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Phase 4 plan + Phase 5 second-model review (PASS-WITH-REVISIONS)

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Phase 4 — Plan first QPU IDCT8 kernel given the M5 32.95us/dispatch
constraint. Frame-level dispatch (8160 WGs for 1080p), 64
invocations/WG = 4 blocks/WG, 3 SSBOs + push constants, reproduces
FFmpeg's transposed column-pass orientation. Predicted M2 ~16 Mblock/s
(BW-limited), R ~ 2.0 -> strong PASS under phase1.md decision rules.

Phase 5 — Second-model review by Claude Sonnet (fresh-context Agent,
no prior session memory). Verdict PASS-WITH-REVISIONS with 2 RED-class
findings + 1 YELLOW that this commit applies:

  RED  finding 5 (dst race condition): int32_t[] dst with 4 lanes
       writing to overlapping 32-bit words = non-atomic concurrent
       writes = Vulkan UB. Fix: uint8_t[] via storageBuffer8BitAccess
       (verified exposed). Applied to phase4.md sec 5 + GLSL declaration.

  RED  finding 7 (early-return before barrier): if (block_idx >=
       n_blocks) return; ahead of barrier() is UB by Vulkan spec.
       For 1080p (32640 blocks, /4) no partial WGs; for any frame
       width not /32 there are. Fix: oob flag, gate work bodies,
       barrier() unconditional. Applied to phase4.md sec 4 pseudocode.

  YELLOW finding 6 (subgroup ops): docs claimed BASIC+VOTE only;
         actual exposed set is BASIC+VOTE+BALLOT+SHUFFLE+
         SHUFFLE_RELATIVE+QUAD per vulkaninfo. Plan doesn't use any
         subgroup ops in v1 so unaffected, but the wrong constraint
         would mislead Phase 6/7. Corrected in phase0.md sec 2,
         phase2.md sec 6, phase4.md sec 1 (C4).

GREEN/YELLOW findings 1-4, 8 (orientation, WG geom, idle lanes, BW
prediction, compute envelope accounting) accepted as-is or deferred to
Phase 7 M6 sweep per plan's existing flagging.

Reviewer verdict post-revisions: "Phase 4 is APPROVED for Phase 6
implementation. No re-review needed; revisions are mechanical and
address verified bugs/errors."

Phase 5 itself just paid for itself: two real UB bugs caught before
any GLSL was written.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
Markus Fritsche
2026-05-18 11:47:03 +00:00
parent dcbbc77038
commit 71db72928f
4 changed files with 550 additions and 2 deletions
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docs/phase0.md
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@@ -68,7 +68,7 @@ Captured live 2026-05-17 via SSH. Full `vulkaninfo` in
| `maxComputeWorkGroupInvocations` | 256 | Standard |
| `maxComputeWorkGroupSize` | 256 / 256 / ? | Standard |
| `subgroupSize` | 16 (fixed) | Matches QPU SIMD width |
| `subgroupSupportedOperations` | BASIC + VOTE only | No arithmetic reductions — accumulate via shared memory |
| `subgroupSupportedOperations` | BASIC + VOTE + BALLOT + SHUFFLE + SHUFFLE_RELATIVE + QUAD | No arithmetic reductions — accumulate via shared memory. `subgroupShuffle` *is* available (corrected per phase5.md finding 6 — earlier text said BASIC+VOTE only). |
| `shaderFloat16` | **false** | Storage only; arithmetic runs FP32 |
| `shaderInt8` | **false** | Storage only; arithmetic on widened ints |
| `shaderInt16` | **false** | Same |