Add GCC OpenACC support for GPU offloading

CLAUDE.md

@@ -42,6 +42,31 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 - Auto-fetched: libneo, fortplot (via FetchContent)
 - Optional: OpenMP (enabled by default), GVEC (experimental, off by default), CGAL (off by default)
 
+### OpenACC GPU Builds
+SIMPLE supports GPU acceleration via OpenACC with two compiler options:
+
+**GCC 16+ with nvptx offload** (experimental):
+- Requires: GCC 16+ built with `--enable-offload-targets=nvptx-none`
+- Location: `/temp/AG-plasma/opt/gcc16`
+- Build: `make gcc-acc`
+- Test: `make gcc-acc-test`
+- Clean: `make gcc-acc-clean`
+- Note: GPU offloading has known memory issues with GCC 16 nvptx. Use `ACC_DEVICE_TYPE=host` for host fallback.
+
+**NVHPC/nvfortran**:
+- Requires: NVIDIA HPC SDK (nvfortran)
+- Build: `make nvfortran-acc`
+- Test: `make nvfortran-acc-test`
+- Clean: `make nvfortran-acc-clean`
+
+**CMake options for OpenACC**:
+```bash
+cmake -S . -B build -GNinja \
+    -DSIMPLE_ENABLE_OPENACC=ON \
+    -DSIMPLE_OPENACC_OFFLOAD_TARGET=nvptx \
+    -DCMAKE_Fortran_COMPILER=/temp/AG-plasma/opt/gcc16/bin/gfortran
+```
+
 ### GVEC Integration
 - Minimal GVEC library automatically built from `thirdparty/gvec/`
 - Provides B-spline and cubic spline functionality for magnetic field interpolation

CMakeLists.txt

@@ -76,7 +76,27 @@ option(ENABLE_OPENMP "Enable OpenMP compiler flags." ON)
 option(ENABLE_COVERAGE "Enable code coverage analysis (Debug/Profile builds only)" OFF)
 option(SIMPLE_DETERMINISTIC_FP "Disable fast-math for reproducible floating-point" OFF)
 option(SIMPLE_ENABLE_PYTHON_TOOLS "Enable Python helpers (tests/data generation)" ON)
-option(SIMPLE_ENABLE_OPENACC "Enable OpenACC offload (NVHPC only)" OFF)
+option(SIMPLE_ENABLE_OPENACC "Enable OpenACC offload (NVHPC or GCC with nvptx)" OFF)
+set(SIMPLE_OPENACC_OFFLOAD_TARGET "none" CACHE STRING "OpenACC offload target for GCC (none|nvptx)")
+set_property(CACHE SIMPLE_OPENACC_OFFLOAD_TARGET PROPERTY STRINGS none nvptx)
+
+# GCC OpenACC support (must be before NVHPC block to set flags early)
+if(CMAKE_Fortran_COMPILER_ID STREQUAL "GNU" AND SIMPLE_ENABLE_OPENACC)
+    message(STATUS "OpenACC enabled for GCC compiler")
+    add_compile_options($<$<COMPILE_LANGUAGE:Fortran>:-fopenacc>)
+    add_link_options(-fopenacc)
+    # Pass OpenACC settings to libneo (must be set before find_or_fetch)
+    set(ENABLE_OPENACC ON CACHE BOOL "Enable OpenACC in libneo" FORCE)
+    set(OPENACC_OFFLOAD_TARGET "${SIMPLE_OPENACC_OFFLOAD_TARGET}" CACHE STRING "OpenACC offload target for libneo" FORCE)
+    if(SIMPLE_OPENACC_OFFLOAD_TARGET STREQUAL "nvptx")
+        message(STATUS "OpenACC offload target: nvptx-none (GPU)")
+        add_compile_options($<$<COMPILE_LANGUAGE:Fortran>:-foffload=nvptx-none>)
+        add_link_options(-foffload=nvptx-none)
+    else()
+        message(STATUS "OpenACC offload target: none (host fallback)")
+    endif()
+endif()
+
 option(SIMPLE_ENABLE_DEBUG_OUTPUT "Enable debug output in hot loops" OFF)
 
 if(SIMPLE_ENABLE_DEBUG_OUTPUT)
@@ -270,21 +290,21 @@ endif()
 if (ENABLE_COVERAGE)
     if (CMAKE_BUILD_TYPE MATCHES "Debug|Profile")
         message(STATUS "Coverage analysis enabled for ${CMAKE_BUILD_TYPE} build")
-        
+
         # Create interface library for coverage flags to scope them to specific targets
         add_library(coverage_flags INTERFACE)
         target_compile_options(coverage_flags INTERFACE --coverage -fprofile-arcs -ftest-coverage)
         target_link_options(coverage_flags INTERFACE --coverage -lgcov)
-        
+
         # Add custom target for coverage data generation
         find_program(LCOV_PATH lcov)
         if(LCOV_PATH)
             add_custom_target(coverage
                 COMMAND ${CMAKE_COMMAND} -E echo "Generating coverage data..."
                 COMMAND ${LCOV_PATH} --capture --directory ${CMAKE_BINARY_DIR} --output-file coverage.info
-                    --rc branch_coverage=1 
-                    --ignore-errors inconsistent 
-                    --ignore-errors mismatch 
+                    --rc branch_coverage=1
+                    --ignore-errors inconsistent
+                    --ignore-errors mismatch
                     --ignore-errors unused
                 COMMAND ${LCOV_PATH} --remove coverage.info
                     "/home/ert/code/libneo/*"

Makefile

@@ -167,3 +167,30 @@ nvfortran-acc-test: nvfortran-acc
 
 nvfortran-acc-clean:
 	rm -rf $(NVHPC_ACC_BUILD_DIR)
+
+# GCC OpenACC build targets (requires GCC 16+ with nvptx offload support)
+GCC16_ROOT := /temp/AG-plasma/opt/gcc16
+GCC_ACC_BUILD_DIR := build_gcc_acc
+
+.PHONY: gcc-acc gcc-acc-test gcc-acc-configure gcc-acc-clean
+
+gcc-acc-configure:
+	cmake -S . -B$(GCC_ACC_BUILD_DIR) -GNinja \
+		-DCMAKE_BUILD_TYPE=Release \
+		-DCMAKE_Fortran_COMPILER=$(GCC16_ROOT)/bin/gfortran \
+		-DCMAKE_C_COMPILER=$(GCC16_ROOT)/bin/gcc \
+		-DSIMPLE_ENABLE_OPENACC=ON \
+		-DSIMPLE_OPENACC_OFFLOAD_TARGET=nvptx \
+		-DENABLE_PYTHON_INTERFACE=OFF \
+		-DCMAKE_COLOR_DIAGNOSTICS=ON \
+		$(FLAGS)
+
+gcc-acc: gcc-acc-configure
+	LD_LIBRARY_PATH=$(GCC16_ROOT)/lib64:$$LD_LIBRARY_PATH cmake --build $(GCC_ACC_BUILD_DIR) --config $(CONFIG)
+
+gcc-acc-test: gcc-acc
+	cd $(GCC_ACC_BUILD_DIR) && LD_LIBRARY_PATH=$(GCC16_ROOT)/lib64:$$LD_LIBRARY_PATH \
+		ctest --test-dir test --output-on-failure $(if $(filter 1,$(VERBOSE)),-V) $(if $(TEST),-R $(TEST)) -LE "python|regression"
+
+gcc-acc-clean:
+	rm -rf $(GCC_ACC_BUILD_DIR)