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perf: fix N=4@64MHz 7/8->8/8 by removing any_pending fan-out (user's idea)

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Real critical-path trace after the flash #1 revert showed a NEW
bottleneck (neural_director.job_out_slot -> dependency_manager.
node_resolved/node_state, 76-84% routing) distinct from every prior
fix this session -- already a flat, parallel 64-way compare, not a
serial chain, so the established restructuring fix class doesn't
apply. Root cause: any_pending (added for FPGA_DATA_READY) reads
node_state[0:N_NODES-1] combinationally every cycle, adding real
fan-out onto the same congested signal.

User's own suggestion: replace the combinational scan with a
synchronous up/down counter. pending_count +1 on registration
acceptance, -1 on dispatch acceptance; any_pending = (pending_count
!= 0) -- mathematically identical (DEC-0008: nodes never reclaimed
mid-run) but reads one small register instead of scanning a 16-wide
array every cycle.

Verified: D-Stress N=4 bit-exact (49927 cycles, data_ready PASS).
Fresh 8-seed P&R: N_SLOTS=4 @ 64MHz now 8/8 PASS (was 7/8 after the
flash revert), worst seed1 64.55MHz, best seed0 72.37MHz.

See decisions.log DEC-0042.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013xXuuRUWZScuo1DeYJxs3v
This commit is contained in:
michele
2026-09-07 19:16:01 +02:00
co-authored by Claude Sonnet 5
parent ee708775c7
commit 481b5d223f
2 changed files with 91 additions and 11 deletions
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hardware/v2/logs/decisions.log
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@@ -2402,3 +2402,63 @@ STATUS: two-flash architecture and FPGA_DATA_READY CLOSED and
verified (bit-exact + real placement). Flash #1's RTL port
(`flash_copy_engine.v` integration into V2's top-level) remains a
real, separate, OPEN task -- ball positions reserved, not wired.
DEC-0042 -- Flash #1 removed (user priority: MHz over on-board
persistence); dependency_manager's any_pending switched from a
combinational scan to a counter, recovering N_SLOTS=4 @ 64MHz to 8/8
DATE: 2026-09-07
CONTEXT: DEC-0042's own predecessor (see the reverted commit) closed
flash #1 functionally but caused a real, disclosed timing regression
(N_SLOTS=4 8/8 -> 3/8). User's explicit decision: prioritize clock
frequency over on-board flash persistence -- "preferisco avere un
Megahertz più alti che avere la flash" -- ESP32 can push data fresh
each session instead. Flash #1 fully reverted (`git revert`, clean,
history-preserving) rather than hand-edited back out.
POST-REVERT REAL BASELINE (fresh 8-seed matrix, first rigorous
measurement of this exact RTL state -- includes the earlier sdram_clk
fix, never previously swept at full seed count): N_SLOTS=4 @ 64MHz
7/8 PASS (worst seed1 62.47MHz FAIL; best seed4 77.21MHz). N_SLOTS=8
@ 64MHz 3/8 PASS. User's explicit direction: focus exclusively on
N_SLOTS=4 for now, treat N_SLOTS=8 as a later/separate effort.
ROOT CAUSE of the remaining N=4 seed1 failure (real critical-path
trace, not guessed): a NEW bottleneck class, distinct from every prior
fix this session (ERR-0027/0028/0029, all serial-to-parallel logic
restructurings) -- `neural_director.job_out_slot` ->
`dependency_manager.node_resolved`/`node_state`, 76-84% routing.
Inspected the actual logic: already a flat, parallel 64-way compare
(N_NODES x MAX_DEPS independent comparators, no serial chain), so the
established "flatten a chain" fix class does not apply. Root cause is
signal FAN-OUT, not logic depth: this session's own earlier
`any_pending` addition (DEC-0041/ERR's own FPGA_DATA_READY work) reads
`node_state[0:N_NODES-1]` combinationally every cycle (a 16-wide
OR-reduce), adding real fan-out load onto the SAME `node_state` array
already sitting on this critical path.
FIX (user's own idea, credited): replace the combinational OR-reduce
with a synchronous up/down counter -- `pending_count` incremented on
a node's own registration acceptance (`reg_valid&&reg_ready`, the
exact edge it enters WAITING/READY) and decremented on its own
dispatch acceptance (`ready_valid&&ready_ready`, the exact edge it
leaves WAITING/READY for DISPATCHED); `any_pending = (pending_count !=
0)`. Mathematically identical to the original OR-reduce (DEC-0008:
nodes are never reclaimed mid-run, so registered-count minus
dispatched-count exactly equals "currently WAITING or READY"), but
reads one small registered counter instead of scanning a 16-wide array
every cycle -- zero added fan-out on the congested node_state signal.
File changed: `hardware/v2/rtl/dependency_manager.v`.
VERIFICATION: D-Stress N_SLOTS=4 re-run, bit-exact (49927 cycles,
identical to before, 256/256 vs golden), `data_ready` PASS (confirms
the counter-based reimplementation is semantically identical in
practice, not just in theory). Fresh 8-seed nextpnr-ecp5 P&R matrix:
**N_SLOTS=4 @ 64MHz now 8/8 PASS** -- worst seed1 64.55MHz (real but
thin margin, +0.55MHz), best seed0 72.37MHz.
STATUS: N_SLOTS=4 @ 64MHz CLOSED, 8/8, real production baseline.
N_SLOTS=8 explicitly deferred by user request, not attempted further
this pass. Flash #1 removed; its RTL (flash_mem_adapter.v,
tb_flash_integration_smoke.v, the OP_FLASH_CMD opcode) remains in git
history (revertible commit `59901a4`) if ever needed again.