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perf(v2): shared activation cache - further 1.66-2.00x real speedup (DEC-0016)

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Implements optimization #2 from the final benchmark campaign's own
recommendation, on top of DEC-0015's word-level burst rewrite: a new
shared activation_cache.v module fetches a given activation (X)
vector from PSRAM once instead of once per neuron sharing it - the
exact redundant traffic pattern the dense-layer workloads in this
project's benchmark suite exhibit.

Each memory_manager's own prefetch_engine now fetches WEIGHTS only;
the activation half is requested from the shared cache instead
(single-tag, tile-granular, N_SLOTS request ports, its own real
word-level PSRAM backend via a new dedicated arbiter port).
dataflow_core.v/slot_mem_arbiter.v/neural_multiprocessor.v widened to
N_SLOTS+1 ports to arbitrate the cache's traffic alongside each
slot's weight traffic.

Two real bugs found and fixed during implementation (ERR-0010): a
target-bank/pending-bank race in memory_manager.v's activation-cache
wiring (the same bug class ERR-0006 already fixed once for
pf_target_bank - a later handoff's queued request can overwrite which
bank an earlier, still-in-flight request's ack applies to), and a
repeat of ERR-0009's N_SLOTS=1 zero-width replication bug in
activation_cache.v itself.

Real, measured results: the full final-benchmark campaign (24/24
workload/config combinations) re-verified bit-exact. D-Stress cycles
fall a further 1.66-2.00x on top of DEC-0015 (~4x combined vs the
original byte-level baseline). But the cache's real Fmax cost is much
steeper than DEC-0015's own: N_SLOTS=2 (the recommended default,
DEC-0014) drops from 133.58 to 87.72 MHz (-34%, margin over 80MHz
shrinks from +67% to +9.7%), and N_SLOTS=4 drops to 65.01 MHz - now
FAILING the 80MHz target it previously passed. Combined real
wall-clock speedup vs the original baseline: N=1 3.86x, N=2 2.45x
(both real net wins); N=4 is a real regression once its own now-failing
Fmax is honestly used, though N=4 was never the recommended
configuration.

N_SLOTS=2 remains the recommended default (DEC-0014 unaffected) with
a thinner but still real Fmax margin. Cache hit-detection pipelining
is flagged as concrete follow-up work if N_SLOTS>2 is ever needed with
the cache active - not attempted this round.

