EXP-0056: N_SLOTS=16 failed timing on LFE5U-85F (23-24MHz vs 64MHz
target). First hypothesis (dependency_manager.v's serial ready-scan)
was wrong but real -- built and verified priority_encoder_lsb.v (a
generic recursive tree encoder) and dependency_manager_fast.v, bit-
exact equivalent to the original, but integrated it made no real
difference (24.26MHz). The real cause, found from nextpnr's own
critical-path report: nms_activation_fill_ctrl_v3.v's balanced max-
tree was only ever extended to N_SLOTS in {1,2,4,8}, silently falling
back to the original slow scan for 16. Added the missing case
(nms_activation_fill_ctrl_v3_n16.v), verified isolated (10017/10017)
and functionally (D-Stress N=16 still 256/256 bit-exact). Real result:
71.01MHz, PASS at 64MHz (single seed so far).
EXP-0057: built layer_weight_buffer.v, a double-buffered per-layer
weight scratchpad (fill one buffer in the background from SDRAM while
compute reads many times from the other -- weight-stationary reuse,
as opposed to D-Stress's own deliberately zero-reuse pattern). Wired
to the real sdram_controller_openrow.v + sdram_model.v, no new
hardware. For the same 32768 bytes of useful data: zero-reuse costs
27048 real cycles, reuse costs 3777 -- 7.16x real measured speedup on
the SAME SDR SDRAM, no DDR3, no clock change. This is the answer to
whether DDR3 is necessary for a workload class that actually has
reuse (e.g. conv-style face recognition, unlike D-Stress) -- it isn't,
at least not for this reason.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MUG92aM9m68TRc4rG55BcC
micheleandClaude Sonnet 5
1ce78dff6e