Better Recursion With Swift

Presented at the Brooklyn Swift Developers Meetup, August 2015.

Transcript

1. BETTER RECURSION WITH SWIFT
2. AKA…
3. RAIDERS OF THE LOST ARC Another reason why Swift value types matter.
7. FAST. MODERN. SAFE
8. FUNCTIONAL-ISH
9. let str = "Favours Immutability"
10. GOOD: CONCURRENCY
11. MEH: CACHE EFFICIENCY
12. CHALLENGE FOR ITERATION
13. func factorial(n: Int) -> Int { var result = 1 var x = n while x > 0 { result = result * x x-- } return result }
14. RECURSION Immutable iteration!
15. func factorialRec(n: Int) -> Int { if n > 0 { return n * factorialRec(n - 1) } return 1 }
16. ELEGANT.
17. YET INEFFICIENT. New call frame on each iteration.
18. STACK OVERFLOW Easier than ever with recursion!
19. All Is not Lost
20. TAIL CALLS Recursive call is performed last.
21. func tco(n: Int, acc: Int) -> Int { if n > 0 { return tco(n - 1, acc * n) } return acc }
22. func tco(n: Int, acc: Int) -> Int { if n > 0 { return tco(n - 1, acc * n) } return acc } func factorialTCO(n: Int) -> Int { return tco(n, 1) }
23. func tco(n: Int, acc: Int) -> Int { if n > 0 { return tco(n - 1, acc * n) } return acc }
24. CAN BE OPTIMIZED To something like this…
25. func factorial(n: Int) -> Int { var result = 1 var x = n while x > 0 { result = result * x x-- } return result }
26. DOES SWIFT DO THIS?
27. DOES SWIFT DO THIS? The internet says: *Sometimes*
29. Or… xcrun swiftc -S
30. AMD64 ABI Args placed in rdi, rsi, rdx, .. Return value -> rax
31. func tco(n: Int, acc: Int) -> Int rdi → n rsi → acc rax → return value
32. IS TCO HAPPENING? call → Bad - Creates new stack frame jmp, jg, jle, j… → Good - Stays in current stack frame
33. __TF4main3tcoFTSiSi_Si: pushq %rbp movq %rsp, %rbp subq $64, %rsp cmpq $0, %rdi movq %rdi, -8(%rbp) movq %rsi, -16(%rbp) jle LBB19_4 . . movq -24(%rbp), %rdi movq -40(%rbp), %rsi callq __TF4main3tcoFTSiSi_Si movq %rax, -56(%rbp) jmp LBB19_5 LBB19_4: movq -16(%rbp), %rax movq %rax, -56(%rbp) jmp LBB19_5 LBB19_5: movq -56(%rbp), %rax addq $64, %rsp popq %rbp retq
34. __TF4main3tcoFTSiSi_Si: pushq %rbp movq %rsp, %rbp subq $64, %rsp cmpq $0, %rdi movq %rdi, -8(%rbp) movq %rsi, -16(%rbp) jle LBB19_4 . . movq -24(%rbp), %rdi movq -40(%rbp), %rsi callq __TF4main3tcoFTSiSi_Si movq %rax, -56(%rbp) jmp LBB19_5 LBB19_4: movq -16(%rbp), %rax movq %rax, -56(%rbp) jmp LBB19_5 LBB19_5: movq -56(%rbp), %rax addq $64, %rsp popq %rbp retq Stack frame setup.
