When working with Fortran code in a C++ project, you may encounter situations where you need to utilize Fortran compiler-specific extensions. These extensions provide additional functionality or optimizations that are not available in standard C++.

In this blog post, we will explore some of the commonly used Fortran compiler-specific extensions in C++ and discuss how they can be beneficial in certain scenarios.

1. Interoperability with Fortran

extern "C"

The extern "C" declaration is used in C++ to specify that a particular function or variable has C linkage, ensuring that the function name or variable name is not subject to C++ name mangling.

When interfacing with Fortran code in C++, it is essential to use extern "C" to ensure that the function signatures match and prevent any naming conflicts.

extern "C" {
    void fortran_function(int* arg);
}

Name Mangling

Fortran and C++ have different name mangling schemes, which can result in incompatible function signatures when linking these two languages.

To address this issue, most Fortran compilers provide a way to control the name mangling scheme. For example, the GNU Fortran compiler (gfortran) allows you to change the name mangling scheme to match the one used by C++. This can be done using the -fno-underscoring compiler flag.

gfortran -fno-underscoring fortran_code.f90

2. Compiler-specific Intrinsics

Intel Compiler Intrinsics

The Intel C++ compiler (icc) provides a set of intrinsics that allow you to directly call specialized functions implemented in the compiler. These intrinsics provide access to low-level optimizations and hardware-specific features, resulting in improved performance.

For example, the _mm_add_ps intrinsic allows you to add two packed single-precision floating-point values using the SIMD (Single Instruction, Multiple Data) instructions supported by Intel processors.

#include <immintrin.h>

void vector_add(float* a, float* b, float* result, int size) {
    for (int i = 0; i < size; i += 4) {
        __m128 vec_a = _mm_load_ps(&a[i]);
        __m128 vec_b = _mm_load_ps(&b[i]);
        __m128 vec_result = _mm_add_ps(vec_a, vec_b);
        _mm_store_ps(&result[i], vec_result);
    }
}

IBM Compiler Intrinsics

The IBM XL C/C++ compiler provides a similar set of intrinsics that allow you to leverage the Power Architecture-specific features.

For example, the vec_add intrinsic can be used to add two double-precision floating-point values using vector instructions supported by IBM Power processors.

#include <builtins.h>

void vector_add(double* a, double* b, double* result, int size) {
    for (int i = 0; i < size; i += 2) {
        vector double vec_a = vec_ld(i * sizeof(double), a);
        vector double vec_b = vec_ld(i * sizeof(double), b);
        vector double vec_result = vec_add(vec_a, vec_b);
        vec_st(vec_result, i * sizeof(double), result);
    }
}

Conclusion

Utilizing Fortran compiler-specific extensions in C++ can help you take advantage of advanced language features, enhance performance, and facilitate seamless integration between Fortran and C++ code.

However, it is important to note that these extensions are highly dependent on the specific compiler and may not be portable across different compiler implementations. Therefore, it is recommended to thoroughly understand the documentation and limitations of the compiler-specific extensions before incorporating them into your projects.

#Fortran #C++