Compiler chapter six .ppt course material
- 1. Chapter six Intermediate Code Generation Rift valley University Harar Campus
- 2. Overview Intermediate code is the interface between front end and back end in a compiler. Ideally the details of source language are confined to the front end and the details of target machines to the back end (a machine model) This intermediate code serves as a bridge between the high-level source code and the final machine code or another target language.
- 3. 3 Overview In a compiler, the front end translates source program into an intermediate representation, and the back end generates the target code from this intermediate representation. The use of a machine independent intermediate code (IC) is: retargeting to another machine is facilitated the optimization can be done on the machine independent code
- 4. 4 Overview Intermediate representations span the gap between the source and target languages: closer to target language; (more or less) machine independent; allows many optimizations to be done in a machine-independent way. Implementable via syntax directed translation, so can be folded into the parsing process.
- 5. Intermediate Representations Decisions in IR design affect the speed and efficiency of the compiler Some important IR properties Ease of generation Ease of manipulation Procedure size Level of abstraction The importance of different properties varies between compilers Selecting an appropriate IR for a compiler is critical 5
- 6. 6 Types of Intermediate Languages High Level Representations (e.g., syntax trees): closer to the source language easy to generate from an input program code optimizations may not be straightforward. Low Level Representations (e.g., 3-address code) closer to the target machine; easier for optimizations, final code generation;
- 7. Intermediate Code Generation Intermediate language can be many different languages, and the designer of the compiler decides this intermediate language. Syntax tree can be used as an intermediate language. Postfix notation can be used as an intermediate language. Three-address code (Quadraples) can be used as an intermediate language We will use three address to discuss intermediate code generation. Three address are close to machine instructions, but they are not actual machine instructions. Some programming languages have well defined intermediate languages. 7
- 8. 8 Syntax Trees A syntax tree shows the structure of a program by abstracting away irrelevant details from a parse tree. Each node represents a computation to be performed; The children of the node represents what that computation is performed on. Syntax trees decouple parsing from subsequent processing.
- 9. 9 Syntax Trees: Example Grammar : E E + T | T T T * F | F F ( E ) | id Input: id + id * id Parse tree: Syntax tree:
- 10. 10 Syntax Trees: Structure Expressions: leaves: identifiers or constants; internal nodes are labeled with operators; the children of a node are its operands. Statements: a node’s label indicates what kind of statement it is; the children correspond to the components of the statement.
- 11. Syntax Tree While parsing the input, a syntax tree can be constructed. A syntax tree (abstract tree) is a condensed form of parse tree useful for representing language constructs. For example, for the string a + b, the parse tree in (a) below can be represented by the syntax tree shown in (b); the keywords (syntactic sugar) that existed in the parse tree will no longer exist in the syntax tree. 11 E E E + a b Parse tree + a b Abstract tree
- 12. Three-Address Code A three address code is: x := y op z where x, y and z are names, constants or compiler- generated temporaries; op is any operator. But we may also use the following notation for three address code (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. We use the term “three-address code” because each statement usually contains three addresses (two for operands, one for the result). 12
- 13. Three-Address Code 13 a:= b * -c + b * -c t1 := - c t2 := b * t1 t3 := - c t4 := b * t3 t5 := t2 + t4 a := t5 t1 := - c t2 := b * t1 t5 := t2 + t2 a := t5
- 14. Postfix Notation Postfix notation is a linear representation of a syntax tree. In the postfix notation, any expression can be written unambiguously without parentheses In postfix notation, the operator appears after the operands, i.e., the operator between operands is taken out & is attached after operands. Example : Translate a ∗ d − (b + c) into Postfix form. Solution ad ∗ bc + −
- 15. Three-Address Code… In three-address code: Only one operator at the right side of the assignment is possible, i.e. x + y * z is not possible. Similar to postfix notation, the three address code is a linear representation of a syntax tree. It has been given the name three-address code because such an instruction usually contains three addresses (the two operands and the result) t1 = y * z t2 = x + t1 15
- 16. Data structures for three address codes Quadruples Has four fields: op, arg1, arg2 and result Triples Temporaries are not used and instead references to instructions are made Indirect triples In addition to triples we use a list of pointers to triples
- 17. Example-1 b * minus c + b * minus c t1 = minus c t2 = b * t1 t3 = minus c t4 = b * t3 t5 = t2 + t4 a = t5 Three address code minus * minus c t3 * + = c t1 b t2 t1 b t4 t3 t2 t5 t4 t5 a arg1 result arg2 op Quadruples minus * minus c * + = c b (0) b (2) (1) (3) a arg1 arg2 op Triples (4) 0 1 2 3 4 5 minus * minus c * + = c b (0) b (2) (1) (3) a arg1 arg2 op Indirect Triples (4) 0 1 2 3 4 5 (0) (1) (2) (3) (4) (5) op 35 36 37 38 39 40
- 18. Example continued… Construct quadruple and triple for the following equation:(a+b) * ( c+ d) - ( a+ b+ c) three Address code: a. Quadruple b. triples op arg1 arg2 result + a b t1 + c d t2 * t1 t2 t3 + t1 c t4 - t3 t4 t5 op arg1 arg2 + a b + c d * (0) (1) + (0) c - (2) (3)
Editor's Notes
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