Explicitly parallel instruction computing (EPIC) is a term coined in 1997 by the HP–Intel alliance to describe a computing paradigm that researchers had been investigating since the early 1980s. This paradigm is also called Independence architectures. It was the basis for Intel and HP development of the Intel Itanium architecture, and HP later asserted that "EPIC" was merely an old term for the Itanium architecture. 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 VLIWEdit
By 1989, researchers at HP recognized that reduced instruction set computer (RISC) architectures were reaching a limit at one instruction per cycle.[clarification needed] They began an investigation into a new architecture, later named EPIC. 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:
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 evolved from VLIW architecture, but retained many concepts of the superscalar architecture.
Moving beyond VLIWEdit
EPIC architectures add several features to get around the deficiencies of
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.
^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, Morrison, Gordon E.; Brooks, Christopher B. & Gluck, Frederick G., "Parallel processing method and apparatus for increasing processing throughout by parallel processing low level instructions having natural concurrencies", published 1989-07-11, assigned to MCC Development Ltd.
^ ab"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.
Historical background for EPIC
Mark Smotherman (2002) "Understanding EPIC Architectures and Implementations"