Synchronous elastic circuits with Early Evaluation and Token Counterflow

1
Synchronous elastic circuitswith Early
Evaluationand Token Counterflow

• Jordi CortadellaUniversitat Politècnica de
  Catalunya, Barcelona
• Mike KishinevskyStrategic CAD Labs, Intel Corp.,
  Hillsboro

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Latency-insensitive (elastic) system
Out
In
B3
B1
B2
Every block onlymakes one step when all inputs
are valid
3
SELF (linear communication)
sender
receiver
Data
Data
En
En
En
En
V
V
V
V
Valid
Valid
1
1
1
1
Stop
Stop
S
S
S
S
4
Elastic netlists
Enable signal to data latches
Fork
Join
Join / Fork
5
Early evaluation
3
x
15
5
2
6
Early evaluation
3
x
6
2
7
Early evaluation
0
x
0
8
Anti-token
8
Example next-PC calculation
PC1
Branch target address
9
Example next-PC calculation
PC1
Branch target address
10
Example next-PC calculation
PC1
Anti-token !
No branch
Branch target address
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Previous work

• Petri nets
• Extensions to model OR causalityKishinevsky et
  al. 1994, Yakovlev et al. 1996
• Asynchronous systems
• Reese et al 2002 Early evaluation
• Brej 2003 Early evaluation with anti-tokens
• Ampalan Singh 2006 preemption using anti-tokens

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Marked Graph model
13
Marked Graph model
14
Marked Graph model
15
Dual Marked Graph model
Enabled !
Place invariants (tokens per cycle) are preserved
16
How to implement anti-tokens ?
17
How to implement anti-tokens ?
18
How to implement anti-tokens ?
Valid
Valid
Valid

Stop
Stop
-
Valid
Valid
Stop
Stop
19
Elastic controllers
L
H
Data
En
En
H
L
V
V
V
V
H
L
S
S
S
S
20
Dual elastic controllers
En
En


V
V


S
S
-
-
V
V
-
-
S
S
21
Fork/join
Dual fork/join
Join with early evaluation

• Protocol properties modeled with temporal logic
  (CTL and LTL)
• Verified with model checking (NuSMV)

22
Example
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Conclusions

• Early evaluation can increase performance beyond
  the min mean cycle ratio
• The duality between tokens and anti-tokens
  suggests a clean and effective implementation
• Early evaluation provides a new paradigm for
  correct-by-construction microarchitectural
  transformations