While Multiflow’s commercial success was small and short-lived, the fact of its technical success and the dissemination of its technology and people had a great effect on the future of computer science and the computer industry. Multiflow’s computers were arguably the most novel ever to be broadly sold, programmed, and used as if they were normal computers (other novel computers either required novel programming, or represented more incremental steps beyond existing computers).
Along with Cydrome, an attached-VLIW minisupercomputer company that had less commercial success, Multiflow demonstrated that the VLIW design style was practical, a conclusion surprising to many. While still controversial, VLIW has since been a force in high-performance embedded systems, and has been finding slow acceptance in general-purpose computing.
VLIW was put forward by Fisher as a way to build general-purpose instruction-level parallel processors exploiting ILP to a degree that would have been impractical using what would later be called superscalar control hardware. Instead, the compiler could, in advance, arrange the ILP to be carried out nearly in lock-step by the hardware, commanded by long instructions or a similar mechanism. While there had previously been processors that achieved significant amounts of ILP, they had all relied upon code laboriously hand-parallelized by the user, or upon library routines, and thus were not general-purpose computers and did not fit the VLIW paradigm.
The practicality of trace scheduling was demonstrated by a compiler built at Yale by Fisher and three of his graduate students, John Ruttenberg, Alexandru Nicolau, and especially John Ellis, whose doctoral dissertation on the compiler won the ACM Doctoral Dissertation Award in 1985. Encouraged by their compiling progress, Fisher’s group started an architecture and hardware design effort called the ELI (Enormously Long Instructions) Project.
Multiflow delivered its first working VLIW minisupercomputers in early 1987 to three beta-sites: Grumman Aircraft, Sikorsky Helicopter, and the Supercomputer Research Center. A Trace 14/200 was demonstrated to the public at a supercomputing conference in May, 1987, in Santa Clara, California.
Multiflow also announced a 28/ model at the outset, and eventually these were built and sold to a few customers. The 28/ had 1024-bit instruction words. Having ordinary programs compiled for computers like these was unquestionably revolutionary, as no earlier computer had offered compiled ILP even like that of the 7/ models. The 28/ systems pushed these limits far beyond either academic or industrial conception. While only a few customer programs contained enough ILP to keep a 28/ busy, when they did the performance was remarkable, since the processor would then initiate close to all 28 operations on average.
In 1988, the company started development of an ECL /500 family, which was to feature a 14/ that could also be used as a multiprocessor of two 7/ models, but that system was not completed before the company ceased operations.
The compiler was particularly noteworthy, as could be expected given Multiflow’s technology. The company built a new compiler, in a similar style to that developed at Yale, but industrial-strength and with the incorporation of much commercially-necessary capability. In addition to implementing aggressive trace scheduling, it was known for its reliability, for its incorporation of the state-of-the-art in optimization, and its ability to handle simultaneously many different language variants and all of the different object-code incompatible models of the Multiflow Traces. (While code from a 7/X00 could run correctly on a 14/X00, the nature of the architecture mandated that it would have to be recompiled to run faster than it did on the 7/.)
The compiler was generating correct code by 1985, and by 1987 it was producing code that found significant amounts of ILP. After 1987, with the press of customers and prospects, its development emphasized features and functionality, though performance-oriented improvement continued.
The compiler was so robust, and so good at exposing ILP independent of the system it was targeted for, that after Multiflow closed, the compiler was licensed by many of the largest computer companies. (It has been reported that this included Intel, Hewlett-Packard, Digital Equipment Corporation, Fujitsu, Hughes, HAL Computer Systems, and Silicon Graphics. Other companies known to have licensed the technology include Equator Technologies, Hitachi and NEC. Compilers built starting from that code base were used for advanced development and benchmark reporting for the most important superscalar processors of the 1990s. Descendants of the compiler were still in wide use 20 years after it first started generating correct code, and are often used as benchmark targets for new compiler development. MIT and The University of Washington are among the universities that received and used the compiler for advanced research purposes.
The Multiflow compiler was written in C. It pre-dated the popular use of C++ (Multiflow was a beta-site for the language). The compiler designers were strong believers in the object-oriented paradigm, however, and the compiler had a rather idiosyncratic style that encapsulated the structures and operations in it. This caused a steep learning curve for the many developers who used it after Multiflow’s demise, but one that was usually considered worth it because of the unique combination of ambitious compiling and rock-solid engineering the compiler offered.
Following Multiflow’s closing, its employees went on to have a widespread effect on the industry. The small core group of engineers and scientists, numbering about 20, produced 4 fellows in major American computer companies (2 of whom were Eckert-Mauchly Award winners), several founders of successful startups, and leaders of major development efforts at large companies. The only nontechnical person in the core group, hired out of business school, went on to lead corporate development at a major research lab. As Multiflow grew, it continued the tradition of hiring highly talented people: as one example, the documentation writer became one of the most influential editors in computer publishing. Multiflow’s effect on the computer industry was very much its people in addition to its technology.
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