Pointer assignment
Compiler Implementation
COP-3402
Pointer assignment in SimpleIR
Dereference assignment
*t1 := 11
t1 is an address. Store the value 11 at that address.
Stack frame layout
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 diagram, same as prior lecture
Assigning to a dereferenced pointer
*t1 := 11
Stack frame, with addresses
- What's the value of t1?
- What's the value of *t1?
Following an address
What assembly instruction stores a value from to address?
Register indirect mov
mov %rbx, (%rax)
(%rax) here means store the value at the address given in %rax.
Computing the address
- Get the value of the variable (the address)
- Get the value of memory at that address
- Store that the value of memory at that address
- Add the offset (it's negative so it's equivalent to subtraction)
- Store the result
Computing the address
(Diagram)
Assembly code
Assume the following offsets
| Variable | Offset |
|---|---|
| t1 | -40 |
Assembly code
# *t1 := 11
mov -40(%rbp), %rax # value of the deref'ed variable (t1)
mov $11, %rbx # value of the right-hand side (11)
mov %rbx, (%rax) # store the result in the address (*t1)