secondary IVs appear as addresses.

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

Issue Details

---

This analysis would be used for IV widening (#7312) as well as to help implement better arm64 post-increment addressing modes.

Author:	BruceForstall
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	9.0.0

[Sergio0694]

Will this allow the JIT to automatically achieve the same codegen for loops over eg. spans or arrays, that you can do today if you manually write them as a while (Unsafe.IsAddressLessThan(...)) and then increment a moving ref, rather than indexing every single iteration? Eg. we adopted this pattern everywhere in ImageSharp and while it did give us a noticeable ~10% perf boost overall (which admittedly is fairly big), but the code ended up being basically unreadable and very error prone and quite not safe 😅

[BruceForstall]

Will this allow the JIT to automatically achieve the same codegen for loops over eg. spans or arrays...

I hope the answer is "yes", either via this or other improvements to the JIT.

we adopted this pattern everywhere in ImageSharp

Hmmm... that's unfortunate. Maybe you could open an issue with this example (before, and after, and why the Unsafe version is faster), if you haven't already?

[jakobbotsch]

#97865 adds an initial framework for IV analysis, but let's keep this issue open for strength reduction which I expect to work on in March.

[jakobbotsch]

I split out strength reduction into #100913. I'm going to consider this one fixed by #97865.