Short Notes: Intermediate Code Generation | Short Notes for Computer Science Engineering - Computer Science Engineering (CSE) PDF Download

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Intermediate codes are machine independent codes, but they are close to machine 
instructions. The given program in a source language is converted to an equivalent 
program in an intermediate language by the intermediate code generator.
• The designer of the compiler decides the intermediate language.
• Syntax trees can be used as an intermediate language.
• Postfix notations, three address code (quadruples) can be used as an 
intermediate language.
Syntax Tree
Syntax tree is a variant of the parse tree, where each leaf represents an operand 
and each interior node represent an operator.
P ro d u ctio n Sem an tic R u le
E — s-E l op E2 
E - < E 1 )
E — -E l 
E - ^ i d
E .val = NODE (op: E l.v a l, E 2.val)
E .val = E l.v a l
E .val = U n ary E 1 .val
E .val = (L E A F (id))
A sentence a * (b +d) would have the following syntax tree
/ \
3 / + \
b a
Example of a syntax tree
P ro d u ctio n S em an tic R u le P ro g ra m F ra g m en t
E — » E l o p E 2 E .code = E l. code Print op
||E2.code||op
E — ? ( E l) E .code = E l. code
E -+ id E .code = id Print id
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Intermediate codes are machine independent codes, but they are close to machine 
instructions. The given program in a source language is converted to an equivalent 
program in an intermediate language by the intermediate code generator.
• The designer of the compiler decides the intermediate language.
• Syntax trees can be used as an intermediate language.
• Postfix notations, three address code (quadruples) can be used as an 
intermediate language.
Syntax Tree
Syntax tree is a variant of the parse tree, where each leaf represents an operand 
and each interior node represent an operator.
P ro d u ctio n Sem an tic R u le
E — s-E l op E2 
E - < E 1 )
E — -E l 
E - ^ i d
E .val = NODE (op: E l.v a l, E 2.val)
E .val = E l.v a l
E .val = U n ary E 1 .val
E .val = (L E A F (id))
A sentence a * (b +d) would have the following syntax tree
/ \
3 / + \
b a
Example of a syntax tree
P ro d u ctio n S em an tic R u le P ro g ra m F ra g m en t
E — » E l o p E 2 E .code = E l. code Print op
||E2.code||op
E — ? ( E l) E .code = E l. code
E -+ id E .code = id Print id
Three-Address Code
When each statement contains three addresses (two for operands and one for 
result), Most general kind of three-address code is
x = yop z
Where x, y and z are names, constants or compiler generated temporaries and op 
any operator.
But we can also use the following notation for quadruples (much better notation 
because it looks like a machine code instruction)
Op y, z, x
Apply operator op to y and z and store the result in x.
Representation of Three-Address Codes
Three-address code can be represented in various forms i.e., quadruples, triples 
and indirect triples. These forms are demonstrated by way of examples below.
e.g., A = - B*(C +0)
Three address code is as follow 
T1 = - B 
T2 = C + D 
T3 = T1*T2 
A = T3
Quadruples
O p e ra to r O p e ra n d 1 O p era n d 2 R esu lt
(1)
- B T1
<2)
+ C D T2
(3)
T1 T2 T3
(4) = A T3
Triples
O p e ra to r O p era n d 1 O p era n d 2
(1)
- B
(2)
+ C D
(3) (1) <2)
(4) = A
(3)
Indirect triple
S tatem en t
(0)
(56)
(1)
(57)
(2) (58)
(3) (59)