Norman Ramsey

Stochastic lambda calculus and monads of probability distributions

Probability distributions are useful for expressing the meanings of probabilistic languages, which support formal modeling of and reasoning about uncertainty. Probability distributions form a monad, and the monadic definition leads to a simple, natural semantics for a stochastic lambda calculus, as well as simple, clean implementations of common queries. But the monadic implementation of the expectation query can be much less efficient than current best practices in probabilistic modeling. We therefore present a language of measure terms, which can not only denote discrete probability distributions but can also support the best known modeling techniques. We give a translation of stochastic lambda calculus into measure terms. Whether one translates into the probability monad or into measure terms, the results of the translations denote the same probability distribution.

Specifying representations of machine instructions

We present SLED, a specification language for Encoding and Decoding, which describes, abstract, binary, and assembly-language representations of machine instructions. Guided by a SLED specification, the New Jersey Machine-Code Toolkit generates bit-manipulating code for use in applications that process machine code. Programmers can write such applications at an assembly language level of abstraction, and the toolkit enables the applications to recognize and emit the binary representations used by the hardware. SLED is suitable for describing both CISC and RISC machines; we have specified representations of MIPS R3000, SPARC, Alpha, and Intel Pentium instructions, and toolkit users have written specifications for the Power PC and Motorola 68000. The article includes representative excerpts from our SPARC and Pentium specifications. SLED uses four elements; fields and tokens describe parts of instructions; patterns describe binary representations of instructions or group of instructions; and constructors map between the abstract and binary levels. By combining the elements in different ways, SLED supports machine-independent implementations of machine-level concepts like conditional assembly, span-dependent instructions, relocatable addresses, object code, sections, and relocation. SLED specifications can be checked automatically for consistency with existing assemblers. The implementation of the toolkit is largely determined by our representations of patterns and constructors. We use a normal form that facilitates construction of encoders and decoders. The article describes the normal form and its use. The toolkit has been used to help build several applications. We have built a retargetable debugger and a retargetable, optimizing linker. Colleagues have built a dynamic code generator, a decompiler, and an execution-time analyzer. The toolkit generates efficient code; for example, the linker emits binary up to 15% faster than it emits assembly language, making it 1.7-2 times faster to produce an a.out directly than by using the assembler.

Literate programming simplified

Literate programming tools let you arrange the parts of a program in any order and extract documentation and code from the same source file. The author argues that language-dependence and feature complexity have hampered acceptance of these tools, then offers a simpler alternative.<<ETX>>

A generalized algorithm for graph-coloring register allocation

Graph-coloring register allocation is an elegant and extremely popular optimization for modern machines. But as currently formulated, it does not handle two characteristics commonly found in commercial architectures. First, a single register name may appear in multiple register classes, where a class is a set of register names that are interchangeable in a particular role. Second, multiple register names may be aliases for a single hardware register. We present a generalization of graph-coloring register allocation that handles these problematic characteristics while preserving the elegance and practicality of traditional graph coloring. Our generalization adapts easily to a new target machine, requiring only the sets of names in the register classes and a map of the register aliases. It also drops easily into a well-known graph-coloring allocator, is efficient at compile time, and produces high-quality code.

C--: A Portable Assembly Language that Supports Garbage Collection

For a compiler writer, generating good machine code for a variety of platforms is hard work. One might try to reuse a retargetable code generator, but code generators are complex and difficult to use, and they limit one’s choice of implementation language. One might try to use C as a portable assembly language, but C limits the compiler writer’s flexibility and the performance of the resulting code. The wide use of C, despite these drawbacks, argues for a portable assembly language. C-— is a new language designed expressly for this purpose. The use of a portable assembly language introduces new problems in the support of such high-level run-time services as garbage collection, exception handling, concurrency, profiling, and debugging. We address these problems by combining the C-— language with a C-— run-time interface. The combination is designed to allow the compiler writer a choice of source-language semantics and implementation techniques, while still providing good performance.

An algebraic approach to file synchronization

A file synchronizer restores consistency after multiple replicas of a filesystem have been changed independently. We present an algebra for reasoning about operations on filesystems and show that it is sound and complete with respect to a simple model. The algebra enables us to specify a file-synchronization algorithm that can be combined with several different conflict-resolution policies. By contrast, previous work builds the conflict-resolution policy into the specification, or worse, does not specify the synchronizers behavior precisely. We classify synchronizers by asking whether conflicts can be resolved at a single disconnected replica and whether all replicas are identical after synchronization. We also discuss timestamps and argue that there is no good way to propagate timestamps when there is severe clock skew between replicas.

A single intermediate language that supports multiple implementations of exceptions

We present mechanisms that enable our compiler-target language, C--, to express four of the best known techniques for implementing exceptions, all within a single, uniform framework. We define the mechanisms precisely, using a formal operational semantics. We also show that exceptions need not require special treatment in the optimizer; by introducing extra dataflow edges, we make standard optimization techniques work even on programs that use exceptions. Our approach clarifies the design space of exception-handling techniques, and it allows a single optimizer to handle a variety of implementation techniques. Our ultimate goal is to allow a source-language compiler the freedom to choose its exception-handling policy, while encapsulating the architecture-dependent mechanisms and their optimization in an implementation of C--that can be used by compilers for many source languages.

Machine Descriptions to Build Tools for Embedded Systems

Because of poor tools, developing embedded systems can be unnecessarily hard. Machine descriptions based on register-transfer lists (RTLs) have proven useful in building retargetable compilers, but not in building other retargetable tools. Simulators, assemblers, linkers, debuggers, and profilers are built by hand if at all—previous machine descriptions have lacked the detail and precision needed to generate them. This paper presents detailed and precise machine-description techniques that are based on a new formalization of RTLs. Unlike previous notations, these RTLs have a detailed, unambiguous, and machine-independent semantics, which makes them ideal for supporting automatic generation of retargetable tools. The paper also gives examples of λ-RTL, a notation that makes it possible for human beings to read and write RTLs without becoming overwhelmed by machine-dependent detail.

A retargetable debugger

We are developing techniques for building retargetable debuggers. Our prototype, 1db, debugs C programs compiled for the MIPS R3000, Motorola 68020, SPARC, and VAX architectures. It can use a network to connect to faulty processes and can do cross-architecture debugging. 1dbs total code size is about 16,000 lines, but it needs only 250–550 lines of machine-dependent code for each target. 1db owes its retargetability to three techniques: getting help from the compiler, using a machine-independent embedded interpreter, and choosing abstractions that minimize and isolate machine-dependent code. 1db reuses existing compiler function by having the compiler emit PostScript code that 1db later interprets; PostScript works well in this unusual context.

The design of a resourceable and retargetable binary translator

Binary translation, the automatic translation of executable programs from one machine to another, requires analyses and transformations that could be used in a wide variety of tools intended to reverse engineer binary codes. Our approach to binary translation, which is designed to allow both source and target machines to be changed at low cost, is based on a combination of machine descriptions, binary-interface descriptions, and machine-independent analyses. This approach is producing components and component generators that should be usable in a variety of tools for reverse engineering binary codes. This paper presents an overview of the full design and gives excerpts from descriptions used in component generators, and presents preliminary results of four static translators instantiated from the UQBT framework described in this paper.