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Intermediate Code Representations

Intermediate Code Representations. Conceptual phases of compiler. Lexical Analysis (scanner). Semantic Analysis. Code generation. Syntax analysis (parser). Code optimization. Optimized code. Sequence of tokens. Intermediate code - IR 1. Intermediate code IR 2. Target code.

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Intermediate Code Representations

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  1. Intermediate Code Representations

  2. Conceptual phases of compiler Lexical Analysis (scanner) Semantic Analysis Code generation Syntax analysis (parser) Code optimization Optimized code Sequence of tokens Intermediate code - IR1 Intermediate code IR2 Target code Back End machine dependent language independent Middle Front End machine independent language dependent

  3. Why use an IR?

  4. IR – Encodes Compiler’s Program Knowledge • Thus, some IR PROPERTIES: • Ease of generation • Ease of manipulation • Size • Freedom of Expression • Level of Abstraction • Selecting IR is critical.

  5. 3 Categories of IRs • Structural/Graphical • - AST and Concrete ST • - call graph • - program dependence graph (PDG) • 2. Linear • - 3-address code • - abstract stack machine code • Hybrid • - control flow graph (CFG)

  6. Level of Abstraction Consider:A[j,i] = @A + j*10 + i [ ] A I J Loadi 1, R1 Sub RJ, R1, R2 Loadi 10, R3 Mult R2, R3, R4 Sub Ri, R1, r5 Add R4, R5, R6 Loadi @A, R7 Add R7, R6, R8 Load R8, RAIJ

  7. Some Design Issues for IRs • Questions to Ponder: • What is the minimum needed in the language’s set of • operators? • What is the advantage of a small set of operators? • What is the concern of designing the operations • Close to actual machine operations? • 4. What is the potential problem of having a small • Set of IR operations?

  8. High LevelGraphical Representations Consider: A -> V := E E -> E + E | E * E | - E | id String: a := b * - c + b * - c Exercise: Concrete ST? AST? DAG?

  9. Linear IRs: Three Address Code • Sequence of instructions of the form X := y op z where x, y and z are variable names, constants, or compiler generated variables (“temporaries”) • Only one operator is permitted on the RHS – expressions computed using temporaries

  10. Simple Linear IRs Write the 3 – address code for: a := b * - c + b * - c ? = -c = b * ? … complete the code from the ast? The dag?

  11. Exercise Give the 3 address code for: Z := x * y + a[j] / sum(b)

  12. More Simple Linear IRs Stack machine code: push, pop, ops Consider: x – 2 * y Advantages?

  13. Hybrid IRs

  14. Exercise – Construct the CFG

  15. Call Graph Representation Node = function or method Edge from A to B : A has a call site where B is potentially called

  16. Exercise: Construct a call graph

  17. Multiple IRs: WHIRL

  18. Key Highlights of IRs

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