llvm-project

0267abce - [OpenMP] Relocate task shared-by-ref captures across taskgraph replay

[OpenMP] Relocate task shared-by-ref captures across taskgraph replay OpenMP 6.0 lets a taskgraph region be recorded once and replayed many times. Each replay creates a fresh instance of the 'args' pointer block passed to __kmpc_taskgraph (and may execute at a different stack location, or even on a different stack), so by-reference captures inside a recorded task must be re-pointed at the live host objects of the current invocation; otherwise the recorded tasks would dereference stale memory from the stack frame of the initial call to __kmpc_taskgraph. This patch introduces the small infrastructure to do that and wires it up for the explicit 'task' construct. A subsequent patch extends the same scheme to 'taskloop'. On the compiler side (CGOpenMPRuntime.cpp), a new helper emitTaskRelocationFunction emits a per-task thunk: void __omp_taskgraph_relocate.NN(kmp_task_t *task, void *outer_captures); The thunk walks the task's captures and overwrites each entry of task->shareds with the address of the corresponding field projected from the freshly reconstructed outer pointer block. Two classes of capture do not need updating and are treated as no-ops by the thunk: captures that correspond to a firstprivate list item (the body reads from the per-task '.kmp_privates.t' snapshot, populated when the task is allocated and -- for non-trivial types -- reset on each replay by the clone helper introduced later), and captures of variables with static storage duration (their address is link-time fixed). Reductions of a local-stack variable are intentionally not in this set: the taskred state is keyed on the recording-time taskgroup hierarchy and is not yet usable on replay, so we prefer to preserve today's relocate-returns-null / runtime-aborts behaviour for that case so the limitation surfaces as a diagnostic. emitTaskCall now emits such a thunk for each taskgraph-recorded task and passes it as the new trailing argument of __kmpc_taskgraph_task. The redundant 'shareds' parameter is dropped, since relocation now provides the supported mechanism for refreshing that pointer. On the runtime side (kmp.h, kmp_tasking.cpp, OMPKinds.def), introduce a new typedef kmp_task_relocate_t and store the callback on each recorded task in kmp_taskgraph_node_t::relocate, together with the outer-record pointer captured at __kmpc_taskgraph entry in kmp_taskgraph_record_t::taskgraph_args. __kmp_omp_tg_task invokes the callback on replay, and aborts with a new fatal diagnostic (OmpTaskgraphBadCapture, i18n/en_US.txt) when a recorded task has a non-null shareds payload but no relocation callback. There is also a fix for a pre-existing bug in __kmp_taskgraph_clone_task -- the cloned task's shareds pointer was left referring to the original's payload -- which becomes observable as soon as the relocation thunk writes through that pointer. New libomp tests cover lexical and non-lexical shared captures, pointer captures, non-trivial types, recursive recordings, stack-depth differences across replays, and the saved/expired- graph cases. Assisted-By: Claude Opus 4.7 Pull Request: https://github.com/llvm/llvm-project/pull/200404