David M. Ungar

Self: The power of simplicity

Self is a new object-oriented language for exploratory programming based on a small number of simple and concrete ideas: prototypes, slots, and behavior. Prototypes combine inheritance and instantiation to provide a framework that is simpler and more flexible than most object-oriented languages. Slots unite variables and procedures into a single construct. This permits the inheritance hierarchy to take over the function of lexical scoping in conventional languages. Finally, because Self does not distinguish state from behavior, it narrows the gaps between ordinary objects, procedures, and closures. Selfs simplicity and expressiveness offer new insights into object-oriented computation.

Generation Scavenging: A non-disruptive high performance storage reclamation algorithm

Many interactive computing environments provide automatic storage reclamation and virtual memory to ease the burden of managing storage. Unfortunately, many storage reclamation algorithms impede interaction with distracting pauses. Generation Scavenging is a reclamation algorithm that has no noticeable pauses, eliminates page faults for transient objects, compacts objects without resorting to indirection, and reclaims circular structures, in one third the time of traditional approaches. We have incorporated Generation Scavenging in Berkeley Smalltalk(BS), our Smalltalk-80 implementation, and instrumented it to obtain performance data. We are also designing a microprocessor with hardware support for Generation Scavenging.

Optimizing Dynamically-Typed Object-Oriented Languages With Polymorphic Inline Caches

Polymorphic inline caches (PICs) provide a new way to reduce the overhead of polymorphic message sends by extending inline caches to include more than one cached lookup result per call site. For a set of typical object-oriented SELF programs, PICs achieve a median speedup of 11%.

Debugging optimized code with dynamic deoptimization

SELFs debugging system provides complete source-level debugging (expected behavior) with globally optimized code. It shields the debugger from optimizations performed by the compiler by dynamically deoptimizing code on demand. Deoptimization only affects the procedure activations that are actively being debugged; all other code runs at full speed. Deoptimization requires the compiler to supply debugging information at discrete interrupt points; the compiler can still perform extensive optimizations between interrupt points without affecting debuggability. At the same time, the inability to interrupt between interrupt points is invisible to the user. Our debugging system also handles programming changes during debugging. Again, the system provides expected behavior: it is possible to change a running program and immediately observe the effects of the change. Dynamic deoptimization transforms old compiled code (which may contain inlined copies of the old version of the changed procedure) into new versions reflecting the current source-level state. To the best of our knowledge, SELF is the first practical system providing full expected behavior with globally optimized code.

Optimizing dynamically-dispatched calls with run-time type feedback

Abstrach Object-oriented programs are difficult to optimize because they execute many dynamically-dispatched calls. These calls cannot easily be eliminated because the compiler does not know which callee will be invoked at runtime. We have developed a simple technique that feeds back type information from the runtime system to the compiler. With this type feedback, the compiler can inline any dynamically-dispatched call. Our compiler drastically reduces the calI frequency of a suite of large SELF applications (by a factor of 3.6) and improves performance by a factor of 1.7. We believe that type feedback could significantly reduce call frequencies and improve performance for most other objectoriented languages (statically-typed or not) as well as for languages with type-dependent operations such as generic arithmetic.

An efficient implementation of SELF a dynamically-typed object-oriented language based on prototypes

We have developed and implemented techniques that double the performance of dynamically-typed object-oriented languages. Our SELF implementation runs twice as fast as the fastest Smalltalk implementation, despite SELFs lack of classes and explicit variables. To compensate for the absence of classes, our system uses implementation-level maps to transparently group objects cloned from the same prototype, providing data type information and eliminating the apparent space overhead for prototype-based systems. To compensate for dynamic typing, user-defined control structures, and the lack of explicit variables, our system dynamically compiles multiple versions of a source method, each customized according to its receivers map. Within each version the type of the receiver is fixed, and thus the compiler can statically bind and inline all messages sent to self. Message splitting and type prediction extract and preserve even more static type information, allowing the compiler to inline many other messages. Inlining dramatically improves performance and eliminates the need to hard-wire low-level methods such as +,==, and ifTrue:. Despite inlining and other optimizations, our system still supports interactive programming environments. The system traverses internal dependency lists to invalidate all compiled methods affected by a programming change. The debugger reconstructs inlined stack frames from compiler-generated debugging information, making inlining invisible to the SELF programmer.

