Pointer ref and deref
Compiler Implementation
COP-3402
Pointer operations in SimpleIR
Reference
t1 := &x
Store the address of x (not the value) in t1.
Dereference
t2 := *t1
t1 is an address. Take the value of memory at the address and store it in t2.
Dereference assignment
*t1 := 11
t1 is an address. Store the value 11 at that address.
Complete example
function main localvars a b x y t1 t2 t3 params a b x := 8 y := 9 t1 := &x t2 := *t1 *t1 := 11 t3 := x return x
What is the value of x that gets returned?
- x gets 8
- t1 gets the address of x
- 11 gets stored at the address given by t1, which is the address of x
- x's value is then 11
Stack-allocated variables
How are function-local variables stored at runtime?
Stack frame layout refresher
function main localvars a b x y t1 t2 t3 params a b x := 8 y := 9 t1 := &x t2 := *t1 *t1 := 11 t3 := x return x
Stack frame layout refresher
(Diagram)
- Stack frame layout for the
mainfunction above. - Give numeric addresses to each frame entry.
- Show %rbp
Assignment
How does assignment work in assembly?
Use an offset from %rbp
mov $8, -24(%rbp)
Getting (referencing) a pointer
t1 := &x
Stack frame, with addresses
How do we get the address?
- What will hold the stack frame address?
- What's the offset?
- Assembly operations to compute the address?
Computing the address
- Start with %rbp
- Add the offset (it's negative so it's equivalent to subtraction)
- Store the result
Does this involve the value of the address in anyway?
A little easier to see in when looking at the stack frame
Computing the address
(Diagram)
Assembly code
Assume the following offsets (see the stack frame)
| Variable | Offset |
|---|---|
| x | -24 |
| t1 | -40 |
Assembly code
mov %rbp, %rax # start with stack frame address
add $-24, %rax # add the offset of the referenced var (x)
mov %rax, -40(%rbp) # store the result in the assigned var (t1)
(Diagram)
Dereferencing a pointer
t2 := *t1
What's the value of t2?
- If t1 = 260
- If t1 = 260 and address 260 holds the value 10?
Following an address
What assembly instructions loads a value from an address?
Register indirect mov
mov (%rax), %rbx
(%rax) here means load the value at the address given in %rax.
Computing the address
- Get the value of the de-ref'ed variable (the address)
- Get the value of memory at that address
- Store that the value of memory at that address
Computing the address
(Diagram)
Assembly code
Assume the following offsets
| Variable | Offset |
|---|---|
| x | -24 |
| t1 | -40 |
| t2 | -48 |
Assembly code
mov -40(%rbp), %rax # value of the deref'ed variable (t1)
mov (%rax), %rbx # the value in memory at address %rax (*t1)
mov %rbx, -48(%rbp) # store the result in the assigned var (t2)
Notice how the value of x is the same as the value of *t1. Why is that?
Show on stack frame why this is the case.