[koladefaj]

Purpose

Fixes #54843.

--use-fp64-gumbel promises fp64 exponential-race noise in spec-decode recovery sampling (#43150 extended the flag to this path). On the CPU backend the flag was silently a no-op: the CPU shim for sample_recovered_tokens_kernel downcast the fp64 inv_q to fp32 before calling the C++ kernel, which only read float*. The comment above the downcast claimed the value was read at its existing dtype, which was not true. With the flag on, CPU and GPU produced different recovered tokens for the same random draws in exactly the lower-tail regime the flag exists for (see the issue for the measured divergence).

This PR makes the CPU kernel honor the flag instead of guarding or warning on it:

• csrc/cpu/spec_decode_utils.cpp: the recovered-token loop is templated on the inv_q scalar type and dispatched with AT_DISPATCH_FLOATING_TYPES on inv_q.scalar_type(). The score and argmax accumulate in the noise dtype, matching the Triton kernel's USE_FP64_GUMBEL specialization. The fp32 instantiation is byte-for-byte the previous code path.
• vllm/utils/cpu_triton_utils.py: pass inv_q through unchanged and correct the comment.
• tests/v1/sample/test_rejection_sampler.py: regression test with adversarial inv_q rows (an fp64-resolvable near-tie, and lower-tail draws whose reciprocals overflow fp32). Random draws, as in the existing fp64 test, do not reliably expose the bug. The fp32 expectations in the same test guard that the fp32 path is unchanged.
• .buildkite/hardware_tests/cpu.yaml: run the new test in the CPU-Kernel Tests job. CPU CI did not previously run anything from test_rejection_sampler.py, and the test skips on GPU, so without this it would never run in CI.

Why this is not a duplicate: no open PR references #54843 or touches this kernel. The other contributor who expressed interest in the issue (@luyixiao95) proposed a warn/reject variant and has since said they are happy for this dispatch-based fix to proceed; @mosafariuk (issue filer) confirmed the dispatch approach is the one they scoped as the proper follow-up and offered to review.

Rebase note (2026-10-01): this branch was rebased onto current upstream/main to clear a merge conflict flagged by Mergify (was ~1,200 commits behind). Picking up #43048 (Triton kernel dispatcher refactor) in the process removed the module-level _FuncWrapper aliases (e.g. cpu_triton_utils.sample_recovered_tokens_kernel) that the new test called directly as kernel[(1, 1)](...). The test now calls cpu_triton_utils._sample_recovered_tokens_kernel_impl(...) directly, matching how vllm/platforms/cpu.py registers the CPU fallback post-refactor. No other file in this PR needed changes beyond that one test call. Rebuilt the CPU extension and re-ran everything below against the rebased commit to confirm nothing else broke.

Test Plan

CPU build (VLLM_TARGET_DEVICE=cpu, torch 2.13.0+cpu, x86-64 AVX2, rebuilt against the rebased commit):

pytest -x -v -s tests/v1/sample/test_rejection_sampler.py -k test_cpu_sample_recovered_tokens_preserves_fp64_inv_q
pre-commit run --files .buildkite/hardware_tests/cpu.yaml csrc/cpu/spec_decode_utils.cpp tests/v1/sample/test_rejection_sampler.py vllm/utils/cpu_triton_utils.py

Plus the reproduction script from #54843 (using current_platform.import_kernels() so the ISA-matched _C variant loads), and a negative control (temporarily restoring the inv_q.to(torch.float32) downcast) to confirm the new test actually catches the bug.

Test Result

New regression test, against the rebased commit (1132df761):

tests/v1/sample/test_rejection_sampler.py::test_cpu_sample_recovered_tokens_preserves_fp64_inv_q[inv_q_row0-1-0] PASSED
tests/v1/sample/test_rejection_sampler.py::test_cpu_sample_recovered_tokens_preserves_fp64_inv_q[inv_q_row1-1-0] PASSED
2 passed, 75 deselected

Negative control (downcast restored): both cases fail with assert 0 == 1, i.e. the kernel returns the fp32 answer when fp64 was requested — confirming the test exercises the fix.

