[all-commits] [llvm/llvm-project] 221ada: [libomptarget] Implement locks for amdgcn

[Jon Chesterfield via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 221ada654b28a524d01dc70ec16d38e0f2484f78
      https://github.com/llvm/llvm-project/commit/221ada654b28a524d01dc70ec16d38e0f2484f78
  Author: Jon Chesterfield <jonathanchesterfield at gmail.com>
  Date:   2020-03-05 (Thu, 05 Mar 2020)

  Changed paths:
    M openmp/libomptarget/deviceRTLs/amdgcn/CMakeLists.txt
    A openmp/libomptarget/deviceRTLs/amdgcn/src/amdgcn_locks.hip

  Log Message:
  -----------
  [libomptarget] Implement locks for amdgcn

Summary:
[libomptarget] Implement locks for amdgcn

The nvptx implementation deadlocks on amdgcn. atomic_cas with multiple
active lanes can deadlock - if one lane succeeds, all the others are locked
out. The set_lock implementation therefore runs on a single lane.

Also uses a sleep intrinsic instead of the system clock for a probably
minor performance improvement. The unset/test implementations may be revised
later, based on code size / performance or similar concerns.

This implements the lock at a per-wavefront scope. That's not strictly as
specified, since openmp describes locks in terms of threads. I think the
nvptx implementation provides true per-thread locking on volta and the same
per-warp locking on other architectures.

Reviewers: jdoerfert, ABataev, grokos

Reviewed By: jdoerfert

Subscribers: jvesely, mgorny, jfb, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D75546