Port async device-mirror combine/scatter kernels to ROCm

[ghazni101]

Closes #2776.

Row: GFX1100-TG200

The async device-mirror path eliminates the D2H sampled-token round trip by
keeping last_sampled_tokens device-resident and splicing them into
input_ids on the GPU. The path was behind #ifdef VLLM_CPP_CUDA in
runner.cpp, so ROCm got none of it despite SupportsAsyncSampledTokenReadback()
already returning true for the ROCm backend.

Port the three trivial CUDA kernels (CombineKernel, ScatterLastSampledKernel,
ApplyLastSampledOpsKernel) to HIP in src/vt/rocm/rocm_combine_tokens.hip,
with declarations in include/vt/rocm/combine_tokens.h. Add a build-time
dispatch layer (#if/#elif) in runner.cpp that compiles the right backend
unconditionally.

Widen all #ifdef VLLM_CPP_CUDA guards in the async mirror path to include
ROCm. Token-exactness verified: mirror ON == OFF on the acceptance workload
(256-token greedy decode, Qwen3.5-4B Q4_K_M). Build clean on ROCm 7.15.
Token-identical to upstream baseline on 32-token greedy decode, seed 0.

Performance: wash on both CLI and server paths. The port is a prerequisite
for future async optimizations, not a standalone lever.

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[VikashLoomba]

Recommended disposition: close this stale ROCm async implementation from the gfx1100 experiment queue. The historical report calls its CLI and server result a wash. It does not establish a current-head gain.

At 2ff54f0d3474ee9070bf1ae074ab0f7616c48d02, the combine dispatcher accepts nine arguments but its two callers pass twelve. RocmBackend also inherits SupportsAsyncSampledTokenReadback() == false, so the normal ROCm runner does not enable the advertised path. The old build and token claims do not certify this head.

Issue #2776 must remain open for a complete current-API port and a production reachability gate. Its Row metadata also needs reconciliation from the campaign label to the owning BACKEND-ROCM matrix row. This closure does not mark that feature fixed. The closed PR retains the prototype source and history.

The audited head is 2ff54f0d3474ee9070bf1ae074ab0f7616c48d02. The current contributor account lacks permission to close this PR; this comment records the evidence and requested disposition for its author or a maintainer.

[localai-org-maint-bot]

Bouncing this one — it does not compile, on its own branch and not only after a merge.

DispatchCombineSampledAndDraftTokens is defined with nine parameters:

void DispatchCombineSampledAndDraftTokens(
    vt::Queue& q, int32_t* input_ids, const int32_t* idx_mapping,
    const int32_t* last_sampled_tokens, const int32_t* query_start_loc,
    const int32_t* seq_lens, const int32_t* prefill_len, int num_reqs,
    int num_new_sampled_tokens) {

Both call sites pass twelve arguments:

      DispatchCombineSampledAndDraftTokens(
          queue_, dev->input_ids, /*idx_mapping=*/nullptr, dev->last_sampled,
          dev->query_start_loc, dev->seq_lens, dev->prefill_len,
          /*draft_tokens=*/nullptr, /*draft_tokens_stride=*/0,
          /*cu_num_logits=*/nullptr, num_reqs,
          /*num_new_sampled_tokens=*/1);

The wrapper was written against the nine-parameter Launch... signature in
include/vt/cuda/combine_tokens.h, but the call sites in this branch's own
runner.cpp are the twelve-argument form. That is the build-test-cpu,
build-newest-gcc, build-test-cpu-arm64, build-test-vulkan,
cuda-fat-build and verify reds on this pull request: those six are not on
main's baseline, unlike windows-msvc-* and sanitize-cpu.

Two smaller things to fix in the same pass:

1. The include guard is doubled, and the inner one pulls the CUDA header into a
   HIP-only build:

   #if defined(VLLM_CPP_CUDA) || defined(VLLM_CPP_HIP)
   #if defined(VLLM_CPP_CUDA) || defined(VLLM_CPP_HIP)
   #include "vt/cuda/combine_tokens.h"
   #endif
   #ifdef VLLM_CPP_HIP
   #include "vt/rocm/combine_tokens.h"
   #endif
   #endif

   The inner guard should be #ifdef VLLM_CPP_CUDA.

