Compiler Talk

Talk by Ivan Godard at 7 PM on Wednesday, June 10, 2015, at the
SFBay Association of C/C++ Users

The particulars:
Wednesday, June 10, 2015
6:30 PM Doors Open, 7:00 PM – Talk
VCAFE building, 350 Ellis St, Mountain View, CA (map)
VCAFE is accessible from the semicircular courtyard between Symantec buildings.

This is the tenth topic publicly presented related to the Mill general-purpose CPU architecture. It covers only the tool chain used to generate executable binaries targeted for any member of the Mill CPU architecture family. The talk assumes a familiarity with aspects of CPU architecture in general and C++ programming in particular.

LLVM meets the truly alien:

The Mill CPU architecture in a multi-target tool chain

The Mill is a new general-purpose CPU architecture family that forms a uniquely challenging target for compilation – and also a uniquely easy target. This talk describes the Mill tool chain from language front end to binary executable.

The Mill tool chain is unusual in that it translates LLVM intermediate representation (IR) not into object code but into a different IR (genAsm), tailored for the Mill architecture family. Then a separate tool, the specializer, converts genAsm input into executable binary code (conAsm) for a particular Mill architecture family member. genAsm is a dataflow language, essentially a programmable representation of a single-assignment compiler IR.

The Mill has no general registers. Instead, intermediate results are placed on the Belt, a fixed-length queue, and these operands are accessed by temporal addressing. A Mill operation in effect says “add the third most recent value to drop on the belt to the fifth most recent, and drop the result at the front of the belt, and discard the oldest value from the other end of the belt”. The Mill is also a (very) wide issue machine, and many of these actions are taking place concurrently in each cycle. The tool chain, or rather the specializer component, must track the location of operands as they move along the belt, because their belt address changes as other operations are executed and drop results. In addition, the Mill is statically scheduled with an exposed pipeline, so an operation may produce its results several cycles after the operation was issued, possibly with intervening control flow.

This belt structure leads to unique needs for operation scheduling and operand spilling. These needs are the rough equivalent of instruction selection, register coloring, and spill on a conventional machine. The talk concludes by explaining the algorithms used.