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 Ch.13: Target code generation

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

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

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

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

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

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)

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

–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

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; –

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

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

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

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

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

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

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

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

Static Method Calls invokestatic –invokestatic java/lang/Math/pow(DD)D

Instance-Specific Method Calls invokevirtual –invokevirtual java/io/PrintStream/print(Z)V

Static Single Assignment Form (omitted)

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