flang-rt

flang-rt/ is the runtime support library for code compiled by Flang. It implements the bits of Fortran semantics that can't be reduced to inline machine code: I/O, allocatable and pointer descriptor management, intrinsic procedures with non-trivial implementations, OpenMP runtime entry points, and similar functionality.

Purpose

A Fortran program lowered through Flang's pipeline ends up calling into flang-rt for any operation whose implementation is large enough or sufficiently runtime-state-dependent that inlining it would bloat every callsite. Examples include READ/WRITE against formatted/unformatted units, ALLOCATE/DEALLOCATE with shape and bounds tracking, derived-type assignment, transformational intrinsics like RESHAPE and MATMUL, and the runtime descriptors that back assumed-shape arrays.

History

The Flang runtime originally lived inside flang/ as flang/runtime/. It was carved out into its own top-level subproject (flang-rt/) so it could be built like other LLVM runtime libraries — through LLVM_ENABLE_RUNTIMES rather than LLVM_ENABLE_PROJECTS, with the option of cross-compiling for offload targets. See flang-rt/README.md for the rationale and the migration plan.

Directory layout

flang-rt/
├── include/flang-rt/   # Public headers (descriptor types, runtime-side interface)
├── lib/                # Implementation — IO, allocations, intrinsics, exceptions, ...
├── unittests/
├── test/               # lit regression tests (driven against built artifacts)
├── cmake/
├── docs/
├── examples/
└── README.md

What it implements

A non-exhaustive list of what flang-rt provides at runtime:

• Descriptors — the C structures that back allocatable, pointer, and assumed-shape arrays. The same shape descriptors are exposed at the C / BIND(C) interop boundary.
• I/O — READ, WRITE, OPEN, CLOSE, INQUIRE, list-directed/formatted/unformatted/namelist transfers, recursive I/O, asynchronous I/O.
• Memory and lifetime — ALLOCATE, DEALLOCATE, MOVE_ALLOC, automatic cleanup at scope exit, finalization of derived types.
• Intrinsics that don't inline — RESHAPE, TRANSFER, MATMUL, DOT_PRODUCT, SUM, MINVAL/MAXVAL, PACK/UNPACK, SPREAD, EOSHIFT/CSHIFT, character-handling intrinsics.
• Exception flags / IEEE features — environment for IEEE-arithmetic state.
• Stop / error handling — STOP, ERROR STOP, image control statements.

Integration points

• Flang lowers calls into flang-rt via well-defined symbol names (_FortranA*, _FortranAio*, ...). The naming convention is documented inside flang-rt/include/flang-rt/ and flang/include/flang/Runtime/.
• MLIR / FIR lowering inserts the runtime calls at lowering time; FIR ops have direct counterparts that emit the right runtime entry points.
• OpenMP / Offload runtimes (openmp, offload) cooperate with flang-rt for offloaded Fortran code.

Entry points for modification

• Adding a runtime intrinsic: add the C++ implementation under flang-rt/lib/, declare the symbol in the corresponding header under flang-rt/include/flang-rt/ (and the matching declaration on the compiler side under flang/include/flang/Runtime/), and emit calls from flang/lib/Lower/.
• Adding an I/O capability: edit the relevant flang-rt/lib/IO/ files.

Reference

• flang-rt/README.md — full design and migration notes
• Flang runtime documentation