[all-commits] [llvm/llvm-project] 547e9d: [OpenMP] Fix endian bug in hierarchical barrier co...

[Ulrich Weigand via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 547e9d0c9175f37c9ddd367bbe443a2fd7d6ab7a
      https://github.com/llvm/llvm-project/commit/547e9d0c9175f37c9ddd367bbe443a2fd7d6ab7a
  Author: Ulrich Weigand <ulrich.weigand at de.ibm.com>
  Date:   2026-07-15 (Wed, 15 Jul 2026)

  Changed paths:
    M openmp/runtime/src/kmp_barrier.cpp

  Log Message:
  -----------
  [OpenMP] Fix endian bug in hierarchical barrier code (#117073)

The "oncore" flavor of the hierarchical barrier code uses a single
uint64 to hold both a barrier state *and* a set of flags for the current
node's "leaf kids" in the tree.

To make this work, the barrier state is placed in the least-significant
byte of the uint64, and the flags for leaf kids are placed in other
bytes of the uint64 starting from the most-significant byte to represent
the first child.

At least, this is how it works on little-endian systems. On big-endian
hosts, the current code unfortunately places the leaf kid flags starting
from the *least* significant byte of the uint64, where they will overlap
and clobber the barrier state byte, resulting in corrupted barrier
behavior.

To fix this, this PR changes the child flag offsets to always start with
the MSB, on both big- and little- endian hosts. This was complicated a
bit by the following issues:

- The byte offset was stored "off-by-one" for some reason. This makes it
impossible to represent the MSB on big- endian systems (i.e. byte 0,
which would have to be represented by -1 if offset by one, which does
not fit into the uint8 offset field).

Fixed by storing and using the "offset" field as the actual byte number,
not off-by-one.

- The "offset" field was never used for the master node, but it was
still computed. That computed value was also invalid as it pointed
outside the target uint64, but that didn't matter as it was unused. For
big-endian systems, however, the computation would have resulted in a
*negative* value, which again cannot be stored in the uint8 field.

Fixed by skipping the (unused) computation on the master node and
setting the "offset" field to a dummy value.

This is intended to be NFC on little-endian machines.



To unsubscribe from these emails, change your notification settings at https://github.com/llvm/llvm-project/settings/notifications