This document discusses tail call elimination in the OpenSmalltalk programming language. It begins by defining what a tail call is and providing an example in Smalltalk. It then describes how tail calls are implemented in the stack interpreter and Cog VM without and with tail call elimination. The document finds that tail call elimination provides significant performance improvements for tail recursive functions and more modest gains for general applications. Future work includes better supporting polymorphic calls and reducing redundant code generation in the Cog VM.

1. Tail Call Elimination in Opensmalltalk
Matthew Ralston Dave Mason
Department of Computer Science
Ryerson University
c 2019 Matthew Ralston

2. Agenda
What is a Tail Call?
Tail Call Elimination
Stack Interpreter Implementation
Cog VM JIT Implementation
Results
Conclusions and Future Work

3. What is a Tail Call?
Call followed by a return
Smalltalk Example
Array class>>new: sizeRequested
^ self basicNew: sizeRequested
Call to basicNew: is a tail call
Immediately followed by a return

4. What is a Tail Call?
Call followed by a return
Smalltalk Example
Array class>>new: sizeRequested
^ self basicNew: sizeRequested
Call to basicNew: is a tail call
Immediately followed by a return

5. What is a Tail Call?
Call followed by a return
Smalltalk Example
Array class>>new: sizeRequested
^ self basicNew: sizeRequested
Call to basicNew: is a tail call
Immediately followed by a return

6. What is a Tail Call?
Call followed by a return
Smalltalk Example
Array class>>new: sizeRequested
^ self basicNew: sizeRequested
Call to basicNew: is a tail call
Immediately followed by a return

7. Tail call in Bytecode
Bytecode for Array class»new:
self
pushTemp: 0
send: basicNew:
returnTop

8. Calling new without TCE
Stack after calling new:
new:’s stack frame
new:’s argument
Sender of new:’s frame

9. Calling new without TCE
Stack after calling new:
new:’s stack frame
new:’s argument
Sender of new:’s frame

10. Calling new without TCE - cont.
Stack after calling basicNew:
basicNew:’s stack frame
basicNew:’s argument
new:’s stack frame
new:’s argument
Sender of new:’s frame

11. Calling new without TCE - cont.
Stack after calling basicNew:
basicNew:’s stack frame
basicNew:’s argument
new:’s stack frame
new:’s argument
Sender of new:’s frame

12. Calling new without TCE - cont.
Stack after returning from basicNew:
basicNew:’s result
new:’s stack frame
new:’s argument
Sender of new:’s frame

13. Calling new without TCE - cont.
Stack after returning from basicNew:
basicNew:’s result
new:’s stack frame
new:’s argument
Sender of new:’s frame

14. Calling new without TCE - cont.
Stack after returning from new:
new:’s result
Sender of new:’s frame

15. Calling new without TCE - cont.
Stack after returning from new:
new:’s result
Sender of new:’s frame

16. Tail Call Elimination
Why return to new:?
Why keep new:’s stack frame?

17. Tail Call Elimination
Why return to new:?
Why keep new:’s stack frame?

18. Calling new with TCE
new:’s stack frame
new:’s argument
Sender of new:’s frame

19. Calling new with TCE - cont’d
basicNew:’s stack frame
basicNew:’s argument
Sender of new:’s frame

20. Calling new with TCE - cont’d
basicNew:’s return
Sender of new:’s frame

21. Tail Recursion Elimination
Special Case of Tail Call Elimination
Recursive Call is also a Tail Call

22. Tail Recursion Elimination
Special Case of Tail Call Elimination
Recursive Call is also a Tail Call

23. Motivation
Well Known Optimization
Support in functional languages, CLR, etc.
Not supported in JVM, Python
Necessary for functional languages
Can be useful for OO as well
In common patterns like Visitor Pattern
Iteration in Smalltalk

24. Motivation
Well Known Optimization
Support in functional languages, CLR, etc.
Not supported in JVM, Python
Necessary for functional languages
Can be useful for OO as well
In common patterns like Visitor Pattern
Iteration in Smalltalk

25. Motivation
Well Known Optimization
Support in functional languages, CLR, etc.
Not supported in JVM, Python
Necessary for functional languages
Can be useful for OO as well
In common patterns like Visitor Pattern
Iteration in Smalltalk

26. Motivation
Well Known Optimization
Support in functional languages, CLR, etc.
Not supported in JVM, Python
Necessary for functional languages
Can be useful for OO as well
In common patterns like Visitor Pattern
Iteration in Smalltalk

27. Motivation
Well Known Optimization
Support in functional languages, CLR, etc.
Not supported in JVM, Python
Necessary for functional languages
Can be useful for OO as well
In common patterns like Visitor Pattern
Iteration in Smalltalk

