Simple GPU - Fortran GPU Computing Library

February 17, 2026 ยท View on GitHub

A simple and easy-to-use library for GPU computing in Fortran, providing transparent access to GPU acceleration through a clean Fortran interface.

Documentation

Online API Documentation - Complete API reference generated with Doxygen

Overview

Simple GPU is a library designed to simplify GPU computing in Fortran applications. It provides:

  • Dual implementation: CPU-only version using standard BLAS, and GPU-accelerated version using NVIDIA cuBLAS or AMD hipBLAS
  • Transparent interface: Same Fortran API for both CPU and GPU versions
  • Memory management: Easy GPU memory allocation and data transfer
  • BLAS operations: Common BLAS operations (GEMM, GEMV, DOT, GEAM) for both single and double precision
  • Stream support: Asynchronous operations through CUDA or HIP streams

Features

Memory Management

  • gpu_allocate: Allocate memory on GPU (or CPU for CPU version)
  • gpu_deallocate: Free allocated memory
  • gpu_upload: Transfer data from CPU to GPU
  • gpu_download: Transfer data from GPU to CPU
  • gpu_copy: Copy data between GPU memory regions

Device Management

  • gpu_ndevices: Query number of available GPU devices
  • gpu_set_device: Select active GPU device
  • gpu_get_memory: Query GPU memory status

BLAS Operations

All BLAS operations have variants that accept 64-bit integers for dimensions. These variants have a _64 suffix (e.g., gpu_ddot_64, gpu_dgemm_64).

Level 1: Vector operations

  • gpu_sdot, gpu_ddot: Dot product (single/double precision)

Level 2: Matrix-vector operations

  • gpu_sgemv, gpu_dgemv: Matrix-vector multiplication

Level 3: Matrix-matrix operations

  • gpu_sgemm, gpu_dgemm: Matrix-matrix multiplication
  • gpu_sgeam, gpu_dgeam: Matrix addition/transposition

Advanced Features

  • Streams: Create and manage CUDA streams for asynchronous execution
  • BLAS handles: Manage cuBLAS library handles
  • Stream synchronization: Control execution flow

Installation

Prerequisites

For CPU version (required):

  • C compiler (gcc, clang, etc.)
  • Fortran compiler (gfortran, ifort, etc.)
  • BLAS library (OpenBLAS, Intel MKL, or reference BLAS)
  • Autotools (autoconf, automake, libtool)

For NVIDIA GPU version (optional):

  • NVIDIA CUDA Toolkit (with nvcc compiler)
  • NVIDIA cuBLAS library
  • CUDA-capable GPU

For AMD GPU version (optional):

  • AMD ROCm platform
  • hipBLAS library
  • ROCm-capable GPU

Building from Source

  1. Generate configure script (if building from git):

    ./autogen.sh

  2. Configure the build:

    ./configure

    The configure script will automatically detect if CUDA is available and enable the NVIDIA GPU library if possible.

    Configuration options:

    • --disable-nvidia: Disable NVIDIA GPU library even if CUDA is available
    • --with-cuda=DIR: Specify CUDA installation directory (default: /usr/local/cuda)
    • --disable-amd: Disable AMD GPU library even if ROCm is available
    • --with-rocm=DIR: Specify ROCm installation directory (default: auto-detect)
    • --with-blas=LIB: Specify BLAS library to use

    Example configurations:

    # CPU version only
./configure --disable-nvidia --disable-amd

# Specify CUDA location
./configure --with-cuda=/opt/cuda

# Specify ROCm location
./configure --with-rocm=/opt/rocm

# Use specific BLAS library
./configure --with-blas="-lmkl_rt"

  3. Build the libraries:

    make

  4. Run tests (optional):

    make check

  5. Install:

    sudo make install

Library Versions

Simple GPU provides three shared libraries:

  1. libsimple_gpu-cpu.so: CPU-only version

    • Uses standard BLAS library
    • No GPU required
    • Useful for development and testing on systems without GPUs

  2. libsimple_gpu-nvidia.so: NVIDIA GPU version (if CUDA is available)

    • Uses NVIDIA cuBLAS library
    • Requires CUDA-capable GPU
    • Provides GPU acceleration for supported operations

  3. libsimple_gpu-amd.so: AMD GPU version (if ROCm is available)

    • Uses AMD hipBLAS library
    • Requires ROCm-capable GPU
    • Provides GPU acceleration for supported operations

All libraries provide the same Fortran interface, allowing seamless switching between CPU and GPU implementations.

Usage

Including the Library in Your Code

Important: To use the Simple GPU library in your Fortran project, you must create a file with the .F90 extension (uppercase) that includes the library header using the C preprocessor:

#include <simple_gpu.F90>

Why Use .F90 Instead of .f90?

The .F90 extension (uppercase) is critical because:

  1. Preprocessor Support: Fortran compilers only run the C preprocessor on files with uppercase extensions (.F90, .F, .FOR). The lowercase .f90 extension bypasses the preprocessor, so the #include directive won't work.

  2. Cross-Compiler Compatibility: The #include <simple_gpu.F90> directive allows the C preprocessor to find the header file in a default location (via CPATH environment variable). This means:

    • When you update the library, no changes are needed in your code
    • The library can be compiled once with one compiler (e.g., gcc/gfortran) and used with any other Fortran compiler (ifort, nvfortran, etc.)
    • No Fortran .mod files are distributed, which are notoriously incompatible between different compilers

  3. Simplified Distribution: By avoiding compiler-specific .mod files, simple_gpu maintains maximum portability across different Fortran compiler ecosystems.

Example of a minimal program:

#include <simple_gpu.F90>

program my_program
  use gpu
  implicit none
  
  ! Your code here
  
end program my_program

Data Types

The library provides multidimensional array types for both single and double precision:

  • gpu_double1: 1-dimensional array of double precision values
  • gpu_double2: 2-dimensional array of double precision values
  • gpu_double3: 3-dimensional array of double precision values
  • gpu_double4, gpu_double5, gpu_double6: 4, 5, and 6-dimensional arrays

Similarly for single precision:

  • gpu_real1 through gpu_real6

Each type contains:

  • c: C pointer to GPU memory
  • f: Fortran pointer for accessing data (e.g., f(:) for 1D, f(:,:) for 2D)

The gpu_allocate function is overloaded and automatically accepts the appropriate number of dimensions:

call gpu_allocate(x, n)        ! 1D array: x is gpu_double1 or gpu_real1
call gpu_allocate(a, m, n)     ! 2D array: a is gpu_double2 or gpu_real2
call gpu_allocate(b, l, m, n)  ! 3D array: b is gpu_double3 or gpu_real3

Basic Example with 1D Arrays

#include <simple_gpu.F90>


program example
  use gpu
  implicit none
  
  type(gpu_blas) :: handle
  type(gpu_double1) :: x, y
  double precision, allocatable :: x_h(:), y_h(:)
  double precision :: result
  integer :: n, i
  
  n = 1000
  
  ! Initialize BLAS handle
  call gpu_blas_create(handle)
  
  ! Allocate vectors (1D arrays)
  call gpu_allocate(x, n)
  call gpu_allocate(y, n)
  
  ! Create and initialize host data
  allocate(x_h(n), y_h(n))
  do i = 1, n
    x_h(i) = dble(i)
    y_h(i) = dble(i) * 2.0d0
  end do
  
  ! Upload to GPU
  call gpu_upload(x_h, x)
  call gpu_upload(y_h, y)
  
  ! Compute dot product on GPU
  ! Note: Use address of first element (x%f(1), not x)
  call gpu_ddot(handle, n, x%f(1), 1, y%f(1), 1, result)
  
  print *, 'Dot product result:', result
  
  ! Clean up
  deallocate(x_h, y_h)
  call gpu_deallocate(x)
  call gpu_deallocate(y)
  call gpu_blas_destroy(handle)
  
end program example

Example with 2D Arrays

#include <simple_gpu.F90>

program example_2d
  use gpu
  implicit none
  
  type(gpu_blas) :: handle
  type(gpu_double2) :: a, b, c
  double precision, allocatable :: a_h(:,:), b_h(:,:), c_h(:,:)
  double precision :: alpha, beta
  integer :: m, n, i, j
  
  m = 100
  n = 200
  
  ! Initialize BLAS handle
  call gpu_blas_create(handle)
  
  ! Allocate matrices (2D arrays)
  call gpu_allocate(a, m, n)
  call gpu_allocate(b, m, n)
  call gpu_allocate(c, m, n)
  
  ! Create and initialize host data
  allocate(a_h(m,n), b_h(m,n), c_h(m,n))
  do j = 1, n
    do i = 1, m
      a_h(i,j) = dble(i + j)
      b_h(i,j) = dble(i * j)
    end do
  end do
  