Issue repro script against the fixed shim:

(a) normal range                   shim(flag=on)=[1]  fp64_ref=[1]  fp32_ref=[1]  -> OK (fp64 parity)
(b1) near-tie below fp32 eps       shim(flag=on)=[1]  fp64_ref=[1]  fp32_ref=[0]  -> OK (fp64 parity)
(b2) overflow steals argmax        shim(flag=on)=[1]  fp64_ref=[1]  fp32_ref=[0]  -> OK (fp64 parity)
(b3) NaN probe (0 * inf entry)     shim(flag=on)=[2]  fp64_ref=[2]  fp32_ref=[0]  -> OK (fp64 parity)
(ctl) flag off                     shim(flag=off)=[0]  fp32_ref=[0]
multi-req draft_probs fp64 path matches ref: True

No regressions: the rest of test_rejection_sampler.py (75 other tests) fails identically with and without this change on this CPU-only build — those tests invoke Triton GPU kernels directly and need torch.cuda support (AssertionError: Torch not compiled with CUDA enabled), unrelated to this change. That's also why CPU CI only wires in the one new test by name rather than the whole file. pre-commit run passes every applicable hook on the 4 changed files: ruff check, ruff format, clang-format, typos, mypy, SPDX-header check, and "Check new tests are wired into Buildkite CI" (confirms the cpu.yaml entry is correctly linked to the new test).

No model-output change on GPU. On CPU with --use-fp64-gumbel, recovered tokens now match the GPU Triton kernel for the same draws; with the flag off, behavior is unchanged.

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AI assistance disclosure: this change was developed with AI assistance (Claude Code). I reviewed every changed line, built the CPU extension locally, and ran the tests and repro above myself. The 2026-10-01 rebase, the test-call fix it required, and the subsequent CPU rebuild/test/pre-commit re-verification were also done with Claude Code, with me reviewing the diff and results.

🤖 Generated with Claude Code

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ℹ️ Recent review info

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Review profile: CHILL

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Run ID: 0a5d27c8-2166-49d3-a73b-b4a1a91ec8f2

📥 Commits

Reviewing files that changed from the base of the PR and between 784cac7 and 9b1fc6b474dd70cf59791040e59a723230e47ded.

📒 Files selected for processing (4)

• .buildkite/hardware_tests/cpu.yaml
• csrc/cpu/spec_decode_utils.cpp
• tests/v1/sample/test_rejection_sampler.py
• vllm/utils/cpu_triton_utils.py

🚧 Files skipped from review as they are similar to previous changes (4)

• vllm/utils/cpu_triton_utils.py
• tests/v1/sample/test_rejection_sampler.py
• csrc/cpu/spec_decode_utils.cpp
• .buildkite/hardware_tests/cpu.yaml

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Walkthrough

The CPU recovered-token sampler now preserves inv_q precision through Python and C++ dispatch. New CPU tests verify fp64 near-tie and overflow behavior while confirming the existing fp32 path. The CPU pipeline runs the new test and tracks its source dependencies.

Changes

CPU fp64 recovered-token sampling

Layer / File(s)	Summary
Preserve inv_q precision in the CPU kernel vllm/utils/cpu_triton_utils.py, csrc/cpu/spec_decode_utils.cpp	The CPU fallback passes inv_q without converting it to fp32. The C++ kernel dispatches scoring based on the inv_q scalar type.
Validate CPU precision behavior tests/v1/sample/test_rejection_sampler.py, .buildkite/hardware_tests/cpu.yaml	A CPU-only test covers fp64 near-tie and overflow cases, fp32 behavior, and the pipeline command tracks the required source dependencies.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to c6157

This change preserves fp64 recovered-token sampling on CPU and adds targeted CPU coverage while retaining fp32 behavior. No concrete merge-blocking risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 28.57% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 3 files. (1 skipped: 1…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The PR satisfies issue #54843 by preventing silent fp64 handling failure, correcting the misleading comment, preserving fp32 behavior, and implementing true fp64 CPU support for parity with GPU sampli…
Out of Scope Changes check	✅ Passed	The changes are limited to the CPU sampling implementation, its shim, regression coverage, and CPU CI configuration, all of which support the linked issue objectives.
Title check	✅ Passed	The title clearly identifies the CPU spec-decode bug and the fp64 Gumbel behavior it fixes.
Description check	✅ Passed	The description directly explains the CPU bug, implementation, regression tests, CI coverage, and expected behavior.

Full details: Docstring Coverage

Explanation

Docstring coverage is 28.57% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 3 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @koladefaj.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork