• 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:
parser
Static semantic
analysis
Machine-specific
Code optimization
Target code
generation
Intermediate
Code
generation
Machine-independent
Code optimization
• 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)
• Three address code:
• A sequence of statement of the form x:=y op z
• Example: a:=b*-c + b * -c
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
• Three address statements are close to the assembly
statements (OP src1 src2 dst)
• Some three-address statements that will be used
later:
– Assignment statements:
• With a binary operation:
• With a unary operation:
• With no operation(copy) :
x := y op z
x:= op y
x := y
– Branch statements
• Unconditional jump:
• Conditional jumps:
goto L
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
– 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