Pass: Passing Array descriptor by value

[Shaikh-Ubaid]

This is a WIP (Work in progress). The tests might not pass.

[Shaikh-Ubaid]

As suggested in this #70 (comment), using this pass, we need to change

subroutine f(a, b)
integer :: a(:,:)
real :: b(:,:,:)
end subroutine

into

subroutine f(na1, na2, a, nb1, nb2, nb3, b)
integer, intent(in) :: na1, na2, nb1, nb2, nb3
integer :: a(na1, na2)
real :: b(nb1, nb2, nb3)
end subroutine

doubt: Please, could someone possibly share how the following c/cpp code is expected to change when this pass is applied.

void my_func(int **a, int ***b) {
    ... // some statements here
}

[Shaikh-Ubaid]

On line 21, do we need to use PassUtils::PassVisitor or is ASR::StatementWalkVisitor also fine/works?

[Shaikh-Ubaid]

(Lines 50 to 52) Since, the arguments passed to visit_FunctionCall() (and others) are const, it was no possible to update them. So, I used a const_cast<> to allow us to update values but this leads to warnings as follows during build:

/home/ubaid/OpenSource/lfortran/src/libasr/pass/arr_dims_propagate.cpp:50:29: warning: variable ‘xx’ set but not used [-Wunused-but-set-variable]
   50 |         ASR::FunctionCall_t xx = const_cast<ASR::FunctionCall_t &>(x);

[certik]

The solution here is to have a visitor that allows to modify the ASR tree in-place. We just have to do it.

[Shaikh-Ubaid]

This segfaults currently when printing ASR (using pickle(asr, true /* use colors */, true /* indent */, true /* with_intrinsic_modules */))

[Shaikh-Ubaid]

As FunctionCall is an expression, SubroutineCall is a statement and Function and Subroutines are symbols, it seems as if we need/might-need several visitors/ASR-visits, one each for replace/modify expression, replace/modify statement, replace/modify symbol. I think I need help in this, please.

[certik]

Creating the general ASR pass might be more complicated, so let's do that later.

For now, just support the same kind of arrays that @czgdp1807 implemented in LFortran to represent with just a pointer.

We will later add an ASR pass to be able to transform more general arrays into that.

Conclusion: just get a(n) style arrays working, and do not support a(:) arrays for now.

[czgdp1807]

Creating the general ASR pass might be more complicated, so let's do that later.

Correct me if I am wrong. So we need to do the following things,

1. Replace procedure signature of the kind f(array1, array2) with f(array1, dim1, dim2, array2, dim3, dim4). Just an example. Programatically this can be generalised to any number of dimensions.
2. And then replace the declarations inside the procedure for these arrays.
3. Replace the the procedure calls according to the updated signature.

If so, then I think it shouldn’t be hard but just lengthy. Can I try to do this? In any case if successful then good otherwise we will know the limitations of this strategy.

Please let me know. I will continue this if you agree.

[certik]

@czgdp1807 yes exactly. Yes, go ahead and try to do this!

This would be very useful optional pass, that we can reuse across all backends. For some, like LLVM, it will give an extra performance in some cases. For some other, like WASM, it would be needed to even get some cases working.

[czgdp1807]

Great. Added this to my priority list. Will start working on this.

[czgdp1807]

N.B. #87. Its a WIP.

[czgdp1807]

Closing in favour of #87