Animation: from cartoons to the user interface

User interfaces are often based on static presentations, a model ill suited for conveying change. Consequently, events on the screen frequently startle and confuse users. Cartoon animation, in contrast, is exceedingly successful at engaging its audience; even the most bizarre events are easily comprehended. The Self user interface has served as a testbed for the application of cartoon animation techniques as a means of making the interface easier to understand and more pleasant to use. Attention to timing and transient detail allows Self objects to move solidly. Use of cartoon-style motion blur allows Self objects to move quickly and still maintain their comprehensibility. Self objects arrive and depart smoothly, without sudden materializations and disappearances, and they rise to the front of overlapping objects smoothly through the use of dissolve. Anticipating motion with a small contrary motion and pacing the middle of transitions faster than the endpoints results in smoother and clearer movements. Despite the differences between user interfaces and cartoons --cartoons are frivolous, passive entertainment and user interfaces are serious, interactive tools -- cartoon animation has much to lend to user interfaces to realize both affective and cognitive benefits. *This work was originally supported by Sun Microsystems Laboratories, an NSF Graduate Fellowship, National Science Foundation Presidential Young Investigator Grant #CCR-8657631, IBM Powell Foundation, Apple Computer, Inc., Cray Laboratories, Tandem, NCR Corporation, Texas Instruments, Inc., and Digital Equipment Corporation.

Mirrors: design principles for meta-level facilities of object-oriented programming languages

We identify three design principles for reflection and metaprogramming facilities in object oriented programming languages. <i>Encapsulation</i>: meta-level facilities must encapsulate their implementation. <i>Stratification</i>: meta-level facilities must be separated from base-level functionality. <i>Ontological correspondence</i>: the ontology of meta-level facilities should correspond to the ontology of the language they manipulate. Traditional/mainstream reflective architectures do not follow these precepts. In contrast, reflective APIs built around the concept of <i>mirrors</i> are characterized by adherence to these three principles. Consequently, mirror-based architectures have significant advantages with respect to distribution, deployment and general purpose metaprogramming.

Making pure object-oriented languages practical

In the past, object-oriented language designers and programmers have been forced to choose between pure message passing and performance. Last year, our SELF system achieved close to half the speed of optimized C but suffered from impractically long compile times. Two new optimization techniques, deferred compilation of uncommon cases and non-backtracking splitting using path objects, have improved compilation speed by more than an order of magnitude. SELF now compiles about as fast as an optimizing C compiler and runs at over half the speed of optimized C. This new level of performance may make pure object-oriented languages practical.

Architecture of SOAR: Smalltalk on a RISC

Smalltalk on a RISC (SOAR) is a simple, Von Neumann computer that is designed to execute the Smalltalk-80 system much faster than existing VLSI microcomputers. The Smalltalk-80 system is a highly productive programming environment but poses tough challenges for implementors: dynamic data typing, a high level instruction set, frequent and expensive procedure calls, and object-oriented storage management. SOAR compiles programs to a low level, efficient instruction set. Parallel tag checks permit high performance for the simple common cases and cause traps to software routines for the complex cases. Parallel register initialization and multiple on-chip register windows speed procedure calls. Sophisticated software techniques relieve the hardware of the burden of managing objects. We have initial evaluations of the effectiveness of the SOAR architecture by compiling and simulating benchmarks, and will prove SOARs feasibility by fabricating a 35,000-transistor SOAR chip. These early results suggest that a Reduced Instruction Set Computer can provide high performance in an exploratory programming environment.