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Functions
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Terms
• foo is the caller
• bar is the callee
• a, b are the actual parameters to bar
• x, y are the formal parameters of bar
• Shorthand:
• argument = actual parameter
• parameter = formal parameter
void foo() {int a, b;...bar(a, b);
}
void bar(int x, int y) { ...}
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Different kinds of parameters
• Value parameters
• Reference parameters
• Result parameters
• Value-result parameters
• Read-only parameters
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Value parameters
• “Call-by-value”
• Used in C, Java, default in C++
• Passes the value of an argument to the function
• Makes a copy of argument when function is called
• Advantages? Disadvantages?
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Value parameters
int x = 1;void main () { foo(x, x); print(x);}
void foo(int y, int z) { y = 2; z = 3; print(x);}
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Value parameters
• What do the print statements print?
int x = 1;void main () { foo(x, x); print(x);}
void foo(int y, int z) { y = 2; z = 3; print(x);}
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Value parameters
• What do the print statements print?
• Answer:
print(x); //prints 1
print(x); //prints 1
int x = 1;void main () { foo(x, x); print(x);}
void foo(int y, int z) { y = 2; z = 3; print(x);}
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Reference parameters
• “Call-by-reference”
• Optional in Pascal (use “var” keyword) and C++ (use “&”)
• Pass the address of the argument to the function
• If an argument is an expression, evaluate it, place it in memory and then pass the address of the memory location
• Advantages? Disadvantages?
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Reference parameters
int x = 1;void main () { foo(x, x); print(x);}
void foo(int &y, int &z) { y = 2; z = 3; print(x); print(y);}
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Reference parameters
• What do the print statements print?
int x = 1;void main () { foo(x, x); print(x);}
void foo(int &y, int &z) { y = 2; z = 3; print(x); print(y);}
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Reference parameters
• What do the print statements print?
• Answer:
print(x); //prints 3
print(x); //prints 3
print(y); //prints 3!
int x = 1;void main () { foo(x, x); print(x);}
void foo(int &y, int &z) { y = 2; z = 3; print(x); print(y);}
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Result parameters
• Return values of a function
• Some languages let you specify other parameters as result parameters – these are un-initialized at the beginning of the function
• Copied at the end of function into the arguments of the caller
• C++ supports “return references”
int& foo( ... )
compute return values, store in memory, return address of return value
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Result parameters
int x = 1;void main () { foo(x, x); print(x);}
void foo(int y, result int z) { y = 2; z = 3; print(x);}
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Result parameters
• What do the print statements print?
int x = 1;void main () { foo(x, x); print(x);}
void foo(int y, result int z) { y = 2; z = 3; print(x);}
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Result parameters
• What do the print statements print?
• Answer:
print(x); //prints 3
print(x); //prints 1
int x = 1;void main () { foo(x, x); print(x);}
void foo(int y, result int z) { y = 2; z = 3; print(x);}
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Value-result parameters
• “Copy-in copy-out”
• Evaluate argument expression, copy to parameters
• After subroutine is done, copy values of parameters back into arguments
• Results are often similar to pass-by-reference, but there are some subtle situations where they are different
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Value-result parametersint x = 1;int w = 1;void main () { foo(w, x); print(x); print(w);}
void foo(int& y, value result int z) {
y = 2; z = 3; print(x); print(w);}
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Value-result parameters
• What do the print statements print?
int x = 1;int w = 1;void main () { foo(w, x); print(x); print(w);}
void foo(int& y, value result int z) {
y = 2; z = 3; print(x); print(w);}
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Value-result parameters
• What do the print statements print?
• Answer:
print(x) //prints 3
print(w) //prints 2
print(x) //prints 1
print(w) //prints 2
int x = 1;int w = 1;void main () { foo(w, x); print(x); print(w);}
void foo(int& y, value result int z) {
y = 2; z = 3; print(x); print(w);}
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What about this?
int x = 1;void main () { foo(x, x); print(x);}
void foo(value result int y, value result int z) {
y = 2; z = 3; print(x);}
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What about this?
• What do the print statements print?
int x = 1;void main () { foo(x, x); print(x);}
void foo(value result int y, value result int z) {
y = 2; z = 3; print(x);}
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What about this?
• What do the print statements print?
• Answer:
print(x); //undefined!
print(x); //prints 1
int x = 1;void main () { foo(x, x); print(x);}
void foo(value result int y, value result int z) {
y = 2; z = 3; print(x);}
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Read only parameters
• Used when callee will not change value of parameters
• Read-only restriction must be enforced by compiler
• This can be tricky when in the presence of aliasing and control flow
• Is this legal? Hard to tell!
