1. Compiler Construction
Sohail Aslam
Lecture 36
compiler: Bottom-up Parsing

2. Three-Address Code
Three-address code is attractive for several reasons:
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compiler: Bottom-up Parsing

3. Three-Address Code
absence of destructive operators gives the compiler freedom to reuse names and values
three-address code is reasonably compact: operations are 1 to 2 bytes; addresses are 4 bytes
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compiler: Bottom-up Parsing

4. Three-Address Code
absence of destructive operators gives the compiler freedom to reuse names and values
three-address code is reasonably compact: operations are 1 to 2 bytes; addresses are 4 bytes
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compiler: Bottom-up Parsing

5. Three-Address Code
many modern processors implement three-address operations, a three-address code models their properties well
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compiler: Bottom-up Parsing

6. Syntax-directed Translation
We now consider syntax-directed translation schemes using three-address code for various programming constructs
We start with the assignment statement
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compiler: Bottom-up Parsing

7. Syntax-directed Translation
We now consider syntax-directed translation schemes using three-address code for various programming constructs
We start with the assignment statement
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compiler: Bottom-up Parsing

8. Production translation scheme S → id = E { p = lookup(id.name);
emit( p, ‘=’, E.place); }
E → E1 + E2
{ E.place = newtemp();
emit( E.place, ‘=’, E1.place,
‘+’, E2.place); }
E → E1  E2
‘’, E2.place); }
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compiler: Bottom-up Parsing

9. Production translation scheme E → – E1 { E.place = newtemp();
emit( E.place, ‘=’, ‘–’ , E1.place); }
E → ( E1 )
{ E.place = E1.place; }
E → id
{ p = lookup(id.name);
emit( E.place, ‘=’, p ); }
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compiler: Bottom-up Parsing

10. Assignment Statement
The tranlation scheme uses a symbol table for identifiers and temporaries
Every time the parser encounters an identifier, it installs it in the symbol table.
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compiler: Bottom-up Parsing

11. Assignment Statement
The tranlation scheme uses a symbol table for identifiers and temporaries
Every time the parser encounters an identifier, it installs it in the symbol table.
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compiler: Bottom-up Parsing

12. Assignment Statement
The symbol table can be implemented as a hash table or using some other efficient data structure for table.
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compiler: Bottom-up Parsing

13. Assignment Statement
The routine lookup(name) checks if there an entry for the name in the symbol table
If the name is found, the routine returns a pointer to entry.
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compiler: Bottom-up Parsing

14. Assignment Statement
The routine lookup(name) checks if there an entry for the name in the symbol table
If the name is found, the routine returns a pointer to entry.
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compiler: Bottom-up Parsing

15. Assignment Statement
The routine newtemp() returns a new temporary in response to successive calls
Temporaries can be placed in the symbol table
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compiler: Bottom-up Parsing

16. Assignment Statement
The routine emit() generates a three-address statement which can either be held in memory or written to a file
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compiler: Bottom-up Parsing

17. Example Here is the bottom-up parse of the assignment statement
a = b*-c + b*-c
and the syntax-directed translation into three-address code
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compiler: Bottom-up Parsing