Explicitly parallel instruction computing


Explicitly parallel instruction computing 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 VLIW

By 1989, researchers at HP recognized that reduced instruction set computer architectures were reaching a limit at one instruction per cycle. 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. 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 by using the compiler to find and exploit additional opportunities for parallel execution.
VLIW has several short-comings that precluded it from becoming mainstream:
EPIC architecture evolved from VLIW architecture, but retained many concepts of the superscalar architecture.

Moving beyond VLIW

EPIC architectures add several features to get around the deficiencies of
VLIW:
The EPIC architecture also includes a grab-bag of architectural concepts to increase ILP:
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: