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BETTER RECURSION WITH SWIFT

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AKA…

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RAIDERS OF THE LOST ARC Another reason why Swift value types matter.

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FAST. MODERN. SAFE

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FUNCTIONAL-ISH

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let str = "Favours Immutability"

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GOOD: CONCURRENCY

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MEH: CACHE EFFICIENCY

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CHALLENGE FOR ITERATION

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func factorial(n: Int) -> Int { var result = 1 var x = n while x > 0 { result = result * x x-- } return result }

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RECURSION Immutable iteration!

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func factorialRec(n: Int) -> Int { if n > 0 { return n * factorialRec(n - 1) } return 1 }

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ELEGANT.

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YET INEFFICIENT. New call frame on each iteration.

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STACK OVERFLOW Easier than ever with recursion!

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All Is not Lost

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TAIL CALLS Recursive call is performed last.

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func tco(n: Int, acc: Int) -> Int { if n > 0 { return tco(n - 1, acc * n) } return acc }

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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) }

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func tco(n: Int, acc: Int) -> Int { if n > 0 { return tco(n - 1, acc * n) } return acc }

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CAN BE OPTIMIZED To something like this…

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func factorial(n: Int) -> Int { var result = 1 var x = n while x > 0 { result = result * x x-- } return result }

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DOES SWIFT DO THIS?

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DOES SWIFT DO THIS? The internet says: *Sometimes*

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Or… xcrun swiftc -S

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AMD64 ABI Args placed in rdi, rsi, rdx, .. Return value -> rax

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func tco(n: Int, acc: Int) -> Int rdi → n rsi → acc rax → return value

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IS TCO HAPPENING? call → Bad - Creates new stack frame jmp, jg, jle, j… → Good - Stays in current stack frame

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__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

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__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.

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__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

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__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

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__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

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__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

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__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

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OPTIMIZE

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WALK: -Onone

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KILL: -O

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-Ounchecked

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__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

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__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

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__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

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SOMETIMES Swift Does This.

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AUTOMATIC REFERENCE COUNTING Imagine compile time garbage collection.

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OLDEN TIMES Memory managed by hand.

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@interface Something() { NSNumber *_numberIvar; } @end @implementation Something - (void)setNumber:(NSNumber *)number { NSNumber *tmp = _numberIvar; _numberIvar = [number retain]; [tmp release]; } @end

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@interface Something() { NSNumber *_numberIvar; } @end @implementation Something - (void)setNumber:(NSNumber *)number { NSNumber *tmp = _numberIvar; _numberIvar = [number retain]; [tmp release]; } @end

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WITH ARC

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- (void)setNumber:(NSNumber *)number { _numberIvar = number; }

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COMPILER MAGIC Inserts retain/release calls.

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ARC in Swift Manages references.

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TCO + ARC

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class Integer: IntegerLiteralConvertible { let val: Int required init(integerLiteral value: Int) { self.val = value } init(_ val: Int) { self.val = val } }

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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 }

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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 }

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__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:

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__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:

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__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

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__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

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Integer created on +,-,* ops

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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) }

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NEEDS RELEASING

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func tc(n: Integer, acc: Integer) -> Integer { if n > 0 { return tc(n - 1, acc * n) } return acc } SO THIS…

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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

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All Is sometimes Lost

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FUNCTIONAL SWIFT Use value types.

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VALUE SEMANTICS ≠ VALUE TYPES

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SO.. BE CAREFULL

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THANKS!

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github.com/haveahennessy @haveahennessy

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http://www.hopperapp.com/