Handle call-by-value parameters

[raehik]

Analysis.BBlock.perBlock is where the decision is made on how to create blocks from call()s. Previously, it would check to see if the expression was assignable: either a plain variable, or a subscript. If so, it would "re-assign" them (I'm not completely clear on this part). Now, we treat bracketed plain variables as non-assignable, figuring that they're used as call-by-value.

Closes #203 .

TODO

• perBlock: do same for the expression function call (pull the pattern out)
• write some tests
• document a bit clearer?

[raehik]

I believe we need to restrict the parameter re-assigning (-> call-by-reference) for more parameters: specifically, bracketed variables (a) do not get re-assigned (so they stay call-by-value).

I think my change works. The flows-from graph looks correct. @mrd , could I check in with you on this?

[raehik]

I'm testing like so:

module m
    implicit none
    contains

    subroutine double(i, i2)
        integer, intent(in)  :: i
        integer, intent(out) :: i2
        print *, "entering double, i =", i
        i2 = 2*i
        print *, "leaving  double, i =", i
    end subroutine double

end module m

program overlapping_arg
    use m, only: double
    implicit none
    integer :: j
    j = 3
    call double(j, j)
    print *, "(1) in main, j =", j
    call double(j, j)
    print *, "(2) in main, j =", j
end program overlapping_arg

And running stack run -- test.f90 --show-flows-from=B13 --dot | dot -Tpng > graph.png. Bracketing various js in the 2 calls to double turns that usage to a call-by-value, which effectively removes that line on the data flow graph.

[raehik]

I ignored the named parameter feature that Fortran has! Almost finished re-jigging the parsers to properly handle that. (I don't think we look at named parameters in the analysis, so I'm not fixing anything there.)

[raehik]

Pending while we resolve some potential issues with the --show-flows-{to,from} flags -- they give unexpected output, which could be flipped directions or just ambiguous naming