Sequential Timing Optimization

Sequential Timing Optimization

Long path timing constraints

• Data must not reach destination FF too late

si + d(i,j) + Tsetup sj + P

si

sj

d(i,j) T setup

dmax(i,j)i j

Short path timing constraints

• FF should not get >1 data set per period

si

sj

dmin(i,j) Thold

si + dmin(i,j) sj + Thold

dmin(i,j)i j

Clock skew optimization

• Another approach for sequential timing optimization

• Deliberately change the arrival times of the clock at various memory elements in a circuit for cycle borrowing– For zero skew, delay from clock source to all FF’s = T

– Positive skew of at FFk

• Change delay from clock source to FFk to T +

– Negative skew of at FFk

• Change delay from clock source to FFk to T –

• Problem statement: set skews for optimized performance

Sequential timing optimization

• Two “true” sequential timing optimization methods– Retiming: moving latches around in a design

– Clock skew optimization: deliberately changing clock arrival times so that the circuit is not truly “synchronous”

Clk Clk

Comb Block 1 Comb Block 2

Clk

FF FF FF FF

Clk

FF FF

Clk Clk

ClkClk

FF FF FF

DelayClkClk Clk

Comb Block 1 Comb Block 2

Clk

FF FF FF

• Represented by the optimization problem below - solve for P and optimal skews

minimize Psubject to

(for all pairs of FF’s (i,j) connected by a combinational path)

si + dmin(i,j) sj + Thold

si + dmax(i,j) + Tsetup sj + P

• If dmax(i,j) and dmin(i,j) are constant – linear program in the variables si and P

Finding the optimal clock period using skews

Graph-based approaches

• For a constant clock period P, the linear program = system of difference constraints

sp - sq constant

• As before, perform a binary search on P

• For each value of P build an equivalent constraint graph

• Shortest path in the constraint graph gives a set of skews for a given value of P

• If P is infeasible, there will be a negative cycle in the graph that will be detected during shortest-path calculations

i jf ( P )

Retiming

Assume unit gate delays, no setup times

Initial Circuit: P=3

Retimed Circuit: P=2

Clk Clk

Comb Block 1 Comb Block 2

Clk

FF FF FF

FF

Clk

FF FF

Clk Clk

Retiming: Definition

• Relocation of flip-flops (FF’s) and latches (usually to achieve lower clock periods)

• Maintain the latency of all paths in circuit, i.e., number of FF stages on any input-output path must remain unchanged

Graph Notation of Circuit

w(euv) = #latencies between u and v

r(u) is # latencies moved across gate u

r(PI) = r(PO) = 0: Merge them both into a “host” node h with r(h) = 0

wr(euv) = w(euv) + r(v) - r(u)

u vw(euv) = 2

r(u) = 1

w(euv) = 1u v

r(v) = 2

u vwr(euv) = 2

u v

delay = d(u) delay = d(v)

For a path from v1 to vk

• Consider a path of vertices

– Define w(v1 to vk) = w12 + w23 + … + w(k-1,k)

– After retiming, wr(v1 to vk) = w12r + w23r + … + w(k-1,k)r

= [w12+r(2)–r(1)]+[w23+r(3)–r(2)]+[w23+r(3)–r(2)]+…+[w(k-1,k)+r(k)–r(k-1)]

= w(v1 to vk) + r(k) – r(1)

– For a cycle, v1 = vk, which implies that wr = w for a cycle

– In other words, retiming leaves the # latencies unchanged on any cycle

v1 v2 v3 vkw12 w23 w34 Wk-1,k

Constraints for retiming

• Non-negativity constraints (cannot have negative latencies)– wr on each edge must be non-negative

– For any edge from vertex u to vertex v,

wr(u,v) = w(u,v) + r(v) – r(u) 0

i.e., r(u) – r(v) w(u,v)

• Period constraints (need a latency if path delay period)– (or more precisely, path delay + Tsetup period)

