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Critical path patterns with corresponding optimization strategies. Use this when users request optimize the RTL code based on the timing analysis results.
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| name | rtl-opt |
| description | Critical path patterns with corresponding optimization strategies. Use this when users request optimize the RTL code based on the timing analysis results. |
| license | Complete terms in LICENSE.txt |
op_is_add, op_is_sub, state_is_idle) computed once and reusedcount_is_max, state_is_active), compute once, reuse across always blocksfull, enable)ptr+1, ptr+2), then select with late muxsel?(A+B):(C+D) structure(sel?A:C)+(sel?B:D) - move mux before addercount == MAX_COUNT)| Path Characteristic | Recommended Strategy | Confidence |
|---|---|---|
| FSM state decode to output | One-hot pre-decode (#1), FSM output registration (#4) | High |
| Counter with comparison | Condition pre-computation (#2), wire extraction | High |
| High-fanout control signal | Register duplication (#3), input registration (#5) | High |
| Arithmetic gated by control | Late mux-select (#6), mux-before-adder (#7) | Medium |
| Large case/lookup table | Hierarchical decomposition (#8) | Medium |
| GF/crypto arithmetic | Logic tree balancing (#9), CSE (#10) | Medium |
| Binary-encoded FSM | One-hot encoding (#11) - with caution | Medium |
| Deeply nested ternary | Wire decomposition (#14) - modest benefit | Low |
| Priority-encoded chain | AVOID parallel flattening | Anti-pattern |
| Memory array paths | SKIP - out of scope | N/A |
| Design Type | Best Strategies | SEC Pass Rate |
|---|---|---|
| FSM-heavy (ticket_machine, vending_machine) | One-hot (#1, #11), mux-before-adder (#7) | 60% |
| Communication (UART, SPI, router) | Input registration (#5), FSM output reg (#4), condition pre-compute (#2) | 70% |
| Pipeline (router, FIFO) | Register duplication (#3), input registration (#5) | 80% |
| Datapath (DSP, LSTM, datapath) | Tree balancing (#9), CSE (#10), wire decomposition (#14) | 65% |
| Encoder/Decoder (pcie) | Hierarchical decomposition (#8) | 85% |
| Controller (cpu_fsm, controller) | One-hot pre-decode (#1), condition pre-compute (#2) | 90% |
| Strategy | Attempts | Successes | Rate | Avg WNS Improvement |
|---|---|---|---|---|
| One-hot pre-decode | 12 | 11 | 92% | +8% |
| Condition pre-computation | 18 | 16 | 89% | +5% |
| Register duplication | 8 | 7 | 88% | +7% |
| FSM output registration | 10 | 9 | 90% | +8% |
| Input signal registration | 14 | 12 | 86% | +10% |
| Late mux-select | 6 | 4 | 67% | +12% |
| Mux-before-adder | 4 | 3 | 75% | +40% |
| Hierarchical decomposition | 5 | 4 | 80% | +35% |
| Logic tree balancing | 8 | 5 | 63% | +6% |
| CSE | 10 | 7 | 70% | +4% |
| One-hot FSM encoding | 8 | 5 | 63% | +35% |
| Parallel logic flattening | 6 | 2 | 33% | +35% (when successful) |
| Count direction change | 4 | 0 | 0% | N/A |
| Wire decomposition | 12 | 10 | 83% | +3% |
Conservative wins over aggressive: High SEC pass rate more important than large improvements. A 5% improvement that passes SEC beats a 30% improvement that fails.
Sequential optimizations are safer: Adding registers (strategies #3, #4, #5) rarely break SEC. Combinational restructuring is higher risk.
Pre-computation beats restructuring: Extracting conditions to wires (#1, #2) is safer than restructuring logic (#7, #11, #12).
Design complexity matters: Simple designs (FIFO, SPI) tolerate more aggressive optimization. Complex designs (LSTM, cpu_pipe) need conservative approaches.
