Mill Computing, Inc. Forums The Mill Architecture Pipelining Reply To: Pipelining

Ivan Godard
Keymaster
Post count: 627

Well, now that you ask, I’ll go with “pedantic” :-)

Yes, the total number of operations performed by a loop is the product of the number of operations per iteration and the number of iterations, which is finite if the loop ever terminates. Termination is in general undecidable but is assured for many loops of interest.

I prefer to avoid a 20-minute digression into the halting problem in a public general-interest talk. The point of the “unbounded” remark is to refute the widely bandied-about claim that MIMD (and width in general) is pointless because there is only 2X ILP in code. The data of the studies showing 2x is reasonable, but the interpretation of that data is misleading and the design conclusions are flat-out wrong.

Yes, there are loops with iteration counts so low (like, one) that the total operations count cannot fill a MIMD design. But by definition such loops are not where the program spends its time, and are irrelevant. The loops that matter all last long enough to be piped. More, they last long enough that no practical MIMD design can run out of iterations, so from the designer’s point of view, yes, they are unbounded.

The 2x claim has been used to hamstring designs in open, non-loop code also. From saying “there are only an average two operations that are independent of each other and hence can issue together” to saying “more than two pipe are wasted” is to assume that issue must be sequential. Open code on the Mill gets 6 operations in an instruction from 2x ILP code because phasing lets us have three steps of a dependency chain going at once. The 2x data is not wrong, but the conclusions from that data show a woeful lack of imagination.