Compiler Construction

1
Compiler Construction

• Sohail Aslam
• Lecture 36

2
Three-Address Code

• Three-address code is attractive for several
  reasons

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

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

5
Three-Address Code

• many modern processors implement three-address
  operations, a three-address code models their
  properties well

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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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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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 E.place newtemp() emit( E.place, , E1.place, ?, E2.place)
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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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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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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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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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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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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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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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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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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