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Chap. 10, Intermediate Representations

Chap. 10, Intermediate Representations. J. H. Wang Dec. 27, 2011. Outline. Overview Java Virtual Machine Static Single Assignment Form. Overview. Ch.7: AST Ch.8-9: Semantic analysis Ch.10: Intermediate representation Ch.11: Code generation for a virtual machine Ch.12: Runtime support

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Chap. 10, Intermediate Representations

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  1. Chap. 10, Intermediate Representations J. H. Wang Dec. 27, 2011

  2. Outline • Overview • Java Virtual Machine • Static Single Assignment Form

  3. Overview • Ch.7: AST • Ch.8-9: Semantic analysis • Ch.10: Intermediate representation • Ch.11: Code generation for a virtual machine • Ch.12: Runtime support • Ch.13: Target code generation

  4. Overview • Semantic gap between high-level source languages and target machine language • Examples • Early C++ compilers • cpp: preprocessor • cfront: translate C++ into C • C compiler

  5. Another Example • LaTeX • TeX: designed by Donald Knuth • dvi: device-independent intermediate representation • Ps: PostScript • pixels • Portability enhanced

  6. Challenges • Challenges • An intermediate language (IL) must be precisely defined • Translators and processors must be crafted for an IL • Connections must be made between levels so that feedback from intermediate steps can be related to the source program • Other concerns • Efficiency

  7. The Middle-End • Front-end: parser • Back-end: code generator • Middle-end: components between front- and back-ends • Compiler suites that host multiple source languages and target multiple instruction sets obtain great leverage from a middle-end • Ex: s source languages, t target languages • s*t vs. s+t

  8. Additional Advantages • An IL allows various system components to interoperate by facilitating access to information about the program • E.g. variable names and types, and source line numbers could be useful in the debugger • An IL simplifies development and testing of system components • The middle-end contains phases that would otherwise be duplicated among the front- and back-ends • It allows components and tools to interface with other products

  9. It can simply the pioneering and prototyping of news ideas • The ILs and its interpreter can serve as a reference definition of a language • Interpreters written for a well-defined IL are helpful in testing and porting compilers • An IL enables the crafting of a retargetable code generator, which greatly enhances its portability • Pascal: P-code • Java: JVM • Ada: DIANA (Descriptive Intermediate Attributed Notation for Ada)

  10. Java Virtual Machine • Class files: binary encodings of the data and instructions in a Java program • Design principles • Compactness • Instructions in nearly zero-address form • A runtime stack is used • Operands are implicit • E.g.: iadd instruction • A loss of runtime performance • Multiple instructions to accomplish the same effect • To push 0 on TOS • iconst_0, ldc_w 0

  11. Safety • An instruction can reference storage only if it is of the type allowed by the instruction, and only if the storage is located in an area appropriate for access • From security’s point of view, purely zero-address form is problematic • The registers that could be accessed by a load instruction may not be known until runtime • JVM: not zero-address • E.g. iload 5 • When a class file is loaded, many other checks are performed by the bytecode verifier

  12. Contents of a Class File • Attributes that contain various information about the compiled class • Types: primitive and reference types • (Fig. 10.4) • Primitive type: a single character • Reference type t: Lt • E.g.: String type in java.lang package: Ljava/lang/String;

  13. Constant pools • tagged union • int, float, java.lang.String • Referenced by its ordinal position, not byte-offset

  14. JVM Instructions • Arithmetic • Register traffic • Registers and types • Static fields • Instance fields • Branching • Other method calls • Stack operations

  15. Arithmetic • int: 32-bit, 2’s complement • iadd • fadd(float) • ladd(long) • dadd(double)

  16. Register Traffic • JVM has an unlimited number of virtual registers • JVM registers typically host a method’s local variables • JVM registers are untyped • iload 2 • iload_2: abbreviated • istore 10 • aload and astore: for reference types

  17. Registers and Types • Static analysis (or bytecode verification) • To ensure that values flow in and our of registers without compromising Java’s type systems • Type conversion • i2f

  18. Static Fields • getstatic • E.g.: getstatic java/lang/System/out Ljava/io/PrintStream; • putstatic

  19. Instance Fields • A class can declare instance field for which instance-specific storage is allocated • getfield • getfiled Point/x I • putfield • putfield Point/x I

  20. Branching • ifeq, ifne, iflt, ifle, ifgt, ifge • if_icmpeq, if_icmpne, if_icmplt, if_icmple, if_icmpgt, if_icmpge

  21. Static Method Calls • invokestatic • invokestatic java/lang/Math/pow(DD)D

  22. Instance-Specific Method Calls • invokevirtual • invokevirtual java/io/PrintStream/print(Z)V

  23. Static Single Assignment Form • (omitted)

  24. Thanks for Your Attention!

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