C++ Named Return Value Optimization

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1 C++ Named Return Value Optimization

2 post. meetup.com/c-user-group-karlsruhe

4 auto create() -> T { T castor /* initialize */; //... return castor; } void use() { T pollux = create(); //... }

5 variable object resource

6 { } Type variable /* initialize */; //... //...

7 { } Type variable /* initialize */; //... return stuff;

8 { } Type variable /* initialize */; //... throw stuff;

9 variable object clean-up scope left destroyed executed

10 class Type { //... }; ~Type() { // clean-up code }

11 { } Type variable /* init */ ; Type other_var = variable; //...

12 variable object clean-up different independent independent

13 { } Type variable /* init */ ; { Type other_var = variable; //... } //...

14 variable object resource

16 int MyInt array<int, 10> vector<int>

17 { int castor = 0; int pollux = castor; assert(castor == pollux); castor = 1729; pollux = -1; } assert(castor!= pollux);

18 { vector<int> castor(1000); vector<int> pollux = castor; assert(castor == pollux); castor.at(42) = 1729; pollux.at(42) = -1; } assert(castor!= pollux);

19 int MyInt array<int, 10> vector<int> special

20 vector<int> memory allocation object resource

21 { vector<int> castor(1000); vector<int> pollux = castor; assert(castor == pollux); castor.at(42) = 1729; pollux.at(42) = -1; } assert(castor!= pollux);

22 variable object resource different independent independent

23 variable object resource initialize from copy copy clone variable object resource

24 auto create() -> vector<int> { vector<int> castor(1000); //... return castor; } void use() { vector<int> pollux = create(); //... }

25 variable object resource

27 int_pair p() { int_pair r = {0}; return r; } int_quadruple q() { int_quadruple r = {0}; return r; } movl ret movl movl ret $0, %eax $0, %eax $0, %edx

28 ... sets register 0 register 1 register 2 reads callee (called function) calls caller

29 int_pair p() { int_pair r = {0}; return r; } int_quadruple q() { int_quadruple r = {0}; return r; } int_octuple o() { int_octuple r = {0}; return r; } movl ret movl movl ret movq movl movl movl movl movl movl movl movl ret $0, %eax $0, %eax $0, %edx %rdi, %rax $0, (%rdi) $0, 4(%rdi) $0, 8(%rdi) $0, 12(%rdi) $0, 16(%rdi) $0, 20(%rdi) $0, 24(%rdi) $0, 28(%rdi)

30 ... reads address register 0 register 1 register 2 sets address callee (called function) calls caller writes to address space for return value

32 RAM register register 0 register 1 register 2

33 RAM register address 2 size 4 register 0 register 1 register 2

34 RAM register address 2 size 4 register 0 register 1 register 2

36 object object

37 trivially copyable

38 ... reads address register 0 register 1 register 2 sets address callee (called function) calls caller writes to address space for return value

39 variable object resource indirect return for non-trivially-copyable

41 void use() { vector<int> pollux = create(); } //... auto create() -> vector<int> { vector<int> castor(1000); //... return castor; }

42 void use() { buffer< vector<int> > future_pollux; create(&future_pollux); vector<int>& pollux = future_pollux; //... } void create(buffer< vector<int> >* ret) { vector<int> castor(1000); //... new(ret) vector<int>(castor); }

43 void create( buffer< vector<int> >* ret) { } vector<int> castor(1000); new(ret) vector<int>(castor); pushq subq movq movl movq call movq movq call movq call movq addq popq ret %rbx $32, %rsp %rdi, %rbx $1000, %esi %rsp, %rdi vector<int>::vector(int) %rsp, %rsi %rbx, %rdi vector<int> ::vector(vector<int> const&) %rsp, %rdi vector<int>::~vector() %rbx, %rax $32, %rsp %rbx

44 void create( buffer< vector<int> >* ret) { new(ret) vector<int>(1000); pushq movq movl call %rbx %rdi, %rbx $1000, %esi vector<int>::vector(int) } movq popq ret %rbx, %rax %rbx

45 NRVO = indirect return - unneccesary operations

46 void use() { vector<int> pollux = create(); } //... auto create() -> vector<int> { vector<int> castor(1000); //... return castor; }

47 pollux castor object resource different same same

48 observable behaviour is guaranteed but not how it is achieved

49 int x = 0; for(int i = 0; i < 100; ++i) x += i; cout << x; mov edi, offset std::cout mov esi, 4950 call ostream::operator<<(int)

50 observable behaviour is guaranteed but not how it is achieved *except NRVO / copy elision

51 resource management is potentially observable

52 limits of NRVO local variable same type as return type

53 auto no_nrvo() -> vector<int> { array< vector<int>, 2 > ogre; } return ogre.at(0);

54 limits of NRVO local variable same type as return type complete object

55 auto nrvo(bool b) -> vector<int> { vector<int> thor(500); vector<int> loki(900); } if(b) return thor; else return loki;

56 limits of NRVO local variable same type as return type complete object compiler smartness

57 auto nrvo() -> vector<int> { vector<int> thor(500); vector<int> loki(900); } if(read_char() == 'a') return thor; else return loki;

58 limits of NRVO local variable same type as return type complete object compiler smartness observable behaviour

60 variable object resource variable object

61 moving is great! but... for some types it is still slow don t want to write it explicitly

62 auto nrvo() -> vector<int> { vector<int> thor(500); vector<int> loki(900); } if(read_char() == 'a') return thor; else return loki;

63 NRVO > move > copy

64 auto no_nrvo() -> vector<int> { vector<int> thor(500); vector<int> loki(900); } if(read_char() == 'a') return move(thor); else return move(loki);

Machine Program: Procedure. Zhaoguo Wang

Machine Program: Procedure. Zhaoguo Wang Machine Program: Procedure Zhaoguo Wang Requirements of procedure calls? P() { y = Q(x); y++; 1. Passing control int Q(int i) { int t, z; return z; Requirements of procedure calls? P() { y = Q(x); y++;