35. __TF4main3tcoFTSiSi_Si: pushq %rbp movq %rsp, %rbp subq $64, %rsp cmpq $0, %rdi movq %rdi, -8(%rbp) movq %rsi, -16(%rbp) jle LBB19_4 . . movq -24(%rbp), %rdi movq -40(%rbp), %rsi callq __TF4main3tcoFTSiSi_Si movq %rax, -56(%rbp) jmp LBB19_5 LBB19_4: movq -16(%rbp), %rax movq %rax, -56(%rbp) jmp LBB19_5 LBB19_5: movq -56(%rbp), %rax addq $64, %rsp popq %rbp retq Stack frame setup. if n > 0
36. __TF4main3tcoFTSiSi_Si: pushq %rbp movq %rsp, %rbp subq $64, %rsp cmpq $0, %rdi movq %rdi, -8(%rbp) movq %rsi, -16(%rbp) jle LBB19_4 . . movq -24(%rbp), %rdi movq -40(%rbp), %rsi callq __TF4main3tcoFTSiSi_Si movq %rax, -56(%rbp) jmp LBB19_5 LBB19_4: movq -16(%rbp), %rax movq %rax, -56(%rbp) jmp LBB19_5 LBB19_5: movq -56(%rbp), %rax addq $64, %rsp popq %rbp retq Stack frame setup. if n > 0
37. __TF4main3tcoFTSiSi_Si: pushq %rbp movq %rsp, %rbp subq $64, %rsp cmpq $0, %rdi movq %rdi, -8(%rbp) movq %rsi, -16(%rbp) jle LBB19_4 . . movq -24(%rbp), %rdi movq -40(%rbp), %rsi callq __TF4main3tcoFTSiSi_Si movq %rax, -56(%rbp) jmp LBB19_5 LBB19_4: movq -16(%rbp), %rax movq %rax, -56(%rbp) jmp LBB19_5 LBB19_5: movq -56(%rbp), %rax addq $64, %rsp popq %rbp retq Stack frame setup. if n > 0 Return value
38. __TF4main3tcoFTSiSi_Si: pushq %rbp movq %rsp, %rbp subq $64, %rsp cmpq $0, %rdi movq %rdi, -8(%rbp) movq %rsi, -16(%rbp) jle LBB19_4 . . movq -24(%rbp), %rdi movq -40(%rbp), %rsi callq __TF4main3tcoFTSiSi_Si movq %rax, -56(%rbp) jmp LBB19_5 LBB19_4: movq -16(%rbp), %rax movq %rax, -56(%rbp) jmp LBB19_5 LBB19_5: movq -56(%rbp), %rax addq $64, %rsp popq %rbp retq Stack frame setup. if n > 0 Return value Restore caller stack frame
39. __TF4main3tcoFTSiSi_Si: pushq %rbp movq %rsp, %rbp subq $64, %rsp cmpq $0, %rdi movq %rdi, -8(%rbp) movq %rsi, -16(%rbp) jle LBB19_4 . . movq -24(%rbp), %rdi movq -40(%rbp), %rsi callq __TF4main3tcoFTSiSi_Si movq %rax, -56(%rbp) jmp LBB19_5 LBB19_4: movq -16(%rbp), %rax movq %rax, -56(%rbp) jmp LBB19_5 LBB19_5: movq -56(%rbp), %rax addq $64, %rsp popq %rbp retq No TCO
40. OPTIMIZE
41. WALK: -Onone
42. KILL: -O
43. -Ounchecked
44. __TTSf4s_s___TF4tcoO3tcoFTSiSi_Si: pushq %rbp movq %rsp, %rbp testq %rdi, %rdi jle LBB33_4 LBB33_1: movq %rdi, %rax decq %rax jo LBB33_5 imulq %rdi, %rsi jo LBB33_5 testq %rax, %rax movq %rax, %rdi jg LBB33_1 LBB33_4: movq %rsi, %rax popq %rbp retq
45. __TTSf4s_s___TF4tcoO3tcoFTSiSi_Si: pushq %rbp movq %rsp, %rbp testq %rdi, %rdi jle LBB33_4 LBB33_1: movq %rdi, %rax decq %rax jo LBB33_5 imulq %rdi, %rsi jo LBB33_5 testq %rax, %rax movq %rax, %rdi jg LBB33_1 LBB33_4: movq %rsi, %rax popq %rbp retq
46. __TTSf4s_s___TF4tcoO3tcoFTSiSi_Si: pushq %rbp movq %rsp, %rbp testq %rdi, %rdi jle LBB33_4 LBB33_1: movq %rdi, %rax decq %rax jo LBB33_5 imulq %rdi, %rsi jo LBB33_5 testq %rax, %rax movq %rax, %rdi jg LBB33_1 LBB33_4: movq %rsi, %rax popq %rbp retq
47. SOMETIMES Swift Does This.
48. AUTOMATIC REFERENCE COUNTING Imagine compile time garbage collection.
49. OLDEN TIMES Memory managed by hand.
50. @interface Something() { NSNumber *_numberIvar; } @end @implementation Something - (void)setNumber:(NSNumber *)number { NSNumber *tmp = _numberIvar; _numberIvar = [number retain]; [tmp release]; } @end
51. @interface Something() { NSNumber *_numberIvar; } @end @implementation Something - (void)setNumber:(NSNumber *)number { NSNumber *tmp = _numberIvar; _numberIvar = [number retain]; [tmp release]; } @end
