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

Three-Address Code Three-address code is attractive for several reasons: This is a note compiler: Bottom-up Parsing

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 This is a note compiler: Bottom-up Parsing

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 This is a note compiler: Bottom-up Parsing

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

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

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

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); } This is a note compiler: Bottom-up Parsing

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 ); } This is a note compiler: Bottom-up Parsing

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. This is a note compiler: Bottom-up Parsing

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. This is a note compiler: Bottom-up Parsing

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

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. This is a note compiler: Bottom-up Parsing

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. This is a note compiler: Bottom-up Parsing

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

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

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 This is a note compiler: Bottom-up Parsing