  ! Upload to GPU
  call gpu_upload(a_h, a)
  call gpu_upload(b_h, b)
  
  ! Matrix addition: C = alpha*A + beta*B
  alpha = 1.5d0
  beta = 0.5d0
  ! Note: Use address of first element (a%f(1,1), not a)
  call gpu_dgeam(handle, 'N', 'N', m, n, &
                 alpha, a%f(1,1), m, beta, b%f(1,1), m, &
                 c%f(1,1), m)
  
  ! Download result from GPU to host
  call gpu_download(c, c_h)
  
  ! Access result in c_h(:,:)
  print *, 'Result at (1,1):', c_h(1,1)
  
  ! Clean up
  deallocate(a_h, b_h, c_h)
  call gpu_deallocate(a)
  call gpu_deallocate(b)
  call gpu_deallocate(c)
  call gpu_blas_destroy(handle)
  
end program example_2d

Important Note about BLAS Function Arguments:

When calling BLAS functions, always use the address of the first element of the array (e.g., x%f(1) for 1D arrays or a%f(1,1) for 2D arrays), otherwise you may encounter type errors:

! Correct:
call gpu_ddot(handle, n, x%f(1), 1, y%f(1), 1, result)

! Incorrect (may cause type error):
call gpu_ddot(handle, n, x, 1, y, 1, result)

Technical Note: The library wrappers use Fortran's c_loc() intrinsic to obtain the memory address of the array element. By passing x%f(1), you're providing the first element as a scalar with the target attribute, which c_loc() then converts to the appropriate C pointer for the underlying BLAS routines.

Using Different Library Versions

To use a specific library version, link your application against the desired library:

# CPU version
gfortran -o myapp myapp.f90 -lsimple_gpu-cpu

# GPU version (NVIDIA)
gfortran -o myapp myapp.f90 -lsimple_gpu-nvidia

# GPU version (AMD)
gfortran -o myapp myapp.f90 -lsimple_gpu-amd

Testing

The library includes comprehensive unit tests that compare CPU and GPU implementations to ensure correctness.

Run tests with:

make check

For verbose test output:

make check-verbose

API Reference

For detailed API documentation, see the comments in:

  • include/simple_gpu.h: C interface declarations
  • include/simple_gpu.F90: Fortran module and type definitions

Performance Notes

  • For small problem sizes, CPU version may be faster due to GPU overhead
  • GPU version shows significant speedup for larger matrices/vectors
  • Use streams for asynchronous operations to overlap computation and data transfer
  • Keep data on GPU between operations to minimize transfer overhead

Troubleshooting

CUDA not detected during configuration

If CUDA is installed but not detected:

./configure --with-cuda=/path/to/cuda

BLAS library not found

Specify BLAS library explicitly:

./configure --with-blas="-lopenblas"

Runtime errors with GPU version

  1. Ensure NVIDIA drivers are properly installed (for NVIDIA GPUs)
  2. Ensure ROCm drivers are properly installed (for AMD GPUs)
  3. Check that CUDA libraries are in your library path: 
    export LD_LIBRARY_PATH=/usr/local/cuda/lib64:$LD_LIBRARY_PATH
  4. Check that ROCm libraries are in your library path: 
    export LD_LIBRARY_PATH=/opt/rocm/lib:$LD_LIBRARY_PATH
  5. Verify GPU is accessible with nvidia-smi (NVIDIA) or rocm-smi (AMD)

Contributing

Contributions are welcome! Please ensure:

  • Code follows existing style and conventions
  • All tests pass before submitting
  • New features include appropriate tests

Adding BLAS Functions

The library currently implements a core set of BLAS operations (DOT, GEMV, GEMM, GEAM). If you need additional BLAS functions (such as AXPY, SCAL, TRSM, SYRK, etc.), contributions are highly encouraged! Adding new BLAS routines follows the established pattern in the codebase and helps make the library more complete and useful for the community.

Building Documentation

The project uses Doxygen to generate API documentation from source code comments.

Prerequisites

  • Doxygen (version 1.9 or later)
  • Graphviz (for generating diagrams)

Generate Documentation Locally

# Install dependencies (Ubuntu/Debian)
sudo apt-get install doxygen graphviz

# Generate documentation
doxygen Doxyfile

# View documentation
# Open docs/html/index.html in your web browser

The documentation is automatically built and published to GitHub Pages when changes are pushed to the main branch.

License

See LICENSE file for details.