1. TARGET CODE -Next Usage
Rajat Sethi
04CS3022
TARGET CODE
-Next Usage

2. Topics to be covered Next Use Information Code Generation
Computation using Backward Scan
Code Generation
Register and Address Descriptor
Algorithm
getreg() function
Example

3. Next Usage
This technique is employed in dead code optimization. The instructions with no ‘next use’ are removed from the block.
If the name in a register is no longer needed, then the register can be assigned to some other name.
We collect next-use information about names in a basic block. Note that we consider only the variables in the same block.

4. Backward Scan: statement i : x := y op z
STEP 1: Attach to statement i, attribute
information of next use of x, y and z.
STEP 2: In symbol table, set x to “no next use”
and “not live”
STEP 3: In symbol table set the next uses of
y and z to I and also to “live”
NOTE : The orders of Step 2 and Step 3 should not
be interchanged because x may be y or z.
e.g.:- x := x+1

5. Register & Address Descriptors
Register Descriptor: It keeps track of what is in each register. It is consulted whenever a new register is needed. E.g.: R0 contains t, y, z.
Address Descriptor: It keeps tract of the location(s) where the current value of the name can be found at the runtime. The location might be register, stack or memory location.

6. Code Generation Algorithm
STEP 1
Find space L to store value of x using
function getreg().
For Y, see whether it is stored in L.
If yes, perform op Z,L
Else perform mov Y,L; op Z,L
Generate : op Z,L
Update address descriptor of X. If X stored
in any other register, remove it.
If the current values of Y and Z have no
next use then free the registers.
STEP 2
STEP 3
STEP 4

7. Function getreg();
STEP 1: If the name y is in a register , that holds the value of no other names (in other words no other names point to the same register as y does), and y is not live and has no next use after the execution of y = x op z, then :-
return L.
Update the address descriptor of y , so that y is no longer in L.
STEP 2: Failing (1), return an empty register for L if it exists.
STEP 3: Failing (2), if x has a next use in the block, or if op requires a register then :-
find an occupied register R.
MOV(R,M) if value of R is not in proper M. If R holds value of many variables, generate a MOV for each of the variables.
STEP 4: Failing (3), select the memory location of x as L.

8. Example: Statements Code Generated Register Descriptor Address t = a-b
d=(a-b) + (a-c) + (a-c)
Statements
Code Generated
Register
Descriptor
Address
t = a-b
MOV a, R0
SUB b, R0
R0 -> t
t -> R0
u= a-c
MOV a, R1
SUB c, R1
R1 -> u
u -> R1
v= t+u
ADD R1,R0
R0 -> v
v -> R0
d= v+u
MOV R0,d
R0 -> d
d -> R0

9. THANK YOU