# Isa Resource Diff > Detect per-kernel GPU resource regressions (VGPR, SGPR, register spills, scratch, static LDS) by diffing the final ISA before and after a change, using its `isa_resource_table.py` helper. Compile-only: needs no GPU and no profiler run, so it works on any target the compiler supports and runs in seconds. Use when asked whether a change increased register pressure, caused spilling, or hurt occupancy, when reviewing a kernel change for resource impact, or as a fast pre-check before spending a profiling run. Usage: /isa-resource-diff [] [--arch ] - Skill: `rocm/isa-resource-diff` (Agent Skill, multi-file: 2 files) - Install (CLI): `npx skillmds@latest add rocm/isa-resource-diff` - Raw SKILL.md: https://api.skillmd.com/api/skills/rocm/isa-resource-diff/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - Author: rocm (https://skillmd.com/u/rocm) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/rocm/isa-resource-diff --- # ISA Resource Diff Compare per-kernel register, spill, scratch, and LDS usage between two builds to catch resource regressions that functional tests do not surface. ## Pick the right skill first | Question | Skill | |---|---| | Did my change increase registers / cause spills / grow LDS? | **this skill** (compile-only, seconds, no GPU) | | *Why* is this kernel slow — which instructions stall, and on what? | `/kernel-trace-analysis` (needs a GPU run + rocprofv3 ATT trace) | | Which commit made it slow? | `/bisect-perf-regression` (needs a runnable benchmark) | | How do I collect a trace at all? | `/capture-kernel-trace` | This skill measures **resources, not time**. A clean result here does not mean performance is unchanged — it means register/LDS/spill pressure is unchanged. A regression here is a strong, cheap signal that is usually worth acting on before profiling, because spilling and occupancy cliffs dominate most kernel slowdowns. See §7 of `docs/kernel_tuning_guide.md` for what to do about one. ## Arguments | Argument | Required | Description | |---|---|---| | `` | No | What to run to produce dumps. Defaults to asking the user. Example: `pytest tests/kernels/test_softmax.py -q` | | `--arch ` | No | Target for compile-only runs, e.g. `gfx950`. Omit to use local hardware | If the user already has two dump directories or two JSON snapshots, skip to Step 3. ## Workflow ### Step 1 — Capture the "before" side Check out or stash to the baseline state first, then: ```bash FLYDSL_DUMP_IR=1 FLYDSL_DUMP_DIR=/tmp/isa-before FLYDSL_RUNTIME_ENABLE_CACHE=0 \ python3 -m pytest tests/kernels/test_softmax.py -q ``` `FLYDSL_RUNTIME_ENABLE_CACHE=0` is **required, not optional** — see Pitfalls. For a target without local hardware, add `ARCH= COMPILE_ONLY=1`: ```bash ARCH=gfx950 COMPILE_ONLY=1 \ FLYDSL_DUMP_IR=1 FLYDSL_DUMP_DIR=/tmp/isa-before FLYDSL_RUNTIME_ENABLE_CACHE=0 \ python3 -m pytest tests/kernels/test_softmax.py -q ``` ### Step 2 — Capture the "after" side Apply the change, then rerun **the identical command** into a *fresh* directory (`/tmp/isa-after`). Same test, same parameters, same arch, same cache setting. ### Step 3 — Diff ```bash python3 ${CLAUDE_SKILL_DIR}/scripts/isa_resource_table.py diff /tmp/isa-before /tmp/isa-after ``` The tool requires **Python 3.10+**. If `python3` is older it exits 2 with a clear message; use `python3.10 ${CLAUDE_SKILL_DIR}/scripts/isa_resource_table.py …` instead. Both sides may independently be a dump directory or a `.json` snapshot, so a baseline can be captured once and reused: ```bash python3 ${CLAUDE_SKILL_DIR}/scripts/isa_resource_table.py summarize /tmp/isa-before --json baseline.json python3 ${CLAUDE_SKILL_DIR}/scripts/isa_resource_table.py diff baseline.json /tmp/isa-after ``` ## Reading the output ``` * = regression trigger; other columns are informational. vgpr = total (arch+acc, LLVM's occupancy number); arch_vgpr/agpr are its split -- do not add them. kernel *vgpr arch_vgpr agpr ... *lds_static_bytes lds_read ... ------------------------------------------------------------------------------------------------- gemm::d128_fmha_fwd_kernel_0 942->960(+18) 942->960(+18) 0 ... 212992->229376(+16384) 12 compared 1 of 1 kernels; 0 unchanged; 1 changed; worsened: 2; improved: 0 RESULT: REGRESSION ``` Only changed and problematic kernels are printed. The **last** stdout line is always the `RESULT:` verdict and matches the exit code exactly — script against that line, not against a fixed number of them: when there are problems, a count of untrustworthy items is printed above it. **Columns marked `*` are regression triggers**; the rest are context. | Column | Trigger | Read from | What it is | |---|---|---|---| | `vgpr` | yes | `.vgpr_count` metadata | The count that decides occupancy: `arch + accumulator` where the register file is unified (gfx90a and later MFMA-capable parts), `max(arch, accumulator)` where it is split (gfx908) or where there are no AGPRs at all | | `arch_vgpr` | no | `.set` symbol `num_vgpr` | The arch VGPR count on its own | | `agpr` | no | `.set` symbol `num_agpr` | The accumulator VGPR count on its own | | `sgpr` | yes | `.sgpr_count` metadata | SGPRs including the fixed extras (VCC, XNACK, FLAT_SCRATCH) | | `numbered_sgpr` | no | `.set` symbol `numbered_sgpr` | SGPRs without those extras; tells a real increase from VCC becoming live | | `vgpr_spill` | yes | `.vgpr_spill_count` metadata | VGPRs the allocator spilled | | `sgpr_spill` | yes | `.sgpr_spill_count` metadata | SGPRs the allocator spilled | | `scratch_bytes` | yes | `.private_segment_fixed_size` metadata | Private segment per work-item; exact on every target | | `lds_static_bytes` | yes | `.group_segment_fixed_size` metadata | Statically allocated LDS per work-group | | `lds_read` / `lds_write` | no | instruction count | `ds_read`/`ds_load` and `ds_write`/`ds_store` sites | | `scratch_store` / `scratch_load` | no | instruction count | `scratch_*` sites; `n/a` where spilling goes through `buffer_*` | | `matrix_ops` | no | instruction count | MFMA / WMMA / sparse-MFMA (`v_smfmac_*`) sites | Three things are easy to misread: - **Do not add `arch_vgpr` and `agpr` to `vgpr`.