1. Intermediate Code Generating machine-independent intermediate form.
Decouple backend from frontend, facilitate retargeting
Machine independent code optimizer can be applied here.
Position of intermediate code generator:
Intermediate
Code
generation
Static semantic
analysis
parser
Machine-specific
Code optimization
Target code
generation
Machine-independent
Code optimization

2. Intermediate languages, many kinds for different purposes
High-level representation for source to source translation to keep the program structure:
Abstract syntax tree
Low-level representation for compiling for target machine.
An intermediate form that is close to low level machine language.
Three address code (more later)
gcc uses RTL, a variation of the three address code.
Other commonly used intermediate language
Control flow graph, Program dependence graph (PDG), DAG (direct acyclic graph)

3. Three address code: A sequence of statement of the form x:=y op z
Example: a:=b*-c + b * -c
Three address statements are close to the assembly statements (OP src1 src2 dst)
t1 := -c
t2 := b * t1
t3 := -c
t4 := b * t3
t5 = t2 + t4
a = t5
t1 := -c
t2 := b * t1
t3 = t2 + t2
a = t3

4. Some three-address statements that will be used later:
Assignment statements:
With a binary operation: x := y op z
With a unary operation: x:= op y
With no operation(copy) : x := y
Branch statements
Unconditional jump: goto L
Conditional jumps: if x relop y goto L
Statement for procedure calls
Param x, set a parameter for a procedure call
Call p, n call procedure p with n parameters
Return y return from a procedure with return value y

5. Example: instructions for procedure call: p(x1, x2, x3, …, xn):
param x1
param x2 …
param xn
call p, n
Indexed assignments:
x := y[i] and x[i] := y
Address and pointer assignments
x := &y, x := *y