Module-focused optimization works: Targeting specific modules (pcie decoder, datapath sBox) more successful than whole-design changes.
Diversity strategy helps discovery: Different path selection (endpoint clustering, high fanout, module-focused) finds different optimization opportunities.
Baseline sensitivity varies: Some designs (simple_spi) are already highly optimized; aggressive changes cause synthesis failures rather than SEC failures.
This file merges and normalizes the uploaded skill libraries into one comprehensive reference. It organizes the skills into High-confidence, Medium-confidence, Low-confidence, and Do not use. Each skill is written in the format:
<pattern, strategy, example>
For every example, both before and after Verilog snippets are included.
<repeated comparisons / repeated control checks, pre-compute shared condition wires and reuse them, example>(state == X), (cmd == Y), (count == Z) or repeated shared condition fragments across branches.always / assign logic. Very safe and consistently effective for FSM decode, counters, and control fanout.// BEFORE
always @(*) begin
if (state == IDLE && cmd == START)
next_state = START;
else if (state == IDLE && cmd == STOP)
next_state = STOP;
else
next_state = state;
end
// AFTER
wire state_is_idle = (state == IDLE);
wire cmd_is_start = (cmd == START);
wire cmd_is_stop = (cmd == STOP);
wire go_start = state_is_idle & cmd_is_start;
wire go_stop = state_is_idle & cmd_is_stop;
always @(*) begin
if (go_start)
next_state = START;
else if (go_stop)
next_state = STOP;
else
next_state = state;
end
<shared sub-conditions reused in multiple branches, build hierarchical shared condition wires before final checks, example>state == DATA && sck == HIGH.// BEFORE
assign fire_done = (state == DATA) && (sck == HIGH) && (count == MAX);
assign fire_shift = (state == DATA) && (sck == HIGH) && (count > THRESH);
// AFTER
wire state_is_data = (state == DATA);
wire sck_is_high = (sck == HIGH);
wire data_and_sck = state_is_data & sck_is_high;
wire count_is_max = (count == MAX);
wire count_is_gt = (count > THRESH);
assign fire_done = data_and_sck & count_is_max;
assign fire_shift = data_and_sck & count_is_gt;
<FSM / opcode / instruction decode used in many places, one-hot pre-decode encoded values before downstream logic, example>state_is_* / op_is_*, then use them directly.// BEFORE
assign do_add = (opcode == 4'b0001) && valid;
assign do_sub = (opcode == 4'b0010) && valid;
assign do_and = (opcode == 4'b0011) && valid;
// AFTER
wire op_is_add = (opcode == 4'b0001);
wire op_is_sub = (opcode == 4'b0010);
wire op_is_and = (opcode == 4'b0011);
assign do_add = op_is_add & valid;
assign do_sub = op_is_sub & valid;
assign do_and = op_is_and & valid;
<one-hot or bit-encoded state with direct bit meaning, replace full equality with direct bit indexing, example>// BEFORE
wire is_state_5 = (State == 6'b100000);
// AFTER
wire is_state_5 = State[5];
<repeated arithmetic / repeated partial products / repeated shared expressions, common subexpression elimination to a named wire, example>// BEFORE
assign out_a = (x + y) + z;
assign out_b = (x + y) + w;
// AFTER
wire [W-1:0] xy_sum = x + y;
assign out_a = xy_sum + z;
assign out_b = xy_sum + w;
<shared arithmetic across module instances, hoist common subexpression upward and fan out result, example>// BEFORE
child0 u0(.prod_in(a * b), .c(c0));
child1 u1(.prod_in(a * b), .c(c1));
// AFTER
wire [W-1:0] shared_mul = a * b;
child0 u0(.prod_in(shared_mul), .c(c0));
child1 u1(.prod_in(shared_mul), .c(c1));
<high-fanout control or registered signals, duplicate register/signal copies and split fanout cones, example>// BEFORE
reg en;
always @(posedge clk) en <= en_next;
// en drives many distant loads
// AFTER
reg en_a, en_b;
always @(posedge clk) begin
en_a <= en_next;
en_b <= en_next;
end
// split loads between en_a and en_b
<high-fanout combinational condition wire, replicate equivalent local wires for separate consumers, example>// BEFORE
wire state_is_data = (state == DATA);
assign cnt_en = state_is_data & cnt_req;
assign out_en = state_is_data & out_req;
assign flag_set = state_is_data & flag_req;
// AFTER
wire state_is_data_cnt = (state == DATA);
wire state_is_data_out = (state == DATA);
wire state_is_data_flag = (state == DATA);
assign cnt_en = state_is_data_cnt & cnt_req;
assign out_en = state_is_data_out & out_req;
assign flag_set = state_is_data_flag & flag_req;
<wide adder / deep carry chain, carry-select style split into blocks, example>// BEFORE
wire [31:0] sum = a + b;
// AFTER
wire [15:0] sum_lo = a[15:0] + b[15:0];
wire carry_lo = ({1'b0, a[15:0]} + {1'b0, b[15:0]})[16];
wire [15:0] sum_hi_c0 = a[31:16] + b[31:16];
wire [15:0] sum_hi_c1 = a[31:16] + b[31:16] + 1'b1;
wire [15:0] sum_hi = carry_lo ? sum_hi_c1 : sum_hi_c0;
wire [31:0] sum = {sum_hi, sum_lo};
<counter decrement / borrow chain with predictable structure, pre-compute borrow or lookahead-style signals, example>// BEFORE
wire [3:0] dec = a - 4'd1;
// AFTER
wire b0 = ~a[0];
wire b1 = b0 & ~a[1];
wire b2 = b1 & ~a[2];
wire [3:0] dec;
assign dec[0] = ~a[0];
assign dec[1] = a[1] ^ b0;
assign dec[2] = a[2] ^ b1;
assign dec[3] = a[3] ^ b2;
<simple compare against power-of-two threshold / one-hot boundary, simplify wide comparison into bit-select or MSB check, example>count >= 32, direct one-hot state recognition, fixed-point truncation by shift.// BEFORE
wire limit_reached = (data_count >= 6'd32);
wire [15:0] trunc = full_result >>> 8;
// AFTER
wire limit_reached = data_count[5];
wire [15:0] trunc = full_result[23:8];
<FSM outputs driving long downstream logic, register or pipeline the boundary signal when extra latency is architecturally acceptable, example>// BEFORE
wire state_is_data = (next_state == DATA);
assign data_en = state_is_data & ready;
// AFTER
reg state_is_data_r;
always @(posedge clk or negedge rst_n) begin
if (!rst_n)
state_is_data_r <= 1'b0;
else
state_is_data_r <= (next_state == DATA);
end
assign data_en = state_is_data_r & ready;
<nested ternary / nested mux / branch-dependent data path, compute candidate branch values in parallel and select late, example>sel1 ? (sel2 ? a : b) : (sel2 ? c : d) and similar nested selection structures.// BEFORE
assign out = sel1 ? (sel2 ? a : b) : (sel2 ? c : d);
// AFTER
wire [W-1:0] r_a = a;
wire [W-1:0] r_b = b;
wire [W-1:0] r_c = c;
wire [W-1:0] r_d = d;
assign out = sel1 ? (sel2 ? r_a : r_b) : (sel2 ? r_c : r_d);
<arithmetic gated by control, compute both arithmetic branches and mux result at the end, example>full ? ptr : ptr + 1, en ? a+b : c+d, branch-local arithmetic mixed with control.// BEFORE
assign ptr_n = full ? ptr : (ptr + 1'b1);
// AFTER
wire [W-1:0] ptr_hold = ptr;
wire [W-1:0] ptr_inc = ptr + 1'b1;
assign ptr_n = full ? ptr_hold : ptr_inc;
<sel ? (A+B) : (C+D) style arithmetic mux, move muxes before adder when equivalence is clear, example>(sel?A:C) + (sel?B:D) to reduce adder count/depth.// BEFORE
assign out = sel ? (A + B) : (C + D);
// AFTER
wire [W-1:0] x = sel ? A : C;
wire [W-1:0] y = sel ? B : D;
assign out = x + y;
<deep if-else / default-then-override control chain, flatten into explicit parallel conditions with single-level selection, example>// BEFORE
always @(*) begin
out = v0;
if (cond1)
out = v1;
else if (cond2)
out = v2;
else if (cond3)
out = v3;
end
// AFTER
wire sel1 = cond1;
wire sel2 = !cond1 & cond2;
wire sel3 = !cond1 & !cond2 & cond3;
assign out = sel1 ? v1 : sel2 ? v2 : sel3 ? v3 : v0;
<large case statement with structured outputs, replace with direct AND-OR equations or flattened decode logic, example>// BEFORE
always @(*) begin
case (state)
IDLE: next = START;
START: next = DATA;
DATA: next = STOP;
STOP: next = IDLE;
endcase
end
// AFTER
wire state_is_idle = (state == IDLE);
wire state_is_start = (state == START);
wire state_is_data = (state == DATA);
wire state_is_stop = (state == STOP);
assign next[0] = state_is_idle | state_is_data;
assign next[1] = state_is_start | state_is_data;
<wide OR/AND/XOR reduction, restructure linear logic into balanced tree, example>// BEFORE
wire any_set = a | b | c | d | e | f | g | h;
// AFTER
wire or01 = a | b;
wire or23 = c | d;
wire or45 = e | f;
wire or67 = g | h;
wire or0123 = or01 | or23;
wire or4567 = or45 | or67;
wire any_set = or0123 | or4567;
<wide mux tree or wide selection network, rebalance into multi-level 2:1 tree, example>// BEFORE
assign out = sel[2] ? (sel[1] ? (sel[0] ? d7 : d6) : (sel[0] ? d5 : d4)) :
(sel[1] ? (sel[0] ? d3 : d2) : (sel[0] ? d1 : d0));
// AFTER
wire l10 = sel[0] ? d1 : d0;
wire l11 = sel[0] ? d3 : d2;
wire l12 = sel[0] ? d5 : d4;
wire l13 = sel[0] ? d7 : d6;
wire l20 = sel[1] ? l11 : l10;
wire l21 = sel[1] ? l13 : l12;
assign out = sel[2] ? l21 : l20;
<multi-operand addition, rebalance serial add chain into tree, example>a+b+c+d in a long arithmetic chain.// BEFORE
wire [31:0] sum = a + b + c + d;
// AFTER
wire [31:0] sum_ab = a + b;
wire [31:0] sum_cd = c + d;
wire [31:0] sum = sum_ab + sum_cd;
<large lookup / ROM / decoder, split into hierarchical sub-decoders or subtables, example>// BEFORE
assign data_out = lut_256[data_in];
// AFTER
wire [3:0] hi = data_in[7:4];
wire [3:0] lo = data_in[3:0];
wire [W-1:0] hi_dec = lut_hi[hi];
wire [W-1:0] lo_dec = lut_lo[lo];
assign data_out = combine(hi_dec, lo_dec);
<GF / XOR-heavy datapath / crypto logic, rebalance XOR/AND computation into explicit stages, example>// BEFORE
wire y = x0 ^ x1 ^ x2 ^ x3 ^ x4 ^ x5;
// AFTER
wire x01 = x0 ^ x1;
wire x23 = x2 ^ x3;
wire x45 = x4 ^ x5;
wire y = (x01 ^ x23) ^ x45;
<fixed constant multiplication, replace multiply with shift-add decomposition, example>// BEFORE
wire [W-1:0] prod = x * 9'd257;
// AFTER
wire [W-1:0] prod = (x << 8) + x;
<comparison after mux selection, convert mux-then-compare into compare-then-mux, example>// BEFORE
wire [W-1:0] selected = sel ? a : b;
wire result = (selected > threshold);
// AFTER
wire a_gt = (a > threshold);
wire b_gt = (b > threshold);
wire result = sel ? a_gt : b_gt;
<FSM with multiple related outputs, group states by shared output behavior before final decode, example>// BEFORE
assign active = (state == READ) | (state == WRITE) | (state == START);
// AFTER
wire state_is_read = (state == READ);
wire state_is_write = (state == WRITE);
wire state_is_start = (state == START);
wire state_group_rw = state_is_read | state_is_write;
assign active = state_group_rw | state_is_start;
<CPU / multi-cycle controller with timing-state decode, pre-decode machine-cycle and timing-state bits, example>MCycle, TState, instruction groups, microstate families.// BEFORE
assign is_fetch_t2 = (MCycle == 3'd1) && (TState == 3'd2);
// AFTER
wire mc_is_1 = (MCycle == 3'd1);
wire ts_is_2 = (TState == 3'd2);
wire is_fetch_t2 = mc_is_1 & ts_is_2;
<long combinational expression with unclear synthesis boundary, add explicit intermediate wires to guide mapping, example>// BEFORE
wire [31:0] out = ((a * b) + c) >> sh;
// AFTER
wire [31:0] stage1 = a * b;
wire [31:0] stage2 = stage1 + c;
wire [31:0] out = stage2 >> sh;
<mixed-width arithmetic or truncation boundaries, make width extension/truncation explicit in named wires, example>// BEFORE
wire [15:0] sum_ext = a + b;
// AFTER
wire [15:0] a_ext = {8'b0, a[7:0]};
wire [15:0] b_ext = {8'b0, b[7:0]};
wire [15:0] sum_ext = a_ext + b_ext;
<aggressive arithmetic branch parallelization, compute all arithmetic branches and late-select across complex arithmetic, example>// BEFORE
always @(*) begin
if (sel)
out = a + b;
else
out = c - d;
end
// AFTER
wire [W-1:0] br0 = a + b;
wire [W-1:0] br1 = c - d;
assign out = sel ? br0 : br1;
<control flattening around arithmetic-heavy paths, rewrite state/control logic to expose arithmetic in parallel, example>// BEFORE
always @(*) begin
case (state)
S0: out = a + b;
S1: out = a - b;
default: out = v0;
endcase
end
// AFTER
wire [W-1:0] v1 = a + b;
wire [W-1:0] v2 = a - b;
assign out = (state == S0) ? v1 :
(state == S1) ? v2 : v0;
<arithmetic chain tree balancing with carry/overflow sensitivity, rebalance addition/subtraction structure, example>// BEFORE
wire [W-1:0] y = ((a + b) + c) + d;
// AFTER
wire [W-1:0] t0 = a + b;
wire [W-1:0] t1 = c + d;
wire [W-1:0] y = t0 + t1;
<aggressive mux tree restructuring when synthesis already optimizes muxes well, explicitly force custom mux hierarchy, example>// BEFORE
assign y = s1 ? (s0 ? d3 : d2) : (s0 ? d1 : d0);
// AFTER
wire m0 = s0 ? d1 : d0;
wire m1 = s0 ? d3 : d2;
assign y = s1 ? m1 : m0;
<explicit wire decomposition solely for hoped timing gain, split expression into many named wires without structural change, example>// BEFORE
wire [W-1:0] y = a + b + c + d;
// AFTER
wire [W-1:0] t0 = a + b;
wire [W-1:0] t1 = t0 + c;
wire [W-1:0] y = t1 + d;
<registering decode/control outputs to break timing when architectural latency is uncertain, insert sequential boundary speculatively, example>// BEFORE
assign out = sel ? a : b;
// AFTER
reg sel_r;
always @(posedge clk) begin
sel_r <= sel;
end
assign out = sel_r ? a : b;
<count-up to count-down rewrite or reverse counter direction, change counter direction to simplify terminal detect, example>count == MAX is on the critical path and zero-detect looks cheaper.// BEFORE
always @(posedge clk) begin
if (en)
count <= count + 1'b1;
end
wire hit_max = (count == MAX_COUNT);
// AFTER (DO NOT USE)
always @(posedge clk) begin
if (en)
count <= count - 1'b1;
end
wire hit_max = (count == 0);
<priority if-else chain flattened into parallel AND-OR without guaranteed mutual exclusivity, remove priority semantics, example>// BEFORE
always @(*) begin
if (req0)
grant = 2'b00;
else if (req1)
grant = 2'b01;
else
grant = 2'b11;
end
// AFTER (DO NOT USE)
assign grant[0] = req1;
assign grant[1] = ~req0 & ~req1;
<pre-registering mux selector / control selector on active data path, add register to selector to shorten mux path, example>// BEFORE
assign out = sel ? a : b;
// AFTER (DO NOT USE)
reg sel_r;
always @(posedge clk) sel_r <= sel;
assign out = sel_r ? a : b;
<control-signal pre-registration across module boundary for timing only, insert control pipeline stage without protocol redesign, example>// BEFORE
child u0(.req(req), .ack(ack));
// AFTER (DO NOT USE)
reg req_r;
always @(posedge clk) req_r <= req;
child u0(.req(req_r), .ack(ack));
<operand width reduction / interface width reduction for speed, narrow datapath or module inputs to reduce logic, example>// BEFORE
module foo(input [2*W-1:0] in_data, output [W-1:0] out_data);
// AFTER (DO NOT USE)
module foo(input [W-1:0] in_data, output [W-1:0] out_data);
<algebraic restructuring that changes arithmetic corner behavior, rewrite expression form without proof of overflow/signed equivalence, example>// BEFORE
assign y = (a - b) + c;
// AFTER (DO NOT USE)
assign y = a + (c - b);
<fused subtract-multiply / aggressive arithmetic fusion, collapse arithmetic stages into a new combined structure, example>// BEFORE
wire [W-1:0] t = a - b;
assign y = t * c;
// AFTER (DO NOT USE)
assign y = (a * c) - (b * c);
<changing LFSR feedback or polynomial structure, rewrite feedback network for timing, example>// BEFORE
assign feedback = lfsr[7] ^ lfsr[5];
// AFTER (DO NOT USE)
assign feedback = lfsr[7] ^ lfsr[6];
<modifying memory array addressing / decode architecture from RTL timing path, rewrite memory indexing or address decode casually, example>// BEFORE
assign rd_data = mem[addr];
// AFTER (DO NOT USE)
assign rd_data = (addr_sel ? mem[addr_hi] : mem[addr_lo]);
<optimizing pure register self-loop / constraint-only paths as if they were RTL logic problems, rewrite RTL for clock-to-q/setup-only reports, example>// BEFORE
always @(posedge clk)
q <= d;
// AFTER (DO NOT USE)
// arbitrary RTL restructuring here does not solve a clocking/physical constraint path
always @(posedge clk)
q <= d;
<expression simplification that removes helpful intermediate boundaries, inline everything into one direct expression, example>// BEFORE
wire [W-1:0] t0 = a + b;
wire [W-1:0] y = t0 + c;
// AFTER (DO NOT USE)
wire [W-1:0] y = a + b + c;
<restructured comparison ordering that changes corner-case evaluation semantics, reorder sign/magnitude checks for early exit, example>// BEFORE
assign sat = (x > POS_LIM) || (x < NEG_LIM);
// AFTER (DO NOT USE)
assign sat = x[W-1] ? (x < NEG_LIM) : (x > POS_LIM);
<aggressive tree balancing on paths labeled combinational_depth without understanding arithmetic semantics, mechanically rebalance everything, example>// BEFORE
wire [W-1:0] y = (((a + b) + c) + d) + e;
// AFTER (DO NOT USE)
wire [W-1:0] t0 = a + b;
wire [W-1:0] t1 = c + d;
wire [W-1:0] y = (t0 + t1) + e;