Returning Complex Numbers - 5.2 English - 57404

The GNU Fortran compiler (gfortran), AOCC (Flang), and Intel Fortran compiler (ifort) have different requirements for returning complex numbers from the C functions as follows:

• GNU (gfortran) compiler returns complex numbers using registers. Thus, the complex numbers are returned as the return value of the function itself. BLIS refers to this as “gnu” complex return, and this is the default choice.

• AOCC (Flang) and Intel® (ifort) compilers return complex numbers using the hidden first argument. The caller must pass the pointer to the return value as the first parameter. BLIS refers to this as “intel” complex return, but it may be referred to elsewhere as “void” or “F2C” complex return.

The BLAS APIs affected are cdotc, cdotu, zdotc, and zdotu APIs.

BLIS implements BLAS using C code. Thus it can support either Fortran complex return convention with any compiler using appropriate configure or cmake options. The options to implement the default Fortran convention for GNU and AOCC compilers are:

gfortran Example:

• Configure Option:

  --complex-return=gnu
  

• cmake Option:

  -DCOMPLEX_RETURN=gnu
  

• API Call:

  ret_value = cdotc_(&n, x, &incx, y, &incy);
  

AOCC flang Example:

• Configure Option:

  --complex-return=intel CC=clang CXX=clang++
  

• cmake Option:

  -DCOMPLEX_RETURN=intel
  

• API Call:

  cdotc_(&ret_value, &n, x, &incx, y, &incy);