Standard conforming BLAS/LAPACK calls from C

[mjacobse]

Adds wrappers using iso_c_binding and bind(c) to allow standard conforming calls to BLAS/LAPACK functions from the SSIDS C++ code. See #4, and #4 (comment) for background, reasoning and alternatives.

On gfortran, without link time optimization, this adds another function call and a little bit of stack management (https://godbolt.org/z/ooKb4cboM). Without bind(c), the stack management even reduces to just adding 1s for the hidden string length arguments that exposed the broken non-standard function calls in #4 in the first place (https://godbolt.org/z/cjfK7Gjv4). I'm not quite sure why the compiler does not do the same with bind(c), perhaps this is a missed optimization that can be leveraged in the future. This PR still proposes to go with bind(c) for standard compliance and to avoid to continue relying on implementation-defined behavior.

With these wrappers, the compiler flag workaround -fno-optimize-sibling-calls from #69 can be removed.

[jfowkes]

Thank you I wasn't aware that issue #4 for SSIDS arose not from CBLAS/LAPACKE, but instead from the manual declaration of the BLAS functions in the SSIDS code itself as you explain in #4 (comment)

I agree entirely that this is the correct approach to calling C functions from Fortran (even if there is a slight overhead). In fact, this is exactly the mechanism we use in our GALAHAD library for calling C functions from Fortran.

However, since this uses ISO C bindings, that limits us to a newer Fortran standard, something like Fortran 2003 or Fortran 2008? I'm happy as long as it's not Fortran 2018.

[mjacobse]

Thanks for taking a look! The iso_c_binding module is already used extensively throughout the source code. In fact, I did not have to add a use of the module myself for this change set, it was already available from here:

spral/src/ssids/cpu/cpu_iface.f90

Line 6 in 7457139

use, intrinsic :: iso_c_binding

It was also indeed introduced with Fortran 2003.

[jfowkes]

Thanks for taking a look! The iso_c_binding module is already used extensively throughout the source code. In fact, I did not have to add a use of the module myself for this change set.

Ah well spotted, excellent merged!