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Better Recursion With Swift
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Matt Isaacs
August 26, 2015
Programming
1
590
Better Recursion With Swift
Presented at the Brooklyn Swift Developers Meetup, August 2015.
Matt Isaacs
August 26, 2015
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Transcript
BETTER RECURSION WITH SWIFT
AKA…
RAIDERS OF THE LOST ARC Another reason why Swift value
types matter.
None
None
None
FAST. MODERN. SAFE
FUNCTIONAL-ISH
let str = "Favours Immutability"
GOOD: CONCURRENCY
MEH: CACHE EFFICIENCY
CHALLENGE FOR ITERATION
func factorial(n: Int) -> Int { var result = 1
var x = n while x > 0 { result = result * x x-- } return result }
RECURSION Immutable iteration!
func factorialRec(n: Int) -> Int { if n > 0
{ return n * factorialRec(n - 1) } return 1 }
ELEGANT.
YET INEFFICIENT. New call frame on each iteration.
STACK OVERFLOW Easier than ever with recursion!
All Is not Lost
TAIL CALLS Recursive call is performed last.
func tco(n: Int, acc: Int) -> Int { if n
> 0 { return tco(n - 1, acc * n) } return acc }
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) }
func tco(n: Int, acc: Int) -> Int { if n
> 0 { return tco(n - 1, acc * n) } return acc }
CAN BE OPTIMIZED To something like this…
func factorial(n: Int) -> Int { var result = 1
var x = n while x > 0 { result = result * x x-- } return result }
DOES SWIFT DO THIS?
DOES SWIFT DO THIS? The internet says: *Sometimes*
None
Or… xcrun swiftc -S
AMD64 ABI Args placed in rdi, rsi, rdx, .. Return
value -> rax
func tco(n: Int, acc: Int) -> Int rdi → n
rsi → acc rax → return value
IS TCO HAPPENING? call → Bad - Creates new stack
frame jmp, jg, jle, j… → Good - Stays in current stack frame
__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
__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.
__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
__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
__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
__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
__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
OPTIMIZE
WALK: -Onone
KILL: -O
-Ounchecked
__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
__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
__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
SOMETIMES Swift Does This.
AUTOMATIC REFERENCE COUNTING Imagine compile time garbage collection.
OLDEN TIMES Memory managed by hand.
@interface Something() { NSNumber *_numberIvar; } @end @implementation Something -
(void)setNumber:(NSNumber *)number { NSNumber *tmp = _numberIvar; _numberIvar = [number retain]; [tmp release]; } @end
@interface Something() { NSNumber *_numberIvar; } @end @implementation Something -
(void)setNumber:(NSNumber *)number { NSNumber *tmp = _numberIvar; _numberIvar = [number retain]; [tmp release]; } @end
WITH ARC
- (void)setNumber:(NSNumber *)number { _numberIvar = number; }
COMPILER MAGIC Inserts retain/release calls.
ARC in Swift Manages references.
TCO + ARC
class Integer: IntegerLiteralConvertible { let val: Int required init(integerLiteral value:
Int) { self.val = value } init(_ val: Int) { self.val = 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) -> Integer { return Integer(lhs.val * rhs.val) } func >(lhs: Integer, rhs: Integer) -> Bool { return lhs.val > rhs.val }
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 }
__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:
__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:
__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
__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
Integer created on +,-,* ops
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) }
NEEDS RELEASING
func tc(n: Integer, acc: Integer) -> Integer { if n
> 0 { return tc(n - 1, acc * n) } return acc } SO THIS…
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
All Is sometimes Lost
FUNCTIONAL SWIFT Use value types.
VALUE SEMANTICS ≠ VALUE TYPES
SO.. BE CAREFULL
THANKS!
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