1. Abstract

We implement a drop-in OpenMP runtime library that uses GHC's Runtime System as its threading infrastructure. Standard C code compiled with gcc -fopenmp runs on GHC Capabilities instead of libgomp's pthreads. The runtime implements the GCC GOMP_* ABI and the omp_* user API, supporting parallel regions, worksharing loops, barriers, critical sections, tasks, and sections.

After lock-free optimization, the runtime achieves performance parity with native libgomp on both microbenchmarks and real numerical workloads (dense matrix multiplication). Haskell programs call OpenMP-parallelized C code via FFI, with both runtimes sharing the same thread pool. OpenMP workers call back into Haskell via FunPtr with automatic Capability acquisition. GHC's stop-the-world garbage collector does not pause OpenMP workers because they do not hold Capabilities. The culmination is type-safe shared memory: using GHC's linear types, Haskell and OpenMP C code operate on disjoint regions of the same array with compile-time proof of safety and zero synchronization overhead.

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