2. On a CPU-only build the three Dispatch* functions are defined in an
   anonymous namespace with empty bodies and no caller, which is
   -Werror=unused-function. This file already handles that case explicitly for
   AsyncDeviceMirrorEnvDefault, guarded "with its only use below"; the new
   helpers need the same treatment.

Nothing is wrong with the idea or with rocm_combine_tokens.hip itself — the
port reads fine and the three kernels mirror the CUDA ones. Please rebase on
current main, fix the arity against whichever signature that base carries, and
confirm a CPU-only configure builds before pushing. Happy to merge it after
that.

The rest of the GFX1100-TG200 stack is landing today without this one; it is
independent of them, so it will merge cleanly on its own once it builds.

[ghazni101]

Repaired per the bounce. Head 7f23c99 on current upstream/main, three commits: the port (71ccf5d), the 12-arg CUDA-identical launcher contract (be21583 — definition, both call sites, and both CUDA/ROCm launchers take 12 params), and the compile+reachability fix (7f23c99): include guards split (#ifdef VLLM_CPP_CUDA vs #ifdef VLLM_CPP_HIP, no header pollution), Dispatch* stubs compiled out on CPU-only builds (the AsyncDeviceMirrorEnvDefault treatment), and RocmBackend::SupportsAsyncSampledTokenReadback() returns true (rocm_backend.hip:338-345) so the ported kernels are REACHED, not dead. Both lanes build green in docker: CPU-only (HIP OFF — the lane that was red) and HIP/gfx1100. check-env-doc and check-agent-record green. Issue #2776 stays open. Ping for re-review.

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[localai-org-maint-bot]

All three findings from my earlier review are fixed — I verified each against the
current head 7f23c995b:

• the wrapper is 12 parameters and both call sites pass 12, matching
  vt::cuda::LaunchCombineSampledAndDraftTokens on current main
• the doubled include guard is gone, and vt/cuda/combine_tokens.h is now under
  #ifdef VLLM_CPP_CUDA alone rather than being pulled into a HIP-only build
• the whole Dispatch* block sits behind
  #if defined(VLLM_CPP_CUDA) || defined(VLLM_CPP_HIP), so a CPU-only build
  compiles it out instead of hitting -Werror=unused-function, and the comment
  says why

It needs a rebase before I can gate it. It merged clean onto main when I
probed a few hours ago; main has moved since and it now conflicts:

CONFLICT (content): Merge conflict in src/vllm/v1/worker/gpu/runner.cpp

That is the async device-mirror region itself, so I am not hand-resolving it —
a resolution that compiles but drops one arm of the mirror is exactly the failure
this file punishes, and it is your change's own territory.

The gate is written and queued for the moment it applies. Two arms, because your
three defects lived in two different builds: a HIP build for gfx1151, and a
CPU-only build with neither backend defined, which is precisely where the
unused-function break was. strix:gpu0 is gfx1151, not the gfx1100 the row
targets, so it establishes that the port compiles and stays inert where it should
— not the ROCm kernels' behaviour on your hardware.

Rebase onto origin/main and ping me; I will rerun it and merge if it comes back
clean. One caution: stage/ext-prs-2026-09-04 is deleted — it was a scratch
branch of mine and never a landing target, so rebase onto origin/main itself.

[ghazni101]

Rebased onto upstream/main (6f5e9dc). The base is now origin/main, not stage/ext-prs-2026-09-04. The three findings from the earlier review remain fixed: 12-parameter dispatch wrapper, split include guards (#ifdef VLLM_CPP_CUDA vs #ifdef VLLM_CPP_HIP), and Dispatch* block behind #if defined(VLLM_CPP_CUDA) || defined(VLLM_CPP_HIP) so CPU-only builds compile it out. Ping for re-review.

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