Reduce allocations and improve performance in syevr!

[BioTurboNick]

Profiling code that made heavy use of eigen, creating the final arrays here was a hot spot.

I imagine the @view is going to get a frown... it's strictly reducing the second dimension so in theory it could be resized in-place, but I don't know of a way to do that? EDIT: No longer using a view

const AAA = [0.9124359407098924 0.9654684977248279 0.9237069034235668 0.4479855053248103; 0.6313189619311209 0.039044817699327994 0.21147802367801194 0.2242156559761067; 0.9346485775560693 0.7639560598623737 0.30303677867123346 0.33048614822558353; 0.5034773545892783 0.21614932409092613 0.37837816060665885 0.6122920572414832]

function f(a)
      for i = 1:1000
           eigen(Symmetric(a))
      end
end

Before:

After:

[andreasnoack]

It would he good to include the before and after profiling results here to give an idea about the impact

[BioTurboNick]

Thanks, added. Figured I should also do Hamiltonian too because it's the exact same situation.

Looks like there are other spots that could benefit from this as well.

EDIT: found an alternative to @view: initialize Z as a vector and then reshape(resize!()) at the end; extra allocations, but still fewer than the original, and similar performance to the view version.

[dkarrasch]

This PR is against the release-1.11 branch. Is that on purpose? Otherwise this seems good to go?

[BioTurboNick]

Oh, sorry - that's an accident. I relied on the 1.11 branch to make sure the line numbers were right and forked from there to edit. Is it possible to adjust, or do I need to make a new PR?

[dkarrasch]

I wouldn't know where to change it the target branch. Perhaps you can cherry-pick the commit (after squashing) and open a new PR? Or simply change the few lines manually in a fresh branch?

[jishnub]

You may click on the target branch icon on GitHub, which will provide options to change it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.92%. Comparing base (8bb9f6b) to head (39e8882).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1176      +/-   ##
==========================================
+ Coverage   91.89%   91.92%   +0.03%     
==========================================
  Files          34       34              
  Lines       15353    15355       +2     
==========================================
+ Hits        14108    14115       +7     
+ Misses       1245     1240       -5     

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