** `vgpr` is LLVM's own occupancy number and is the only VGPR-family trigger; the other two are shown so you can tell *which half* moved, not so you can total them. On a unified register file `vgpr` already *is* their sum; on gfx908, where the files are split, it is `max(arch, agpr)` — which is still the occupancy number there, since a wave that needs 64 arch VGPRs and 60 AGPRs occupies 64 slots in each of two 256-entry files. Either way, moving accumulators into AGPRs — which the tuning guide recommends — deliberately does not count as a regression. - **`n/a` is not `0`.** It means the quantity does not exist on this target, e.g. `scratch_store`/`scratch_load` on a target that spills through `buffer_*`. Use `scratch_bytes`, which is exact everywhere. - **`?` means unparsed** — the tool could not read something it reports. Any `?` forces exit 2. Never treat it as unchanged. ### Exit codes | Code | stdout | Meaning | What an agent should do | |---|---|---|---| | `0` | `RESULT: OK` | Everything comparable, no trigger increased | Proceed | | `1` | `RESULT: REGRESSION` | Everything comparable, a trigger increased | Investigate — this is a real finding | | `2` | `RESULT: NOT TRUSTWORTHY` | The tool cannot answer | **Fix the inputs and rerun. Do not report "no regression"** | Exit `1` is a claim about the code under test; exit `2` is a claim about the tool's own confidence. Everything that would leave the answer partial reports `2` and never `1`: a crash, an empty dump directory, a dump file that does not parse or does not decode, a metadata entry with no kernel identity or a duplicated one, a negative resource count, a kernel present on only one side, and a target that differs — or that the tool cannot name — on either side. For scripting, `-q/--quiet` drops the table (the untrustworthy count still prints above the verdict), and `--json PATH` writes the full comparison as machine-readable JSON: ```bash python3 ${CLAUDE_SKILL_DIR}/scripts/isa_resource_table.py diff /tmp/isa-before /tmp/isa-after -q --json report.json case $? in 0) echo "no resource regression" ;; 1) echo "REGRESSION — see report.json" ;; *) echo "inconclusive — inputs are bad, do not claim a clean result" ;; esac ``` ## Acting on a regression Map the column that moved to a cause, then follow `docs/kernel_tuning_guide.md`: | Column increased | Usual cause | |---|---| | `vgpr` | More live values; larger tiles or deeper pipelining/unrolling | | `vgpr_spill` / `sgpr_spill` / `scratch_bytes` | Register pressure crossed the budget — normally the most damaging of these signals | | `lds_static_bytes` | Bigger shared tiles or added double buffering; may cross an occupancy step | | `sgpr` alone, by 2, with `numbered_sgpr` flat | Usually just VCC becoming live — rarely meaningful | If a resource regression is confirmed but the kernel is not actually slower, say so rather than "fixing" it: these are proxies for occupancy, not timings. Confirm with `/kernel-trace-analysis` before reworking a kernel. ## Pitfalls These silently produce a *confident wrong answer* if ignored: - **Dump directories are keyed by kernel name only, with no specialization key.** Every JIT specialization of one kernel writes to the same directory and overwrites the previous one, so a parametrized test leaves only its last variant. Diff one shape at a time when the answer must be exact. - **A cache hit produces no dump at all.** Hence `FLYDSL_RUNTIME_ENABLE_CACHE=0` on both sides. Without it a kernel can silently vanish from one side, which the tool reports as `ONLY IN BEFORE` and exit 2. - **`lds_static_bytes` is static LDS only.** A kernel using `SharedAllocator(static=False)` reports `0` no matter how much LDS it takes at dispatch; the tool prints a `dynamic LDS in use` note for it. LDS regressions in those kernels are invisible here. - **The stage number in `NN_final_isa.s` varies between runs.** Nothing cleans the dump directory, so reusing one can leave two files side by side. The tool uses the highest-numbered one and warns; prefer a fresh directory per run. - **Diffs across targets are refused** (exit 2). Register files and LDS banking differ between architectures, and `xnack`/`sramecc` change code generation within one, so both sides must report the same processor *and* the same target features. A target the tool cannot name is refused for the same reason. The triple's environment field is normalized, so `amdgcn-amd-amdhsa--gfx942` and `amdgcn-amd-amdhsa-unknown-gfx942` are the same target. - **Warnings on stderr never change the exit code.** They describe the input tree and are worth reading before trusting a clean result.