Semantics1

1

• Scheduling Primitives for Bluespec
• Arvind
• Computer Science Artificial Intelligence Lab
• Massachusetts Institute of Technology
• Work in progress

2
Motivation
rule recirculate x mem.peek() y
fifo.first() if done?(x) then fifo.deq()
mem.deq() outQ.enq(x) else
mem.deq() mem.req(addr(x))
fifo.deq() fifo.enq(y)
Need a way of - saying deq happens before enq
within the same rule - building such a FIFO
3
BS A Language of Atomic Actions
A program is a collection of instantiated modules
m1 m2 ...
Module Module name State variable
r Rule R a Action method g (x)
a Read method f (x) e
e r c t Op(e , e) e ? e
e (t e in e) m.f(e) e
when e
a r e if e then a a a a
a (t e in a) m.g(e) a
when e
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LPM in Bluespecusing the sequential connective
Different made up syntax

• module lpm
• rule recirculate
• action method enter(x) mem.req(addr(x))
  fifo.enq(x)

(x mem.peek() in (y fifo.first() in (if
done?(x) then fifo.deq() mem.deq()
outQ.enq(x) else (mem.deq()
mem.req(addr(x)))
(fifo.deq() fifo.enq(y)))
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Guards vs Ifs

• Guards affect the surroundings
• (a1 when p1) a2 gt (a1 a2) when p1
• Effect of an if is local
• (if p1 then a1) a2 gt if p1 then (a1 a2)
  else a2
• p1 has no effect on a2

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Conditionals Cases

• if p then a1 else a2
• ? if p then a1 if !p then a2
• Similarly for cases

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Semantics of a rule execution

• Specify which state elements the rule modifies
• Let ? represent the value of all the registers
  before the rule executes
• Let U be the set of updates implied by the rule
  execution

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BS Action Rules
9
BS Action Rules cont
Expression rules are similar
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Guard Semantics

• If no rule applies, e.g., if e in (a when e)
  returns false, then the system is stuck and the
  effect of the whole atomic action of which (a
  when e) is a part is no action.

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Execution model

• Repeatedly
• Select a rule to execute
• Compute the state updates
• Make the state updates

Implementation concern Schedule multiple rules
concurrently without violating one-rule-at-a-time
semantics
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LPM Scheduling issues
The compilers scheduling is not good enough for
this level of specificity
Can a rule calling method enter execute
concurrently with the recirculate rule ? If
not who should have priority?

• module lpm
• rule recirculate
• action method enter(x) mem.req(addr(x))
  fifo.enq(x)

(x mem.res() in (y fifo.first() in (if
done?(x) then fifo.deq() mem.deq()
outQ.enq(x) else (mem.deq()
mem.req(addr(x)))
(fifo.deq() fifo.enq(y)))
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Concurrent Scheduling Rule composition
Rule R1 a1 when p1 Rule R2 a2 when p2

• Scheduling Primitives
• 1. S par(R1,R2)
• where the meaning of rule S is
• Rule S ((if p1 then a1)(if p2 then a2))
  when (p1 xor p2)
• 2. S compose(R1,R2)
• where the meaning of rule S is
• Rule S ((a1 when p1)(a2 when p2))
• 3. S restrict(R1,R2)
• where the meaning of rule S is
• Rule S (a2 when (!p1 p2))

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Schedules

• Grammar
• S R
• compose(S, S)
• par(S, S)
• restrict(S, S)

1. The user must specify a schedule 2. A rule
can occur multiple times in a schedule
Such schedules combined with rule-splitting lets
the user specify concurrency safely in execution
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LPM Split the Internal rule

• module lpm
• rule recirculate
• (x mem.res() in (y fifo.first() in
• (if !done?(x) then (mem.deq()
  mem.req(addr(x)))
• (fifo.deq()
  fifo.enq(y)))
• rule exit
• (x mem.res() in (y fifo.first() in
• (if done?(x) then fifo.deq() mem.deq()
  outQ.enq(x)
• action method enter(x) mem.req(addr(x))
  fifo.enq(x)

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LPM Schedules

• Schedule-1
• rc restrict(enter,recirculate)
• ee compose(exit,enter)
• ex restrict(ee,exit)
• en restrict(exit,enter)
• S1 par(rc,ee,en,ex)
• Schedule-2
• er restrict(recirculate, enter)
• ee compose(exit,er)
• ex restrict(ee,exit)
• en restrict(er,exit)
• S2 par(ee,en,ex)

enter has priority over recirculate no dead
cycles
recirculate has priority over enter no dead
cycles
Difficult to pick between S1 and S2!
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Final words

• Market forces are demanding a much greater
  variety of SoCs
• The design cost for SoCs has to be brought down
  dramatically by facilitating IP reuse
• High-level synthesis tools are essential for
  architectural exploration and IP development

Bluespec and associated tools are a promising
solution to this problem
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Resources

• The Blusepec Inc Web site http//bluespec.com
  http//bluespec.com/products/Papers.htm
• Contact Bluespec Inc to get a free copy of the
  BSV language manual. For other publications
  http//csg.lcs.mit.edu/pubs/bluespecpubs.html
• The 6.375 subject at MIT Complex Digitial
  Systems http//csg.csail.mit.edu/6.375/

Thanks for listening