compiler: Improve IndexDerivative lowering

[FabioLuporini]

Tiny enhancement to avoid useless stupid temporary variables in the generated code

[FabioLuporini]

"Fabio, have you thought about just performing a round of cse() instead"

"Yes I have, unfortunately it's not that easy because CSE is a Cluster-local pass, which doesn't work well here"

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (c888cee) 86.76% compared to head (df4e53d) 86.76%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2288 +/- ## ======================================= Coverage 86.76% 86.76% ======================================= Files 229 229 Lines 42884 42893 +9 Branches 7951 7953 +2 ======================================= + Hits 37207 37216 +9 Misses 5002 5002 Partials 675 675 ```

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[georgebisbas]

"Fabio, have you thought about just performing a round of cse() instead"

"Yes I have, unfortunately it's not that easy because CSE is a Cluster-local pass, which doesn't work well here"

Why does it not work well? Not sure I understand TBH

[FabioLuporini]

Why does it not work well? Not sure I understand TBH

lower_index_derivative might produce:

Cluster([
  r1 = 0
  r1 += <lowered index derivative>
  r0 = r1
])

CSE would be able to see that r0 is unnecessary here and safely removable. But r0 is needed in later Clusters, so it would produce wrong code ultimately

Note that this is just a simplified example

You would need to understand lower_index_derivatives fully to appreciate all the different flavors of this issue

[georgebisbas]

Why does it not work well? Not sure I understand TBH

lower_index_derivative might produce:

Cluster([
  r1 = 0
  r1 += <lowered index derivative>
  r0 = r1
])

CSE would be able to see that r0 is unnecessary here and safely removable. But r0 is needed in later Clusters, so it would produce wrong code ultimately

Note that this is just a simplified example

You would need to understand lower_index_derivatives fully to appreciate all the different flavors of this issue

Thanks