The military maxim amateurs study tactics, professionals study logistics applies to CPU architecture as well as to armies. Less than 10% of the area and power budget of modern high-end cores is devoted to real work by the functional units such as adders; the other 90% marshalls instructions and data for those units and figures out what to do next.
A large fraction of this logistic overhead comes from instruction fetch and decode. Instruction encoding has subtle and far reaching effects on performance and efficiency throughout a core; for example, the intractable encoding used by x86 instructions is why the x86 will never provide the performance/power of other architectures having friendlier encoding.
Some 80% of executed operations are in loops. A software-pipelined loop has instruction-level parallelism (ILP) bounded only by the number of functional units available and the ability to feed them. The limiting factor is often decode; few modern cores can decode more than four instructions per cycle, and none more than 10. The Mill is a new general-purpose CPU architecture that breaks this barrier; high-end Mill family members can fetch, decode, issue and execute over 30 instructions per cycle.
This talk explains the fetch and decode part of the Mill architecture.
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About the speaker:
Ivan Godard has designed, implemented or led the teams for 11 compilers for a variety of languages and targets, an operating system, an object-oriented database, and four instruction set architectures. He participated in the revision of Algol68 and is mentioned in its Report; was on the Green team that won the Ada language competition; designed the Mary family of system implementation languages; and was founding editor of the Machine Oriented Languages Bulletin. He is a Member Emeritus of IFIPS Working Group 2.4 (Implementation languages), and was a member of the committee that produced the IEEE and ISO floating-point standard 754-2011.
Ivan is currently CTO at Out-of-theBox Computing, a startup now emerging from stealth mode. OOTBC has developed the Mill, a clean-sheet rethink of general-purpose CPU architectures. The Mill is the subject of his talk today.