Logged: simulation/synthesis/timing/benchmark/decisions (DEC-0016)/
experiments (EXP-0016)/errors (ERR-0010)/development.log, ROADMAP.md
updated.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013xXuuRUWZScuo1DeYJxs3v
This commit is contained in:
michele
2026-09-05 20:59:14 +02:00
co-authored by Claude Sonnet 5
parent e4a5540b6e
commit 63cac6a7e5
16 changed files with 931 additions and 211 deletions
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hardware/v2/sim/tb_dataflow_core.v
+27 -14
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@@ -96,11 +96,21 @@ module tb;
reg [ADDR_WIDTH-1:0] reg_x_base, reg_w_base, reg_result_addr;
reg [15:0] reg_n_tiles;
wire [N_SLOTS-1:0] slot_mem_req, slot_mem_wr;
wire [ADDR_WIDTH*N_SLOTS-1:0] slot_mem_addr;
wire [16*N_SLOTS-1:0] slot_mem_wdata, slot_mem_rdata;
wire [N_SLOTS-1:0] slot_mem_lb_n, slot_mem_ub_n;
wire [N_SLOTS-1:0] slot_mem_ready;
// Arrays sized N_SLOTS+1 post-M10 (decisions.log DEC-0016) -- index
// N_SLOTS is the shared activation_cache's own backend port. Each
// index still gets its OWN independent behavioral memory (matches
// this testbench's own pre-existing scope: real shared-PSRAM
// arbitration across slots is M8's job, not exercised here) --
// X data is poked ONCE into memory index N_SLOTS (the cache's own,
// single shared backing store) rather than duplicated per-slot,
// since X now genuinely flows through ONE shared path regardless
// of which slot a job lands on; W data is still poked into every
// slot's own memory (unchanged), since W is not shared.
wire [N_SLOTS:0] slot_mem_req, slot_mem_wr;
wire [ADDR_WIDTH*(N_SLOTS+1)-1:0] slot_mem_addr;
wire [16*(N_SLOTS+1)-1:0] slot_mem_wdata, slot_mem_rdata;
wire [N_SLOTS:0] slot_mem_lb_n, slot_mem_ub_n;
wire [N_SLOTS:0] slot_mem_ready;
dataflow_core #(
.DATA_WIDTH(DATA_WIDTH), .P_IN(P_IN), .ACC_WIDTH(ACC_WIDTH), .ADDR_WIDTH(ADDR_WIDTH),
@@ -118,7 +128,7 @@ module tb;
genvar g;
generate
for (g = 0; g < N_SLOTS; g = g + 1) begin : GEN_MEM
for (g = 0; g < N_SLOTS+1; g = g + 1) begin : GEN_MEM
sim_word_mem #(.ADDR_WIDTH(ADDR_WIDTH), .DEPTH(4096)) u_mem (
.clk(clk), .rst(rst),
.req(slot_mem_req[g]), .wr(slot_mem_wr[g]),
@@ -143,6 +153,8 @@ module tb;
else tb.GEN_MEM[0].u_mem.mem[word_addr][15:8] = val;
1: if (byte_addr[0]==1'b0) tb.GEN_MEM[1].u_mem.mem[word_addr][7:0] = val;
else tb.GEN_MEM[1].u_mem.mem[word_addr][15:8] = val;
2: if (byte_addr[0]==1'b0) tb.GEN_MEM[2].u_mem.mem[word_addr][7:0] = val; // shared activation_cache backing store (N_SLOTS index)
else tb.GEN_MEM[2].u_mem.mem[word_addr][15:8] = val;
default: ;
endcase
end
@@ -155,6 +167,7 @@ module tb;
case (slot)
0: peek = (byte_addr[0]==1'b0) ? tb.GEN_MEM[0].u_mem.mem[word_addr][7:0] : tb.GEN_MEM[0].u_mem.mem[word_addr][15:8];
1: peek = (byte_addr[0]==1'b0) ? tb.GEN_MEM[1].u_mem.mem[word_addr][7:0] : tb.GEN_MEM[1].u_mem.mem[word_addr][15:8];
2: peek = (byte_addr[0]==1'b0) ? tb.GEN_MEM[2].u_mem.mem[word_addr][7:0] : tb.GEN_MEM[2].u_mem.mem[word_addr][15:8];
default: peek = 8'sdx;
endcase
end
@@ -193,15 +206,15 @@ module tb;
rst = 0;
@(posedge clk);
// Pre-load PSRAM-equivalent memory for both slots (a job could
// land on either slot, first-free, so both need the data).
// Pre-load PSRAM-equivalent memory. W (per-slot, not shared)
// still needs to land in EVERY slot's own memory (a job could
// land on either slot, first-free). X (post-DEC-0016) flows
// through the ONE shared activation_cache instead -- poked
// once into memory index N_SLOTS(=2)'s backing store.
for (i = 0; i < 8; i = i + 1) begin
poke(0, 23'h10+i, 8'sd2); poke(0, 23'h20+i, 8'sd3); // node0: x=2,w=3
poke(1, 23'h10+i, 8'sd2); poke(1, 23'h20+i, 8'sd3);
poke(0, 23'h30+i, 8'sd1); poke(0, 23'h40+i, 8'sd1); // node1: x=1,w=1
poke(1, 23'h30+i, 8'sd1); poke(1, 23'h40+i, 8'sd1);
poke(0, 23'h50+i, 8'sd1); poke(0, 23'h60+i, 8'sd5); // node2: x=1,w=5
poke(1, 23'h50+i, 8'sd1); poke(1, 23'h60+i, 8'sd5);
poke(2, 23'h10+i, 8'sd2); poke(0, 23'h20+i, 8'sd3); poke(1, 23'h20+i, 8'sd3); // node0: x=2,w=3
poke(2, 23'h30+i, 8'sd1); poke(0, 23'h40+i, 8'sd1); poke(1, 23'h40+i, 8'sd1); // node1: x=1,w=1
poke(2, 23'h50+i, 8'sd1); poke(0, 23'h60+i, 8'sd5); poke(1, 23'h60+i, 8'sd5); // node2: x=1,w=5
end
// node0, node1: no dependencies. node2: depends on BOTH.