Proper handling of static arrays

[avik-pal]

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[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (bda160c) 65.29% compared to head (a31f99f) 65.36%.

Files	Patch %	Lines
src/common.jl	89.28%	3 Missing ⚠️
src/default.jl	80.00%	1 Missing ⚠️
src/factorization.jl	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #444      +/-   ##
==========================================
+ Coverage   65.29%   65.36%   +0.06%     
==========================================
  Files          27       27              
  Lines        2072     2096      +24     
==========================================
+ Hits         1353     1370      +17     
- Misses        719      726       +7     

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[ChrisRackauckas]

Could we add an inference and no alloc check on static array usage too?

[avik-pal]

There is this 1 allocation while creating the LinearCache which I can't seem to track

[ChrisRackauckas]

This is kind of the same thing as the SimpleNonlinearSolve/SimpleDiffEq case. If someone calls solve and not init, then with static things doing something simpler is advantageous. I would think that if someone calls solve on a StaticArray we should basically just do A\b and bypass the rest of LinearSolve.jl. I did a bunch of testing when making https://github.com/SciML/OrdinaryDiffEq.jl/blob/master/src/derivative_utils.jl#L1-L19 and for sufficiently small problems caching the LU with a static array (or the inv) is simply not helpful (track down the PR with the old benchmarks). This could be size dependent too.

Now if they do init I think this all makes sense though, just solve should have a fast path.

[avik-pal]

Yeah that makes sense. Though I feel even for init with SArrays we shouldn't really cache anything, just pretend to cache it. I have found operations mixing MArrays and SArrays to be slower than just doing the uncached SArrays

[ChrisRackauckas]

Agreed