28. Motivation
Well Known Optimization
Support in functional languages, CLR, etc.
Not supported in JVM, Python
Necessary for functional languages
Can be useful for OO as well
In common patterns like Visitor Pattern
Iteration in Smalltalk

29. Motivation
Well Known Optimization
Support in functional languages, CLR, etc.
Not supported in JVM, Python
Necessary for functional languages
Can be useful for OO as well
In common patterns like Visitor Pattern
Iteration in Smalltalk

30. Frequency of Tail Calls
Static Frequency
Platform Packages Tail Calls Total Percentage
Squeak - All 25162 407971 6.17
Squeak - Compiler 863 8747 9.87
Higher Dynamic Frequency
Action Tail Calls Total Percentage
Startup 47669 219054 21.76
Recompile 92041349 551250016 16.70

31. Frequency of Tail Calls
Static Frequency
Platform Packages Tail Calls Total Percentage
Squeak - All 25162 407971 6.17
Squeak - Compiler 863 8747 9.87
Higher Dynamic Frequency
Action Tail Calls Total Percentage
Startup 47669 219054 21.76
Recompile 92041349 551250016 16.70

32. Implementations
Stack Interpreter
Cog VM

33. Implementations
Stack Interpreter
Cog VM

34. Stack Interpreter
Interpreter only
Look ahead to next bytecode for return
Switch to tail call eliminating implementation
Remove the existing stack frame and arguments
Pushes the new arguments and calls the next method

35. Stack Interpreter
Interpreter only
Look ahead to next bytecode for return
Switch to tail call eliminating implementation
Remove the existing stack frame and arguments
Pushes the new arguments and calls the next method

36. Stack Interpreter
Interpreter only
Look ahead to next bytecode for return
Switch to tail call eliminating implementation
Remove the existing stack frame and arguments
Pushes the new arguments and calls the next method

37. Stack Interpreter
Interpreter only
Look ahead to next bytecode for return
Switch to tail call eliminating implementation
Remove the existing stack frame and arguments
Pushes the new arguments and calls the next method

38. Stack Interpreter
Interpreter only
Look ahead to next bytecode for return
Switch to tail call eliminating implementation
Remove the existing stack frame and arguments
Pushes the new arguments and calls the next method

39. Cog JIT Compiler
Not optimizing interpreted calls
JIT compile tail calls as jumps
Cost of tail call check moved to JIT compile time

40. Cog JIT Compiler
Not optimizing interpreted calls
JIT compile tail calls as jumps
Cost of tail call check moved to JIT compile time

41. Cog JIT Compiler
Not optimizing interpreted calls
JIT compile tail calls as jumps
Cost of tail call check moved to JIT compile time

42. Cog VM - Inline Caching
Cog VM - levels of inline caching
No Inline Cache
Monomorphic Send Sites
Polymorphic Send Sites
Megamorphic Send Sites

43. Cog VM - Inline Caching
Cog VM - levels of inline caching
No Inline Cache
Monomorphic Send Sites
Polymorphic Send Sites
Megamorphic Send Sites

44. Cog VM - Inline Caching
Cog VM - levels of inline caching
No Inline Cache
Monomorphic Send Sites
Polymorphic Send Sites
Megamorphic Send Sites

45. Cog VM - Inline Caching
Cog VM - levels of inline caching
No Inline Cache
Monomorphic Send Sites
Polymorphic Send Sites
Megamorphic Send Sites

46. Cog VM - Inline Caching
Cog VM - levels of inline caching
No Inline Cache
Monomorphic Send Sites
Polymorphic Send Sites
Megamorphic Send Sites

47. Cog VM - Inline Caching - cont.
Bypass for unlinked send sites
Activate for monomorphic send sites
Bypass for polymorphic and megamorphic send sites
Need tail call and non-tail call JIT code for each send
Copy method lookup code to sender in tail calls

48. Cog VM - Inline Caching - cont.
Bypass for unlinked send sites
Activate for monomorphic send sites
Bypass for polymorphic and megamorphic send sites
Need tail call and non-tail call JIT code for each send
Copy method lookup code to sender in tail calls

49. Cog VM - Inline Caching - cont.
Bypass for unlinked send sites
Activate for monomorphic send sites
Bypass for polymorphic and megamorphic send sites
Need tail call and non-tail call JIT code for each send
Copy method lookup code to sender in tail calls

50. Cog VM - Inline Caching - cont.
Bypass for unlinked send sites
Activate for monomorphic send sites
Bypass for polymorphic and megamorphic send sites
Need tail call and non-tail call JIT code for each send
Copy method lookup code to sender in tail calls

