Idea: `PerformanceWarning`

[aaron-skydio]

Some thoughts from discussing this:

• It might be nice to have a warning system for “things that are bad for performance”. In particular, the first example I’ve come across is std::pow(x, y) where y is a symbol at codegen time, but is a constant parameter at runtime (in particular an integer, in this case and the worst case 1.0). I can imagine maybe also wanting warnings for integer powers if fast-math isn’t on? Although maybe our codegen should handle this, or assume fast-math.
• Probably if pow(x, y) is the only example, this doesn’t make sense? Not sure if we have other examples
• Misc thoughts:
  • Docs page would be good
    • probably want this regardless
  • Compilers like ispc and numba have things like this
  • Want to be able to configure as warning, or error, or ignored
    • Need to do this per subexpression for big things like OptimizationProblem that encompass lots of expressions / residuals
    • Extra attribute per expression?
    • Code printer probably checks

[jpreiss]

I noticed my generated code has a lot of std::pow calls where y = 2 is known at codegen time. Is there anything I can do to make symforce emit something like _tmp2 = _tmp1 * _tmp1 instead?

[aaron-skydio]

For a constant exponent of 2 known at codegen time, your compiler should optimize the std::pow to a multiply, so we don't bother special casing that to generate a multiply - the logic for which powers we special-case is here: https://github.com/symforce-org/symforce/blob/main/symforce/codegen/backends/cpp/cpp_code_printer.py#L68

If you have a compiler where this doesn't get optimized and actually results in a function call or something else that's not just a multiplication instruction that would be good to know though and we can also special-case that to generate a multiplication