OOPS - INTERMEDIATE CODE GENERATION, Study notes of Object Oriented Programming

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Intermediate Code Generation

Intermediate Code Generation

 Translating source program into an “intermediate

language.”

 Simple

 CPU Independent,

 This completes the “Front-End” of Compilation.

Benefits

1. Retargeting is facilitated

2. Machine independent Code Optimization

can be applied.

Types of Intermediate Languages

 (^) Graphical Representations.  (^) Consider the assignment a:=b-c+b-c:

assign

a +

uminus

b uminus

c c

b

assign

a

uminus

b c

Three Address Code

 (^) Statements of general form x:=y op z  (^) As a result, x:=y + z * w should be represented as t 1 :=z * w t 2 :=y + t 1 x:=t 2  (^) Observe that given the syntax-tree or the dag of the graphical representation we can easily derive a three address code for assignments as above.  (^) In fact three-address code is a linearization of the tree.  (^) Three-address code is useful: related to machine-language/ simple/ optimizable.

Types of Three-Address Statements.

Assignment Statement: x:=y op z Assignment Statement: x:=op z Copy Statement: x:=z Unconditional Jump: goto L Conditional Jump: if x relop y goto L Stack Operations:Push/pop

M ore Advanced:

Procedure: param x 1 param x 2 … param xn call p,n Index Assignments: x:=y[i] x[i]:=y Address and Pointer Assignments: x:=&y x:=*y *x:=y

Syntax-Directed Translation into 3-address code.  (^) First deal with assignments.  (^) Use attributes  (^) E. place: the name that will hold the value of E  (^) Identifier will be assumed to already have the place attribute defined.  (^) E. code: hold the three address code statements that evaluate E (this is the `translation’ attribute).

 Use function newtemp that returns a new temporary variable that we

can use.

 Use function gen to generate a single three address statement given the

necessary information (variable names and operations).

What about things that are not assignments?

 E.g. while statements of the form “while E do S”

(intepreted as while the value of E is not 0 do S)

Extension to the previous syntax-dir. Def. PRODUCTION S  while E do S 1 Semantic Rule S.begin = newlabel ; S.after = newlabel ; S. code = gen (S. begin ‘:’) || E. code || gen (‘if’ E. place ‘=’ ‘0’ ‘goto’ S. after ) || S 1. code || gen(‘goto’ S. begin ) || gen(S. after ‘:’)

Implementations of 3-address statements

 (^) Quadruples t 1 :=- c t 2 :=b * t 1 t 3 :=- c t 4 :=b * t 3 t 5 :=t 2 + t 4 a:=t 5 op arg1 arg2 result (0) uminus c t 1 (1) * b t 1 t 2 (2) uminus c (3) * b t 3 t 4 (4) + t 2 t 4 t 5 (5) := t 5 a Temporary names must be entered into the symbol table as they are created.

Other types of 3-address statements

 (^) e.g. ternary operations like x[i]:=y x:=y[i]  (^) require two or more entries. e.g. op arg1 arg (0) [ ] = x i (1) assign (0) y op arg1 arg (0) [ ] = y i (1) assign x (0)

Implementations of 3-address statements, III

Indirect Triples

op arg1 arg (14) uminus c (15) * b (14) (16) uminus c (17) * b (16) (18) + (15) (17) (19) assign a (18) op (0) (14) (1) (15) (2) (16) (3) (17) (4) (18) (5) (19)