Explicitly parallel instruction computing

Explicitly parallel instruction computing (EPIC) is a term coined in 1997 by the HP–Intel alliance^[1] to describe a computing paradigm that researchers had been investigating since the early 1980s.^[2] This paradigm is also called Independence architectures. It was the basis for Intel and HP development of the Intel Itanium architecture,^[3] and HP later asserted that "EPIC" was merely an old term for the Itanium architecture.^[4] EPIC permits microprocessors to execute software instructions in parallel by using the compiler, rather than complex on-die circuitry, to control parallel instruction execution. This was intended to allow simple performance scaling without resorting to higher clock frequencies.

Roots in VLIW

By 1989, researchers at HP recognized that reduced instruction set computer (RISC) architectures were reaching a limit at one instruction per cycle. They began an investigation into a new architecture, later named EPIC.^[3] The basis for the research was VLIW, in which multiple operations are encoded in every instruction, and then processed by multiple execution units.

One goal of EPIC was to move the complexity of instruction scheduling from the CPU hardware to the software compiler, which can do the instruction scheduling statically (with help of trace feedback information). This eliminates the need for complex scheduling circuitry in the CPU, which frees up space and power for other functions, including additional execution resources. An equally important goal was to further exploit instruction level parallelism (ILP), by using the compiler to find and exploit additional opportunities for parallel execution.

VLIW (at least the original forms) has several short-comings that precluded it from becoming mainstream:

VLIW instruction sets are not backward compatible between implementations. When wider implementations (more execution units) are built, the instruction set for the wider machines is not backward compatible with older, narrower implementations.
Load responses from a memory hierarchy which includes CPU caches and DRAM do not have a deterministic delay. This makes static scheduling of load instructions by the compiler very difficult.

EPIC architecture has evolved from VLIW architecture, while retaining many concepts of the superscalar architecture.

Moving beyond VLIW

EPIC architectures add several features to get around the deficiencies of VLIW:

Each group of multiple software instructions is called a bundle. Each of the bundles has a stop bit indicating if this set of operations is depended upon by the subsequent bundle. With this capability, future implementations can be built to issue multiple bundles in parallel. The dependency information is calculated by the compiler, so the hardware does not have to perform operand dependency checking.
A software prefetch instruction is used as a type of data prefetch. This prefetch increases the chances for a cache hit for loads, and can indicate the degree of temporal locality needed in various levels of the cache.
A speculative load instruction is used to speculatively load data before it is known whether it will be used (bypassing control dependencies), or whether it will be modified before it is used (bypassing data dependencies).
A check load instruction aids speculative loads by checking whether a speculative load was dependent on a later store, and thus must be reloaded.

The EPIC architecture also includes a grab-bag of architectural concepts to increase ILP:

Predicated execution is used to decrease the occurrence of branches and to increase the speculative execution of instructions. In this feature, branch conditions are converted to predicate registers which are used to kill results of executed instructions from the side of the branch which is not taken.
Delayed exceptions, using a not a thing bit within the general purpose registers, allow speculative execution past possible exceptions.
Very large architectural register files avoid the need for register renaming.
Multi-way branch instructions improve branch prediction by combining many alternative branches into one bundle.

The Itanium architecture also added rotating register files, a tool useful for software pipelining since it avoids having to manually unroll and rename registers.

Other research and development

There have been other investigations into EPIC architectures that are not directly tied to the development of the Itanium architecture.

The IMPACT project at University of Illinois at Urbana-Champaign, led by Wen-mei Hwu, was the source of much influential research on this topic.
The PlayDoh architecture from HP-labs was another major research project.
Gelato is an open source development community in which academic and commercial researchers are working to develop more effective compilers for Linux applications running on Itanium servers.

References

↑ Schlansker and Rau (February 2000). "EPIC: An Architecture for Instruction-Level Parallel Processors" (PDF). HP Laboratories Palo Alto, HPL-1999-111. Retrieved 2008-05-08.
↑ US 4847755
1 2 "Inventing Itanium: How HP Labs Helped Create the Next-Generation Chip Architecture". HP Labs. June 2001. Retrieved 2007-12-14.
↑ De Gelas, Johan (November 9, 2005). "Itanium–Is there light at the end of the tunnel?". AnandTech. Retrieved 2008-05-08.

External links

Historical background for EPIC
Mark Smotherman (2002) "Understanding EPIC Architectures and Implementations"

CPU technologies

Architecture	Von Neumann Harvard (Modified Harvard) Dataflow TTA

Instruction set	ASIP CISC RISC EDGE EPIC MISC OISC VLIW NISC ZISC TRIPS Comparison

Word size	1-bit 4-bit 8-bit 9-bit 10-bit 12-bit 15-bit 16-bit 18-bit 22-bit 24-bit 25-bit 26-bit 27-bit 31-bit 32-bit 33-bit 34-bit 36-bit 39-bit 40-bit 48-bit 50-bit 60-bit 64-bit 128-bit 256-bit 512-bit Variable

Execution	Instruction pipelining Bubble Operand forwarding Out-of-order execution Register renaming Speculative execution Branch predictor Memory dependence prediction Hazards

Parallel level	Bit Bit-serial Word Instruction Scalar Superscalar Task Thread Process Data Vector Memory

Multithreading	Temporal Simultaneous Preemptive Cooperative

Flynn's taxonomy	SISD SIMD MISD MIMD SPMD Addressing mode

Types	Digital signal processor (DSP) GPGPU Microcontroller Physics processing unit System on a chip (SoC) Cellular

Components	Address generation unit (AGU) Arithmetic logic unit (ALU) Barrel shifter Floating-point unit (FPU) Back-side bus Multiplexer Demultiplexer Registers Memory management unit (MMU) Translation lookaside buffer (TLB) Cache Register file Microcode Control unit Clock rate

Power management	APM ACPI Dynamic frequency scaling Dynamic voltage scaling Clock gating

CPU hardware security	Non-executable memory (NX bit) Bounds checking (Intel MPX) Hardware restriction (firmware) Trusted Execution Technology Secure cryptoprocessor Hardware security module Hengzhi chip

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