LECTURE 16
Intermediate Code
INTERMEDIATE CODE
• The generation of a machine-independent intermediate form decouples the backend from front-end, and facilitates retargeting.
• Machine independent code optimizations can be applied here.
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.
• Intermediate form is close to low level machine language.
• Three-address code (more on this later).
• gcc uses RTL, a variation of the three-address code.
• Other commonly used intermediate languages
• Control flow graph, Program dependence graph (PDG), DAG (directed acyclic graph).
THREE-ADDRESS CODE
A sequence of statements of the form x = y op z
Three-address statements closely resemble assembly statements (OP src1 src2 dst).
Example: a = b * -c + b * -c
t1 = -c
t2 = b * t1
t3 = -c
t4 = b * t3
t5 = t2 + t4
a = t5
or
t1 = -c
t2 = b * t1
t3 = t2 + t2
a = t3
THREE-ADDRESS CODE
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
 call p, n
 return y
set a parameter for a procedure call
call procedure p with n parameters
return from a procedure with return value y
THREE-ADDRESS CODE
 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