Compilers 2008
Topic 6: Code Generation
6.5 Functions
Code Generation
Functions
The Call Stack
•
•
•
•
•
•
Apart from addressable memory and registers, assembler
makes use of a stack to hold information about
procedure/function calling and nesting in order to switch to
the context of the called function and restore to the caller
function when the calling finishes.
Before calling the function, parameters are pushed onto the
stack.
Then, the program address to return to is pushed on the
stack.
On entering the routine, space is allocated for local
variables of that routine.
On leaving the routine, the part of the stack used by the
routine can be ‘popped’.
Recursive routines thus have separate memory space.
2
1
Code Generation
Functions
The Call Stack
• Uses the highest part of free
memory
• Starts from highest address, and
each subsequent element is below
• Elements consume different number
of bytes (2, 4, 8)
• SP: stack pointer: a special register,
points to the current top element on
the stack.
• BP: Base Pointer: a special register,
points to the start of the stack space
for THIS routine.
b
a
999-1000
997-998
return address 995-996
BP
old_bp
i
993-994
991-992
j
k
989-990
987-988
r
983-986
SP
Free
Memory
3
Code Generation
Functions
Generating Assmebler for functions
•
For the compiler to generate code for functions, we need to
consider the following questions:
1. How are the arguments to the function passed to the invoked
function?
2. How are the results of the function call returned to the main
program?
4
2
Code Generation
Functions
Passing of Arguments
1. How are the arguments to the function passed to the invoked
function?
•
Answer:
•
•
The calling code pushes the function's parameters onto the
stack, one after another, in reverse order (the first argument
specified to the function is pushed last).
The caller then executes a CALL instruction to pass control to
the function.
5
Source program:
...
rutina(a, b)
...
int rutina (int a, char *b)
{
int i, j, k;
double r;
...
}
Object program:
rutina:
PUSH b
PUSH a
CALL rutina
‘rutina’ can now
access its
parameters on the
stack.
6
3
Code Generation
Functions
Functions and the Stack
•
•
•
•
On entering a subroutine, the subroutine usually resets the
base pointer (BP) to point at the start of the local variable
space for the subroutine.
The old value of BP is first pushed onto the stack.
The BP is then set to the current top of the stack (so the old
BP is the first element on the stack).
When we return from the routine, we restore the old value
of BP.
7
Source program:
...
rutina(a, b)
...
int rutina (int a, char *b)
{
int i, j, k;
double r;
...
}
Object program:
PUSH b
PUSH a
CALL rutina
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV SP,BP
POP BP
RET
8
4
BP, SP
PUSH b
PUSH a
CALL rutina
ADD SP,4
Free
Memory
Initially,
Stack is
empty
…
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV SP,BP
POP BP
RET
9
PUSH b
PUSH a
CALL rutina
ADD SP,4
…
BP
SP
b
Free
Memory
Param b is
pushed onto
stack (last
param first)
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV SP,BP
POP BP
RET
10
5
BP
PUSH b
PUSH a
CALL rutina
ADD SP,4
…
SP
b
a
Free
Memory
Param a is
pushed onto
stack, stack
pointer
increments.
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV SP,BP
POP BP
RET
11
BP
PUSH b
PUSH a
CALL rutina
ADD SP,4
…
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV SP,BP
POP BP
RET
SP
Function call
has side effect
of pushing
return address
onto stack.
b
a
return address
Free
Memory
12
6
BP
PUSH b
PUSH a
CALL rutina
ADD SP,4
b
a
return address
SP
…
Control passes to
rutina.
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV SP,BP
POP BP
RET
First thing the
function must do is
preserve the Base
Pointer of the main
routine.
old_bp
Free
Memory
13
PUSH b
PUSH a
CALL rutina
ADD SP,4
…
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV SP,BP
POP BP
RET
b
a
return address
BP,SP
Second thing is to
set the BP for the
current routine to
the current stack
pointer value.
old_bp
Free
Memory
14
7
PUSH b
PUSH a
CALL rutina
ADD SP,4
…
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV SP,BP
POP BP
RET
b
a
return address
BP
We now advance the
stack pointer to
leave space for the
local variables:
i, j, k (int: 2 bytes
each)
old_bp
i
j
k
r
SP
Free
Memory
r (double: 8 bytes)
15
PUSH b
PUSH a
CALL rutina
ADD SP,4
…
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV SP,BP
POP BP
RET
b
a
[BP+8]
[BP+6]
return address [BP+2]
BP
Note that the stack
grows DOWN from
higher addresses to
lower, not UP.
old_bp
i
[BP]
[BP-2]
j
k
[BP-4]
r
[BP-14]
[BP-6]
SP
Free
Memory
16
8
PUSH b
PUSH a
CALL rutina
ADD SP,4
Set SP to the
current BP (the
bottom of the top
scope is the top of BP, SP
the previous scope)
…
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV SP,BP
POP BP
RET
b
a
[BP+8]
[BP+6]
return address [BP+2]
old_bp
i
[BP]
[BP-2]
j
k
[BP-4]
r
[BP-14]
[BP-6]
Free
Memory
17
PUSH b
PUSH a
CALL rutina
ADD SP,4
…
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV SP,BP
POP BP
RET
BP
Then ‘pop’ the
current top of stack
into the BP register
This Restores the
old BP.
It also increases SP
SP
b
a
[BP+8]
[BP+6]
return address [BP+2]
old_bp
i
[BP]
[BP-2]
j
k
[BP-4]
r
[BP-14]
[BP-6]
Free
Memory
18
9
PUSH b
PUSH a
CALL rutina
ADD SP,4
…
BP
The ‘RET’
operation reads the
return address from
the top of the stack
and executes code
from that point.
SP
b
a
[BP+6]
return address [BP+2]
old_bp
i
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV SP,BP
POP BP
RET
[BP+8]
[BP]
[BP-2]
j
k
[BP-4]
r
[BP-14]
[BP-6]
Free
Memory
19
PUSH b
PUSH a
CALL rutina
ADD SP,4
BP, SP
This ‘ADD’
operation takes ‘a’
and ‘b’ off the stack
(2 bytes each)
b
a
[BP+8]
[BP+6]
[BP+2]
[BP]
[BP-2]
…
[BP-4]
[BP-6]
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV SP,BP
POP BP
RET
[BP-14]
20
10
Code Generation
Functions
Returning Value
2. How are the results of the function call returned to the main
program?
•
Answer:
•
Depends on the compiler, but conventionally:
•
•
if the result fits in the EAX register, leave the result there.
Otherwise, place the values on the stack, or in memory.
21
Source Program:
...
rutina(a, b)
...
int rutina (int a, char *b)
{
int i, j, k;
double r;
...
return k;
}
Object Program:
PUSH b
PUSH a
CALL rutina
ADD SP,4
rutina:
PUSH BP
MOV BP,SP
SUB SP,14
...
MOV EAX, [BP-6]
MOV SP,BP
POP BP
RET
Move K
into EAX
22
11