The StreamIt Compiler: Targeting Raw

1. The StreamIt Compiler: Targeting Raw Michael Gordon Bill Thies Michal Karczmarek Saman Amarasinghe

2. StreamIt Overview • Filter is the basic unit of computation • Filters communicate with neighboring blocks using typed FIFO channels • The channels support three operations: • pop(): remove item from end of input channel • peek(i): get value i spaces from end of input channel • push(val): push value onto output channel

3. Filter • Each filter contains: • An init(…) function which is called an initialization time. • A work() function to describe the execution of the filter in the steady state • Other helper functions called by init() or work() • Variables persistent over executions of the work() function

4. unstructured structured • Pipeline • Split/Join • Feedback Composing Filters

5. Pipeline • Sequence of streams • Each stream can be Filter, Pipeline, SplitJoin, or FeedbackLoop Stream 1 Stream 2 Stream N

6. SplitJoin • Independent parallel streams Splitter Stream N Stream 1 Stream 2 Joiner • Splitter and Joiner types are pre-defined: • Duplicate (Splitter) – send item to all streams • Weighted RoundRobin– route in pattern

7. Feedback Loop • For introducing cycles Queue ofinitial inputs Joiner Loop Stream Body Stream Splitter • Splitters and Joiners are same as in SplitJoin

8. Other StreamIt Features (not implemented) • Messaging • dynamic, low-volume messages sent from within a work() function • Message timing that allows a filter to specify when a message will be received • Broadcast support • Re-Initialization • Allows the stream graph to be modified during runtime • Achieved through init() calls

9. class Adder extends Filter { int N; void init (int N) { this.N = N; input = new Channel(Float.TYPE, N); output = new Channel(Float.TYPE, 1); } void work() { float sum = 0; for (int i=0; i<N; i++) { sum += input.popFloat(); } output.pushFloat(sum); } } public class Equalizer extends Pipeline { void init(float samplingRate, int N) { add(new SplitJoin() { void init() { int bottom = 2500; int top = 5000; setSplitter(DUPLICATE()); for (int i=0; i<N; i++, bottom*=2, top*=2) { add(new BandPassFilter(samplingRate, bottom, top)); } setJoiner(ROUND_ROBIN()); }}); add(new Adder(N)); } } class FMRadio extends StreamIt{ void init() { add(new DataSource()); add(new LowPassFilter(samplingRate, cutoffFrequency, numTaps)); add(new FMDemodulator(samplingRate, maxAmplitude, bandwidth)); add(new Equalizer(samplingRate, 4)); add(new Speaker()); } } Code Example - FM Radio

10. The Current Version of StreamIt • Currently the StreamIt compiler only supports static rates. • The number of items peeked, popped, and pushed by each filter remains constant over the life of the filter • Channels can communicate only scalar data types

11. Compiler Flow StreamIt Code KOPIfront-end Parse tree Conversion to StreamIt IR SIR (parameterized) Graph Expansion UNIPROCESSOR RAW SIR (expanded) Fusion /Fission RawBackend Conversionto C Conversion to Low IR

12. Fission / Fusion • Fission • Split a filter into a pipeline for load balancing • Duplicate a filter, placing the duplicates in a SplitJoin to expose parallelism. • Fusion • Merge filters into one filter for load balancing and synchronization removal

13. Raw Backend (Expanded) Layout FlatIR Flattener Scheduler Initial, Steady State Router Switch Code Generation Simulator sw0.s, sw1.s, … Synchronization Schedule Joiner Scheduler Tile Code Generation tile0.c, tile1.c, … Makefile Generation Makefile

14. Raw Backend - Layout • Layout • At this point layout is done by hand • Will be automated soon (before ASPLOS) • After partitioning, each filter is mapped to one Raw tile • Splitters are folded into their corresponding upstream filter (no tile needed) • Some joiners require their own tile • Neighboring Joiners are collapsed

15. Raw Backend - Switch Code • To generate the switch code we use a simulator to simulate the execution of the graph over the layout. • The switch code is generated as the simulator runs • In its current form, StreamIt is totally static and can be simulated (hopefully partitioning has balanced the load of each filter).

16. Simulator • First we produce an initialization schedule and a steady state schedule. • The steady state schedule is periodic, preserving the number of items on each channel. • We simulate the graph first on the initialization schedule then on the steady state schedule. • We use each schedule to calculate the number of times each filter can execute, but ordering is independent of the schedule.

17. Joiners • Joiners require special attention because they could lead to deadlock, if we program the switch to receive in the order specified by the joiner: Pop 20, Push 20 1 1 1 1 Pop 1, Push 1

18. Joiners • To resolve the deadlock, the joiner receives items as calculated by the simulator (ignoring the joiner weights). • The joiner buffers these items internally and pushes data in the order given by the joiner weights.

19. Communication • Filter tiles act as data routers as well • The compiler creates router nodes as necessary (tiles that are not allocated) • The communication model cannot handle some forms of circular communication at this time.

20. Raw Backend - Tile Code • Tile Code is pretty much a direct translation from the Java code • Loop work() and introduce buffers to handle channels: • Each filter buffers its input until it has received pop items, then it fires (done for simplicity). • pop() and peek() are reads from the buffer • A push() is a static network send

21. class Foo extends Filter { public void init() { input = new Channel (Integer.TYPE, 1); output = new Channel (Integer.TYPE, 1); } public void work () { int j, x = 0, pop; pop = input.popInt(); for(j=0; j<50; j++) { x = x + pop; } output.pushInt (x); } } class HelloWorld6 extends StreamIt { public void init () { int i; add (new Source()); for (i = 0; i < 14; i++) add(new Foo()); add (new Sink()); } } Simple Example 1

22. Simple Example 1 • Pipeline of 16 filters with equal rates and work (with peeking):

23. class Foo extends Filter { int loop; public void init(int i) { loop = i; input = new Channel (Integer.TYPE, 1); output = new Channel (Integer.TYPE, 1); } public void work () { int j, x = 0, pop; pop = input.popInt(); for(j=0; j<5*loop; j++) { x = x + pop; } output.pushInt(x); } } Simple Example 2 class HelloWorld6 extends StreamIt { public void init () { int i; add (new Source()); for (i = 0; i < 14; i++) add(new Foo(i)); add (new Sink()); } }

24. Simples Example 2 • Pipeline of 16 filters with unequal work (work increases as we get downstream):

25. FFT

26. FFT

27. FFT

28. FFT - sv(2000);

29. FMRadio

30. FMRadio

31. FMRadio (plus Equalizer)

32. FMRadio (plus Equalizer)