History
The term VLIW, and the concept of VLIW architecture itself, were invented by Josh Fisher in his research group at Yale University in the early 1980s. His original development of trace scheduling as a compilation technique for VLIW was developed when he was a graduate student at New York University. Prior to VLIW, the notion of prescheduling functional units and instruction-level parallelism in software was well established in the practice of developing horizontal microcode. Fisher's innovations were around developing a compiler that could target horizontal microcode from programs written in an ordinary programming language. He realized that to get good performance and target a wide-issue machine, it would be necessary to find parallelism beyond that generally within a basic block. He developed region scheduling techniques to identify parallelism beyond basic blocks. Trace scheduling is such a technique, and involves scheduling the most likely path of basic blocks first, inserting compensation code to deal with speculative motions, scheduling the second most likely trace, and so on, until the schedule is complete.
Fisher's second innovation was the notion that the target CPU architecture should be designed to be a reasonable target for a compiler — the compiler and the architecture for VLIW must be co-designed. This was partly inspired by the difficulty Fisher observed at Yale of compiling for architectures like Floating Point Systems' FPS164, which had a complex instruction set architecture (CISC) that separated instruction initiation from the instructions that saved the result, requiring very complicated scheduling algorithms. Fisher developed a set of principles characterizing a proper VLIW design, such as self-draining pipelines, wide multi-port register files, and memory architectures. These principles made it easier for compilers to write fast code.
The first VLIW compiler was described in a Ph.D. thesis by John Ellis, supervised by Fisher. The compiler was christened Bulldog, after Yale's mascot. John Ruttenberg also developed certain important algorithms for scheduling.
Fisher left Yale in 1984 to found a startup company, Multiflow, along with co-founders John O'Donnell and John Ruttenberg. Multiflow produced the TRACE series of VLIW minisupercomputers, shipping their first machines in 1987. Multiflow's VLIW could issue 28 operations in parallel per instruction. The TRACE system was implemented in an MSI/LSI/VLSI mix packaged in cabinets, a technology that fell out of favor when it became more cost-effective to integrate all of the components of a processor (excluding memory) on a single chip. Multiflow was too early to catch the following wave, when chip architectures began to allow multiple issue CPUs. The major semiconductor companies recognized the value of Multiflow technology in this context, so the compiler and architecture were subsequently licensed to most of these companies.
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