Add stub functions for certain std:: functions

[WardBrian]

This is related to stan-dev/stanc3#997 / stan-dev/stanc3#1011

There are several functions which we don't define in stan::math but rather use std:: (or don't qualify them at all) for when the argument is only reals or ints. As far as I can tell, these are:

• pow
• fmod
• atan2
• min, max (the versions that accept 2 scalars, not the ones for arrays etc)
• ceil

It would be great to redirect these within the library, so we can codegen all functions as fully qualified, e.g. stan::math::pow.

We may also need more definitions for norm and pow of complex numbers, see this log for what currently fails

[rok-cesnovar]

Are you sure about ceil()?

Running this compiles fine for me:

#include <stan/math.hpp>
#include <iostream>

int main() {

   double a = 2.15;
   int b = 2;
   int a_ceil = stan::math::ceil(a);
   int b_ceil = stan::math::ceil(a);

   std::cout << a_ceil << ", " << b_ceil << std::endl;
}

So I guess the question is how do I test this? The stanc3 PR branch and try to compile the model I guess? For stuff like atan2 its clear there is no implementation.

[WardBrian]

ceil, min, and max are all current special cased in the code gen. It's possible this special-casing isn't required, but it looks like this:

  | "ceil" ->
      let std_prefix_data_scalar f = function
        | [ Expr.({ Fixed.meta=
                      Typed.Meta.({adlevel= DataOnly; type_= UInt | UReal; _}); _
                  }) ] ->
            "std::" ^ f
        | _ -> "stan::math::" ^ f
      in
      Some
        (fun ppf es ->
          let f = std_prefix_data_scalar f es in
          pp_call ppf (f, pp_expr, es) )

[WardBrian]

What I'll do is remove this special casing in the stanc3 branch and then we can test against that

[rok-cesnovar]

Oh, thats probably there from before the various Math refactors, one of which also added support for that in ceil().

I can confirm easily for the rest with a simple program, ceil() seems to work fine.

[WardBrian]

Removing the std:: special cases has lead to:

'../test/integration/good/int_overloads.stan' had fails '[]' and errors '['Did not compile!']'
'../test/integration/good/min-max-types.stan' had fails '[]' and errors '['Did not compile!']'
'../test/integration/good/optimization/rosenbrock.stan' had fails '[]' and errors '['Did not compile!']'
'../test/integration/good/function-signatures/math/functions/pow.stan' had fails '[]' and errors '['Did not compile!']'
'../test/integration/good/function-signatures/math/functions/min.stan' had fails '[]' and errors '['Did not compile!']'
'../test/integration/good/function-signatures/math/functions/fmod.stan' had fails '[]' and errors '['Did not compile!']'
'../test/integration/good/function-signatures/math/functions/atan2.stan' had fails '[]' and errors '['Did not compile!']'
'../test/integration/good/function-signatures/math/functions/max.stan' had fails '[]' and errors '['Did not compile!']'
'../test/integration/good/code-gen/complex_scalar.stan' had fails '[]' and errors '['Did not compile!']'
'../test/integration/good/code-gen/shadowing.stan' had fails '[]' and errors '['Did not compile!']'

so it seems like min and max do need this treatment, but ceil was indeed already done

[bob-carpenter]

If we only have stan::math::foo(const var& x) defined, then a double will get promoted when called. We need to make sure the double-based implementations get called because promotion in this context is super inefficient.

[rok-cesnovar]

Yeah, we do need to check that.

Given that we already have all the infrastructure to generate tests on-the-fly for expression testing, I think its worth it to actually make a test with all the scalar signatures and then after calling all the functions check that no var was created.

[bob-carpenter]

@rok-cesnovar --- that sounds perfect. @syclik and I introduced tests like this ages ago and I don't even know if they're sitting around, but the memory manager lets you inspect the allocations if you need to write from scratch.

[WardBrian]

@rok-cesnovar have you had a chance to look at this yet?

[rok-cesnovar]

Yes, I am in the process of making a script to generate the tests mentioned above. I can fix the functions first if this is blocking you though. That can be done quick.