fla-correctness-coverage

Testing & Quality

Guidelines for kernel correctness testing and coverage in fla/ops/** and related modules, including common Triton grid/addressing pitfalls. Helps decide what tests to add or run before an MR.

QUICK START

How to use this skill

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Source SKILL.md: https://github.com/fla-org/flash-linear-attention/blob/HEAD/.agents/skills/fla-correctness-coverage/SKILL.md

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FLA Correctness & Coverage Skill

Use this skill when adding or modifying a kernel in fla/ops/ (e.g., KDA, GDN, GLA, DeltaNet, NSA, etc.) and you need to verify correctness or close a coverage gap.

Workflow

List the current coverage matrix for the op you are touching.
Compare against the axes below.
Add tests for missing combinations that are reachable by user code.
Run the relevant tests and make sure they pass.

Public reference docs

When a task needs operator math or protocol details, read only the relevant reference file:

references/cp.md — context parallelism for linear attention, including KDA/GDN CP formulation.
references/delta-rule.md — Delta Rule operator background.
references/generalized-delta-rule.md — Generalized Delta Rule operator background.
references/simple-gla.md — Simple GLA operator background.

Do not load every reference by default; use these only when the touched code or test depends on that operator's math or distributed protocol.

Coverage axes

For each kernel, check coverage across these dimensions:

Axis	Values to cover
Sequence layout	dense, variable-length (`varlen`)
Direction	forward, backward
Gate mode	safe gate, non-safe gate (if applicable)
Beta mode	raw beta, post-sigmoid beta (if applicable)
QK normalization	with L2 norm, without L2 norm
State	initial state, final state (if the op supports state passing)
GVA	grouped value attention (GVA) enabled vs disabled
Head dimensions	`D != Dv` (different qk and v head dims)
Backend verifier	reference implementation, `torch.autograd.gradcheck`, and backend-specific sanity checks

Kernel implementation safety checks

Before adding or changing a Triton kernel, check these implementation details in addition to numerical tests:

Treat program IDs and grid-derived values as potentially narrow. On NVIDIA, non-first grid dimensions may be narrow; on AMD, Ascend, or other non-NVIDIA backends, every grid dimension may be narrow. Cast to tl.int64 before using them in address arithmetic.
Keep tensor address arithmetic in tl.int64, including block bases, strides, varlen sequence offsets, head offsets, and element offsets. Do not rely on int16 or int32 overflow behavior.
Do not introduce new tl.make_block_ptr use. Triton marks it deprecated; use TensorDescriptor / tl.make_tensor_descriptor when descriptor semantics are needed, or explicit tl.load / tl.store pointer arithmetic following an existing validated kernel pattern.
If a change touches grid shape, program-id mapping, varlen offsets, or pointer math, run a shape that exercises the changed path on NVIDIA and any supported non-NVIDIA backend, or add a precise verifier/skip for unsupported platforms.

Code style constraints

Use fla.utils.device and fla.utils.device_platform in tests instead of adding new hard-coded device strings.
Use IS_NVIDIA, IS_NVIDIA_HOPPER, IS_NVIDIA_BLACKWELL, IS_AMD, and IS_INTEL from fla.utils for platform-specific skips or branches.
Do not add new direct torch.cuda platform checks in correctness tests. If no existing helper covers the condition, add a small helper in fla.utils first.

Default open-source test paths

Use these paths when looking for existing tests or deciding where to add new ones:

tests/ops/test_kda.py — KDA kernel tests
tests/context_parallel/ — context-parallel variants (e.g., test_cp_kda.py, test_cp_gdn.py)
tests/models/test_modeling_kda.py — end-to-end model tests for KDA

Adapt the path to the specific op you are working on (replace kda with gdn, gla, nsa, delta, etc.).

What NOT to put in this skill

Internal-only test paths, local machine paths, private model names, and private workload identifiers.
The open-source skill only points to public tests and public operator docs.

Running tests

# Single op test
pytest tests/ops/test_kda.py -v

# Context parallel tests for the same op
pytest tests/context_parallel/test_cp_kda.py -v

# Model-level test
pytest tests/models/test_modeling_kda.py -v

# All dependent tests (see fla-mr-readiness skill)
python scripts/find_dependent_tests.py <changed_files>