– For any path from vertex v1 to vertex vk, under clock period P,

wr(v1 to vk) = w(v1 to vk) + r(vk) – r(v1) 1 if delay(v1 to vk) > P

i.e., r(v1) – r(vk) w(v1 to vk) – 1 if delay(v1 to vk) > P

Example

• Circuit graph:– Vertex weights = gate delays

– Edge weights = # latencies

• Non-negativity constraints1. r(h) – r(G1) 0

2. r(G1) – r(G2) 0

3. r(G2) – r(G3) 0

4. r(G3) – r(G4) 1

5. r(G4) – r(h) 0

• Period constraints for P = 26. r(h) – r(G3) -1

7. r(G1) – r(G3) -1

8. r(G2) – r(G4) 0

9. r(G2) – r(h) 0

Clk Clk

Comb Block 1 Comb Block 2

Clk

FF FF FF

G1 G3G2 G4

0

11

1 1

0 0

1

0

0

G1

G2 G3

G4

h

Graph-based approaches

• System of difference constraintsr(u) – r(v) c

• Equivalent constraint graph

• Shortest path in the constraint graph gives a set of valid r values for a given value of P (note that period constraints change for different values of P)

• If P is infeasible, there will be a negative cycle in the graph that will be detected during shortest-path calculations

v uc

Corresponding shortest path problem

• Find shortest path from host to get– r(h) = 0

– r(G1) = 0

– r(G2) = 0

– r(G3) = 1

– r(G4) = 0

• This gives the solution

0 0

1

0

0

G1

G2 G3

G4

h

-1

-1

00

Clk Clk

Comb Block 1 Comb Block 2

Clk

FF FF FF FF

Clk

FF FF

Clk Clk

Overall scheme for minimum period retiming

• Objective: to find a retiming that minimizes the clock period (the assignment of r values may not be unique due to slack in the shortest path graph!)– Binary search over P = [0,Punretimed]– Punretimed = period of unretimed circuit = upper bound on optimal P– Range in some iteration of the search = [Pmin, Pmax]– Build shortest path graph with non-negativity constraints (independent of

P)– At each value of P

• Add period constraints to shortest path graph (related to W, D matrices discussed in class – will not describe here)

• Solve shortest path problem• If negative cycle found, set Pmin = P; else set Pmax = P• Iterate until range of P is sufficiently small

Finding shortest paths

• Dijkstra’s algorithm– O(VlogV + E) for a graph with V vertices and E edges– Applicable only if all edge weights are non-negative– The latter condition does not hold in our case!

• Bellman-Ford algorithm– O(VE) for a graph with V vertices and E edges– Outline

for I = 1 to V – 1 for each edge (u,v) E update neighbor’s weights as r(v) = min[r(u) + d(u,v),r(v)]

for each edge (u,v) E if r(u) + d(u,v) > r(v) then a negative cycle exists

• Basic idea: in iteration I, update lowest cost path with I edges• After V – 1 iterations, if any update is still required, a negative cycle exists

“Relaxation” algorithm for retiming

• Perform a binary search on clock period P as before

• At each value of P check feasibility as follows– Repeat V-1 times (where V = # vertices)

1. Set r(u) = 0 for each vertex

2. Perform timing analysis to find clock period of the circuit

3. For any vertex u with delay > P, r(u)++

4. If no such vertex exists, P is feasible

5. Else, retime the circuit using these values of r; update the circuit and go to step 1

– If Clock period > P after V – 1 iterations, then P is infeasible

The retiming-skew relationship

• Skew

• Retiming

• Both borrow one unit of time from Comb Block 2 and lend it to Comb Block 1

• Magnitude of optimal skew = amount of delay that the FF has to move across

• Can be generalized for another approach to retiming

FF

Clk

FF FF

Clk Clk

Clk

Comb Block 1 Comb Block 2

Clk

FF FF FF

Delay = 1Clk

Can move from skews to retiming

• Moving a flip-flop across a gate G– left right increasing its

skew by delay(G)–

– right left reducing its skew by delay(G)

–

• More generally,

Old skew=sDelay=d

New skew = s+d

s1

s2

s3

s4

sj = max1 i 4 (si+MAX(i,j))

sk = max1 i 4 (si+MAX(i,k))

FF j

FF k

Another approach to retiming

• Two-phase approach– Phase A: Find optimal skews

(complexity depends on the number of FF’s, not the number of gates)

– Phase B: Relocate FF’s to retime circuit(since most FF movements are seen to be local in practice, this does not

take too long)

– Not provably better than earlier approach in terms of complexity, but practically works very well