Fix tests for optimizations

[rok-cesnovar]

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• Run unit tests
• Documentation
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@WardBrian can we add a "simple" optimization bug to stress test this? Something that will produce the wrong results.

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[WardBrian]

I've introduced a bug which swaps the order of arguments to fma in the partial evaluator. If it still passes tests that is probably because none of the tests we're running feature a float multiply add. We next need to address stan-dev/performance-tests-cmdstan#38, e.g. we need more tests in this set that get optimized.

[rok-cesnovar]

Thanks!

We next need to address stan-dev/performance-tests-cmdstan#38, e.g. we need more tests in this set that get optimized.

Agreed. Where do we want to put these? In the perf-cmdstan repo or stanc3?

We can then do the same for stan-dev/performance-tests-cmdstan#30

[WardBrian]

It may be arbitrary, but to me I think anything which is testing that the model compiles (e.g., what you're proposing in stan-dev/performance-tests-cmdstan#30) can live here in stanc3, but anything which we want to actually run should be in performance-tests-cmdstan. Sound reasonable?

[rok-cesnovar]

Yeah, good call.

[WardBrian]

We already have https://github.com/stan-dev/stanc3/tree/master/test/integration/downstream, which is a good place to potentially put more of those models we want to make sure compile. I don't believe we're currently compiling that folder all the way down to a binary

[WardBrian]

Seems like example-models/bugs_examples/vol1/seeds/seeds_stanified.stan performs some float multiply adds, which the optimizer detected and then did the wrong thing. The tests did their job here!