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Learn how and when to remove this template message A processor that executes every instruction one after the other i. The performance can be improved by executing different substeps of sequential instructions simultaneously termed pipelining , or even executing multiple instructions entirely simultaneously as in superscalar architectures.
Further improvement can be achieved by executing instructions in an order different from that in which they occur in a program, termed out-of-order execution.
These three methods all raise hardware complexity. Before executing any operations in parallel, the processor must verify that the instructions have no interdependencies.
Modern out-of-order processors have increased the hardware resources which schedule instructions and determine interdependencies. In contrast, VLIW executes operations in parallel, based on a fixed schedule, determined when programs are compiled.
Since determining the order of execution of operations including which operations can execute simultaneously is handled by the compiler, the processor does not need the scheduling hardware that the three methods described above require. Design[ edit ] In superscalar designs, the number of execution units is invisible to the instruction set. Each instruction encodes one operation only. For most superscalar designs, the instruction width is 32 bits or fewer.
In contrast, one VLIW instruction encodes multiple operations, at least one operation for each execution unit of a device. For example, if a VLIW device has five execution units, then a VLIW instruction for the device has five operation fields, each field specifying what operation should be done on that corresponding execution unit. To accommodate these operation fields, VLIW instructions are usually at least 64 bits wide, and far wider on some architectures. In one cycle, it does a floating-point multiply, a floating-point add, and two autoincrement loads.
Superscalar CPUs use hardware to decide which operations can run in parallel at runtime, while VLIW CPUs use software the compiler to decide which operations can run in parallel in advance. Because the complexity of instruction scheduling is moved into the compiler, complexity of hardware can be reduced substantially. Most modern CPUs guess which branch will be taken even before the calculation is complete, so that they can load the instructions for the branch, or in some architectures even start to compute them speculatively.
If the CPU guesses wrong, all of these instructions and their context need to be flushed and the correct ones loaded, which takes time. This has led to increasingly complex instruction-dispatch logic that attempts to guess correctly , and the simplicity of the original reduced instruction set computing RISC designs has been eroded.
VLIW lacks this logic, and thus lacks its energy use, possible design defects, and other negative aspects. In a VLIW, the compiler uses heuristics or profile information to guess the direction of a branch. This allows it to move and preschedule operations speculatively before the branch is taken, favoring the most likely path it expects through the branch.
If the branch takes an unexpected way, the compiler has already generated compensating code to discard speculative results to preserve program semantics. Before VLIW, the notion of prescheduling execution units and instruction-level parallelism in software was well established in the practice of developing horizontal microcode.
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 also developed region scheduling methods to identify parallelism beyond basic blocks. Trace scheduling is such a method, and involves scheduling the most likely path of basic blocks first, inserting compensating code to deal with speculative motions, scheduling the second most likely trace, and so on, until the schedule is complete.
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 emit fast code. The TRACE system was implemented in a mix of medium-scale integration MSI , large-scale integration LSI , and very large-scale integration VLSI , packaged in cabinets, a technology obsoleted as it grew more cost-effective to integrate all of the components of a processor excluding memory on one chip.
Multiflow was too early to catch the following wave, when chip architectures began to allow multiple-issue CPUs. Implementations[ edit ] Cydrome was a company producing VLIW numeric processors using emitter-coupled logic ECL integrated circuits in the same timeframe late s.
This company, like Multiflow, failed after a few years. These two would lead computer architecture research at Hewlett-Packard during the s. Along with the above systems, during the same time — , Intel implemented VLIW in the Intel i , their first bit microprocessor, and the first processor to implement VLIW on one chip.
This simple chip had two modes of operation: a scalar mode and a VLIW mode. In the VLIW mode, the processor always fetched two instructions and assumed that one was an integer instruction and the other floating-point. They found that the CPU could be greatly simplified by removing the complex dispatch logic from the CPU and placing it in the compiler. Compilers of the day were far more complex than those of the s, so the added complexity in the compiler was considered to be a small cost.
Contemporary VLIWs usually have four to eight main execution units. The compiler analyzes this code for dependence relationships and resource requirements.
It then schedules the instructions according to those constraints. In this process, independent instructions can be scheduled in parallel. Because VLIWs typically represent instructions scheduled in parallel with a longer instruction word that incorporates the individual instructions, this results in a much longer opcode termed very long to specify what executes on a given cycle. It uses a similar code density improvement method called configurable long instruction word CLIW.
However, EPIC architecture is sometimes distinguished from a pure VLIW architecture, since EPIC advocates full instruction predication, rotating register files, and a very long instruction word that can encode non-parallel instruction groups.
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Learn how and when to remove this template message A processor that executes every instruction one after the other i. The performance can be improved by executing different substeps of sequential instructions simultaneously termed pipelining , or even executing multiple instructions entirely simultaneously as in superscalar architectures. Further improvement can be achieved by executing instructions in an order different from that in which they occur in a program, termed out-of-order execution. These three methods all raise hardware complexity. Before executing any operations in parallel, the processor must verify that the instructions have no interdependencies. Modern out-of-order processors have increased the hardware resources which schedule instructions and determine interdependencies. In contrast, VLIW executes operations in parallel, based on a fixed schedule, determined when programs are compiled.
Very long instruction word
Technical Report Fisher, J. Specifically, we are building a very long probably over bits instruction word machine, the ELI A machine with this much irregular parallelism can reasonably be coded only in high-level languages; this requires state-of-the-art techniques in compiling horizontal microcode. An effective approach to this problem, trace scheduling, has been developed at Yale over the past three years. Without this or similar work, there is little chance that more usable wide-word architectures will be commercially developed.
Bulldog : a compiler for VLIW architectures