Walter Bright wrote:
Don wrote:
The code below compiles to a single machine instruction, yet the
results are CPU manufacturer-dependent.
This is awesome work, Don. Kudos to you, David and Dmitry.
BTW, I've read that fine-grained CPU detection can be done, beyond what
CPUID gives, by examining slight differences in FPU results. I expect
that *, +, -, / should all give exactly the same answers. But the
transcendentals, and obviously yl2x, vary.
I believe that would have once been possible, I doubt it's true any more.
Basic arithmetic and sqrt all give correctly rounded results, so they're
identical on all processors. The 387 gives greatly improved accuracy,
compared to the 287. But AFAIK there have not been intentional changes
since then.
The great tragedy was that an early AMD processor gave much accurate sin
and cos than the 387. But, people complained that it was different from
Intel! So, their next processor duplicated Intel's hopelessly wrong trig
functions.
I haven't seen any examples of values which are calculated differently
between the processors. I only found one vague reference in a paper from
CERN.
