Mirrors the proven all_reduce ring kernel: uses a separate ring_buffer
on the symmetric heap for receiving data, simple 0/1 flag toggling,
no step-counting. Each step: send current data to next rank's
ring_buffer, wait for predecessor to write into our ring_buffer,
copy received data to correct output slot, forward it next step.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Triton's % operator uses C truncated-division semantics where
(-1) % 4 = -1, not Python's floored-division where (-1) % 4 = 3.
This caused recv_rank_idx and source_rank_idx to be negative for
ranks where group_rank < step, writing to invalid output locations.

Fix: add world_size before the modulo to ensure non-negative operands.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Align with codebase convention (README, benchmarks, docs all use ctx).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Pre-allocates ring_buffer and flags on the symmetric heap once.
Pass the workspace to all_gather to avoid per-call allocation overhead
(~13ms saved per call in benchmarks).

Follows the same pattern as AllReduceWorkspace / all_reduce_preamble.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

The tracing API's record_event_start requires block pointers (for tl.min
reduction), not scalar pointers. Restructure to trace full ring steps
using the ring_buffer tile address instead of individual atomic ops.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

… flags

Replace O(tiles * steps) remote atomic operations with O(steps) by using
a global barrier. All CUs write tiles in parallel for each step, then
synchronize via a local counter + single remote signal. This eliminates
the massive atomic contention that caused ~10x slowdown vs RCCL.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Previous global barrier had two bugs:
1. Race between step reset and CU entry (fixed: monotonic counter)
2. Write-after-read hazard on single ring_buffer (fixed: double-buffer)

Step k reads from buf[k%2], writes to next rank's buf[(k+1)%2].

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Replace per-tile flag handshake with per-chunk handshake to amortize
atomic synchronization overhead. The shard is split into NUM_CHUNKS
row-bands, each with one flag. CUs process all tiles within a chunk
before signaling, reducing atomics from 4*total_tiles*steps to
4*num_chunks*steps. Different CUs work on different chunks at
different ring steps, creating a pipeline that keeps XGMI links busy.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Defer local flag reset until AFTER reading ring_buffer for forwarding
in the next step's send phase. Previously, resetting the flag at the
end of the receive phase allowed the predecessor to overwrite ring_buffer
before we read it for forwarding, causing data corruption.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>