• gcc will not let the assignment happen
void foo(const int x, int y) {int * p;if (...) p = &x else p = &y*p = 4
}
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Esoteric: “name” parameters• “Call-by-name”
• Usually, we evaluate the arguments before passing them to the function. In call-by-name, the arguments are passed to the function before evaluation
• Not used in many languages, but Haskell uses a variant
int x = 2;void main () { foo(x + 2);}
void foo(int y) { z = y + 2; print(z);}
int x = 2;void main () { foo(x + 2);}
void foo(int y) { z = x + 2 + 2; print(z);}
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Why is this useful?
• Consider the code on the left
• Normally, we must evaluate bar() before calling foo()
• But what if bar() runs for a long time?
• In call by name, we only evaluate bar() if we need to use it
int x = 2;void main () { foo(bar());}
void foo(int y) {if ( ... ) {z = y;
} else {z = 3;
} print(z);}
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Other considerations
• Scalars
• For call by value, can pass the address of the actual parameter and copy the value into local storage within the procedure
• Reduces size of caller code (why is this good?)
• For machines with a lot of registers (e.g., MIPS), compilers will save a few registers for arguments and return types
• Less need to manipulate stack
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Other considerations
• Arrays
• For efficiency reasons, arrays should be passed by reference (why?)
• Java, C, C++ pass arrays by reference by default (technically, they pass a pointer to the array by value)
• Pass in a fixed size dope vector as the actual parameter (not the whole array!)
• Callee can copy array into local storage as needed
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Dope vectors
• Remember: store additional information about an array
• Where it is in memory
• Size of array
• # of dimensions
• Storage order
• Can sometimes eliminate dope vectors with compile-time analysis
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Strings
• Requires a descriptor
• Like a dope vector, provides information about string
• May just need to pass a pointer (if string contains information about its length)
• May also need to pass information about length
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Calling a function
• What should happen when a function is called?
• Set the frame pointer (sets the base of the activation record)
• Allocate space for local variables (use the function’s symbol table for this)
• What about registers?
• Callee might want to use registers that the caller is using
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Saving registers
• Two options: caller saves and callee saves
• Caller saves
• Caller pushes all the registers it is using on to the stack before calling function, restores the registers after the function returns
• Callee saves
• Callee pushes all the registers it is going to use on the stack immediately after being called, restores the registers just before it returns
• Why use one vs. the other?
• Simple optimizations are good here: don’t save registers if the caller/callee doesn’t use any
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Activation recordsReturn value
Actual parameters
Caller's return address
Caller's frame pointer
Static links (other FPs)
Register save area
Local variables
FP register
Sta
ck G
row
th
Calle
r's r
esponsib
ility
Calle
e's
responsib
ility
Is this record generated for callee-saves or caller-saves? How would the other record look?
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The frame pointer
• Manipulate with instructions like link and unlink
• Link: push current value of FP on to stack, set FP to top of stack
• Unlink: read value at current address pointed to by FP, set FP to point to that value
• In other words: link pushes a new frame onto the stack, unlink pops it off
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24
Example Subroutine Call and Stack Frame
int SubOne(int a, int b) { int l1, l2; I1 = a; l2 = b; return l1+l2; };
z = SubOne(x,2*y);
Lower addr
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Example Subroutine Call and Stack Frame
int SubOne(int a, int b) { int l1, l2; I1 = a; l2 = b; return l1+l2; };
z = SubOne(x,2*y);
return valuex
2*yreturn addresssaved frame ptr
l1l2
. . .
stac
k
R6
Lower addr
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Example Subroutine Call and Stack Frame
int SubOne(int a, int b) { int l1, l2; I1 = a; l2 = b; return l1+l2; };
z = SubOne(x,2*y);
return valuex
2*yreturn addresssaved frame ptr
l1l2
. . .
stac
k
R6
pushpush xmul 2 y t1push t1jsr SubOnepoppoppop z
link 3move $P1 $L1move $P2 $L2add $L1 $L2 t2move t2 $Runlinkret
3-address code:
Lower addr
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Example Subroutine Call and Stack Frame
int SubOne(int a, int b) { int l1, l2; I1 = a; l2 = b; return l1+l2; };
z = SubOne(x,2*y);
return valuex
2*yreturn addresssaved frame ptr
l1l2
. . .
stac
k
R6
pushpush xmul 2 y t1push t1jsr SubOnepoppoppop z
link 3move $P1 $L1move $P2 $L2add $L1 $L2 t2move t2 $Runlinkret
3-address code:pushpush xload y R1muli 2 R1push R1jsr SubOnepoppoppop R1store R1 z
link R6 3load 3(R6) R1store R1 -1(R6)load 2(R6) R2store R2 -2(R6)load -1(R6) R1add -2(R6) R1store R1 4(R6)unlinkret
assembly code:
Lower addr
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