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

Ivan Godard
Post count: 689

The FP error flags are still live until realized by a non-speculative operation (or fall off the belt), so it is not possible to reuse them for integer. Consider:

float f = 10.0;
int i = int(f/3.0) + 0x7fffffff;

The float divide will set the inexact flag, while the add will integer overflow. Neither is real until out of speculative context, so both must be preserved. There are many (including me) who feel that the inexact flag is bogus, should never have been introduced, and should be deprecated now, but as a commercial product we have to live with the stands we have, and in any case code could have set the other flags instead.

As for “large enough”, a major focus of the Mill design is to avoid corner cases, especially those that get fobbed off on the compiler. We have tried hard to make everything in the operation set uniform. You have identified one case where we failed: the FP flags only exist for FP-sized values, so code like:

float f = ..;
uint8_t b = f < 0.0;

is a problem, because the float flags propagate through the eql operation and subsequent narrowing to a one-byte value. This is not a problem in scalar, because there is a flag-set in the operand even though it is small. But it is a problem with vector narrowing, because we couldn’t see paying for five bits of flags in every byte of a vector. Hence we gritted our teeth and defined vector narrow to be a realizing operation that cannot be speculated. 🙁

It would certainly be possible to have a carry metabit in each byte, and we considered it. After all, there is already a NaR bit in each byte. However, we could not find enough usage for such a design to justify the cost. Perhaps we overlooked some important use for carry – did you have a use-case in mind?

  • This reply was modified 10 years, 4 months ago by  Ivan Godard.
  • This reply was modified 10 years, 4 months ago by  staff.