Intermediate Code

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-independent Code optimization
Target code generation
Machine-specific 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 xy op z
• Example ab-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 yi and xi y
• Address and pointer assignments
• x y, x y