Chap. 10, Intermediate Representations J. H. Wang Dec. 14, 2015

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 synthesis for virtual machines 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 Java class files: binary encodings of the data and instructions in a Java program Design principles (borrowed from JVM reference) –Compactness Instructions in nearly zero-address form –A runtime stack is used –Operands are implicit »E.g.: iadd instructionL pops two items and pushes the sum onto TOS (tops of stack) –A loss of runtime performance Multiple instructions to accomplish the same effect –To push 0 on TOS » iconst_0 : 1 byte » ldc_w 0 : 3 bytes

–Safety An instruction can reference storage only if it’s 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 A class file is organized into sections called 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 : L t; –E.g.: String type in java.lang package: Ljava/lang/String; Array: [ a

–Constant pools tagged union –int, float, java.lang.String Referenced by its ordinal position, not byte-offset –1 byte for some instructions, e.g. ldc –2 bytes for some instructions, e.g. ldc_w

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

Arithmetic Popping operands from the runtime stack, computing result, and pushing the result on TOS –E.g. iadd int: 32-bit, 2’s complement –For other primitive types fadd(float) ladd(long) dadd(double) –Subtraction, multiplication, division, …

Register Traffic JVM has an unlimited number of virtual registers –Usually allocated in a method’s stack frame JVM registers typically host a method’s local variables –Registers starting from 0 are set aside for a method’s parameters JVM registers are untyped –iload 2: push iload_2: abbreviated (2 bytes) –istore 10: pop –fload n: for float values –aload and astore: for reference types (32 bits for object references)

Registers and Types Static analysis (or bytecode verification) –To ensure that values flow in and our of registers without compromising Java’s type systems JVM appears to be stricter than Java language –E.g. Type conversion i2f: from 2’s complement to IEEE floating point format

Static Fields A class’s static fields are present in every instance of the class getstatic name type : push –E.g.: getstatic java/lang/System/out Ljava/io/PrintStream; –Only 3 bytes in representation One: getstatic opcode Two: 16-bit integer specifying a constant-pool entry putstatic: pop

Instance Fields A class can declare instance field for which instance-specific storage is allocated getfield name type : push –E.g.: getfield Point/x I putfield: pop –putfield Point/x I

Branching Instructions to alter the control flow of the executing program –Unconditional goto: (3 bytes) goto_w: 5 bytes –Conditional branches Comparison against 0: ifeq, ifne, iflt, ifle, ifgt, ifge Comparison of non-zero values: if_icmpeq, if_icmpne, if_icmplt, if_icmple, if_icmpgt, if_icmpge

Static Method Calls Static methods: are common to all instances of some type t –E.g.: Math.pow(double a, double b) invokestatic –invokestatic java/lang/Math/pow(DD)D –Parameters pushed on the stack in left-to-right order –3 bytes Method signature: (DD)D

Instance-Specific Method Calls invokevirtual –invokevirtual java/io/PrintStream/print(Z)V –An instance must be pushed on the stack before the method’s parameters: this

Other Method Calls invokespecial A constructor call is special –An uninitialized reference to an object instance is pushed on TOS method –No return value Methods called by invokespecial are dispatched based on the actual (runtime) type of the instance Invokespecial can also be used to invoke a private method, for efficiency reasons only

Stack Operations Instructions specifically for manipulating items near the TOS –To facilitate shorter instruction sequences for common program fragments –dup2: duplicate the top two cells to accommodate long and double types –dup: nicely accommodates multiple assignments x=y=z=value –Pop, swap –dup_x1: in embedded assignment

Static Single Assignment Form (omitted)

Thanks for Your Attention!