52. WITH ARC
53. - (void)setNumber:(NSNumber *)number { _numberIvar = number; }
54. COMPILER MAGIC Inserts retain/release calls.
55. ARC in Swift Manages references.
56. TCO + ARC
57. class Integer: IntegerLiteralConvertible { let val: Int required init(integerLiteral value: Int) { self.val = value } init(_ val: Int) { self.val = val } }
58. func +(lhs: Integer, rhs: Integer) -> Integer { return Integer(lhs.val + rhs.val) } func -(lhs: Integer, rhs: Integer) -> Integer { return Integer(lhs.val - rhs.val) } func *(lhs: Integer, rhs: Integer) -> Integer { return Integer(lhs.val * rhs.val) } func >(lhs: Integer, rhs: Integer) -> Bool { return lhs.val > rhs.val }
59. func factorialTC(n: Integer) -> Integer { return tc(n, 1) } func tc(n: Integer, acc: Integer) -> Integer { if n > 0 { return tc(n - 1, acc * n) } return acc }
60. __TTSf4g_g___TF4tcoO2tcFTCS_7IntegerS0__S0_: pushq %rbp movq %rsp, %rbp . . leaq __TMdC4tcoO7Integer+16(%rip), %rdi callq _swift_getInitializedObjCClass movq %rax, %rbx . . movq %rbx, %rdi callq _swift_allocObject movq %rax, %r15 . . movq %r15, %rdi movq %rbx, %rsi callq __TTSf4g_g___TF4tcoO2tcFTCS_7IntegerS0__S0_ movq %rax, %r14 movq %r15, %rdi callq _swift_release movq %rbx, %rdi callq _swift_release jmp LBB34_7 LBB34_7: movq %r14, %rax . . popq %rbp retq LBB34_8:
61. __TTSf4g_g___TF4tcoO2tcFTCS_7IntegerS0__S0_: pushq %rbp movq %rsp, %rbp . . leaq __TMdC4tcoO7Integer+16(%rip), %rdi callq _swift_getInitializedObjCClass movq %rax, %rbx . . movq %rbx, %rdi callq _swift_allocObject movq %rax, %r15 . . movq %r15, %rdi movq %rbx, %rsi callq __TTSf4g_g___TF4tcoO2tcFTCS_7IntegerS0__S0_ movq %rax, %r14 movq %r15, %rdi callq _swift_release movq %rbx, %rdi callq _swift_release jmp LBB34_7 LBB34_7: movq %r14, %rax . . popq %rbp retq LBB34_8:
62. __TTSf4g_g___TF4tcoO2tcFTCS_7IntegerS0__S0_: pushq %rbp movq %rsp, %rbp . . leaq __TMdC4tcoO7Integer+16(%rip), %rdi callq _swift_getInitializedObjCClass movq %rax, %rbx . . movq %rbx, %rdi callq _swift_allocObject movq %rax, %r15 . . movq %r15, %rdi movq %rbx, %rsi callq __TTSf4g_g___TF4tcoO2tcFTCS_7IntegerS0__S0_ movq %rax, %r14 movq %r15, %rdi callq _swift_release movq %rbx, %rdi callq _swift_release jmp LBB34_7 LBB34_7: movq %r14, %rax . . popq %rbp retq LBB34_8: No TCO
63. __TTSf4g_g___TF4tcoO2tcFTCS_7IntegerS0__S0_: pushq %rbp movq %rsp, %rbp . . leaq __TMdC4tcoO7Integer+16(%rip), %rdi callq _swift_getInitializedObjCClass movq %rax, %rbx . . movq %rbx, %rdi callq _swift_allocObject movq %rax, %r15 . . movq %r15, %rdi movq %rbx, %rsi callq __TTSf4g_g___TF4tcoO2tcFTCS_7IntegerS0__S0_ movq %rax, %r14 movq %r15, %rdi callq _swift_release movq %rbx, %rdi callq _swift_release jmp LBB34_7 LBB34_7: movq %r14, %rax . . popq %rbp retq LBB34_8: No TCO Last Action Performed
64. Integer created on +,-,* ops
65. func +(lhs: Integer, rhs: Integer) -> Integer { return Integer(lhs.val + rhs.val) } func -(lhs: Integer, rhs: Integer) -> Integer { return Integer(lhs.val - rhs.val) } func *(lhs: Integer, rhs: Integer) -> Integer { return Integer(lhs.val * rhs.val) }
66. NEEDS RELEASING
67. func tc(n: Integer, acc: Integer) -> Integer { if n > 0 { return tc(n - 1, acc * n) } return acc } SO THIS…
68. func tc(n: Integer, acc: Integer) -> Integer { let _n1 = n - 1 let _accn = acc * n if n > 0 { let tmp = tc(_n1, _accn) swift_release(_accn) swift_release(_n1) return tmp } return acc } IS REALLY SOMETHING LIKE
69. All Is sometimes Lost
70. FUNCTIONAL SWIFT Use value types.
71. VALUE SEMANTICS ≠ VALUE TYPES
72. SO.. BE CAREFULL
73. THANKS!
74. github.com/haveahennessy @haveahennessy
75. http://www.hopperapp.com/