51. Cog VM - Inline Caching - cont.
Bypass for unlinked send sites
Activate for monomorphic send sites
Bypass for polymorphic and megamorphic send sites
Need tail call and non-tail call JIT code for each send
Copy method lookup code to sender in tail calls

52. Results
Tail Recursive Tests
Real World Tests

53. Results
Tail Recursive Tests
Real World Tests

54. Factorial 500 x 1000
si
sitce
cog
cogtce0
50
100
150
200
250
Time(milliseconds)
Version Mean %Imp %SD Std Dev Median
si 258.43 1.41 258.00
sitce 251.61 2.6 0.7 1.02 251.00
cog 223.18 15.46 217.00
cogtce 199.01 10.8 6.9 1.04 199.00

55. Factorial 5000
si
sitce
cog
cogtce
0
2,000
4,000
6,000
Time(milliseconds)
Version Mean %Imp %SD Std Dev Median
si 6284.60 12.52 6283.00
sitce 1372.40 78.2 0.4 21.88 1356.00
cog 6587.72 16.45 6591.00
cogtce 1333.28 79.8 0.5 27.36 1314.00

56. Factorial 5000 Memory
si
sitce
cog
cogtce
0
20
40
60
MemoryUsage(megabytes)
Version Mean %Imp %SD Std Dev Median
si 61.11 3.28 60.33
sitce 47.37 22.5 7.7 3.35 47.22
cog 61.92 6.17 59.50
cogtce 46.99 24.1 11.3 3.33 46.89

57. Compile All Execution
si
sitce
cog
cogtce0
50
100
150
Time(seconds)
Version Mean %Imp %SD Std Dev Median
si 136.49 2.30 135.52
sitce 129.68 5.0 1.7 0.33 129.60
cog 109.59 0.90 109.29
cogtce 105.86 3.4 0.9 0.45 105.84

58. Browse Number Class Execution
si
sitce
cog
cogtce0
500
1,000
1,500
2,000
Time(milliseconds)
Version Mean %Imp %SD Std Dev Median
si 1842.60 51.99 1850.00
sitce 1759.90 4.5 4.2 56.60 1754.00
cog 1115.40 62.26 1124.00
cogtce 1127.20 -1.1 7.5 55.21 1124.00

59. Method Analyzer Execution
si
sitce
cog
cogtce
0
200
400
600
800
1,000
1,200
Time(milliseconds)
Version Mean %Imp %SD Std Dev Median
si 1215.38 29.02 1200.00
sitce 1180.62 2.9 3.1 23.67 1169.00
cog 254.14 38.37 243.00
cogtce 247.10 2.8 21.1 37.33 237.00

60. Conclusions
Significant improvements in execution time for tail recursive cases
Improvements in execution time for most general applications
Memory usage is only significantly affected for deep recursive call
chains
Stack Interpreter outperforms Cog in some tests
Stack Interpreter supports polymorphic calls
Increased JIT compiled method size leads to overhead

61. Conclusions
Significant improvements in execution time for tail recursive cases
Improvements in execution time for most general applications
Memory usage is only significantly affected for deep recursive call
chains
Stack Interpreter outperforms Cog in some tests
Stack Interpreter supports polymorphic calls
Increased JIT compiled method size leads to overhead

62. Conclusions
Significant improvements in execution time for tail recursive cases
Improvements in execution time for most general applications
Memory usage is only significantly affected for deep recursive call
chains
Stack Interpreter outperforms Cog in some tests
Stack Interpreter supports polymorphic calls
Increased JIT compiled method size leads to overhead

63. Conclusions
Significant improvements in execution time for tail recursive cases
Improvements in execution time for most general applications
Memory usage is only significantly affected for deep recursive call
chains
Stack Interpreter outperforms Cog in some tests
Stack Interpreter supports polymorphic calls
Increased JIT compiled method size leads to overhead

64. Conclusions
Significant improvements in execution time for tail recursive cases
Improvements in execution time for most general applications
Memory usage is only significantly affected for deep recursive call
chains
Stack Interpreter outperforms Cog in some tests
Stack Interpreter supports polymorphic calls
Increased JIT compiled method size leads to overhead

65. Conclusions
Significant improvements in execution time for tail recursive cases
Improvements in execution time for most general applications
Memory usage is only significantly affected for deep recursive call
chains
Stack Interpreter outperforms Cog in some tests
Stack Interpreter supports polymorphic calls
Increased JIT compiled method size leads to overhead

66. Conclusions and Future Work
Support polymorphic caches (partially complete!)
Reduce redundant code generation for Cog

67. Conclusions and Future Work
Support polymorphic caches (partially complete!)
Reduce redundant code generation for Cog

68. Questions?