On 28/08/2024 00:37, Segher Boessenkool wrote:
Hi!
On Tue, Aug 27, 2024 at 10:26:29AM +0200, David Brown wrote:
I think expression templates are what you need here.
The "-ffast-math" optimisations are very specific to floating point
calculations, and are quite dependent on the underlying implementation
of floating point. Contrary to Segher's pessimism, "-ffast-math" is
I'm not pessimistic! Merely realistic :-)
For something like this, it is probably best to err on the side of
pessimism rather than optimism - correctness trumps efficiency every time!
extremely useful and gives significantly better end results in
situations where it is appropriate.
... but I do not think it is appropriate to use in nearly as many cases
as people do use it in.
If you say so - you have probably seen a lot more floating point code
than I have, over a wider range of uses. And I have no real idea how
often the flag is used.
I know that in my particular field - small-systems embedded and
microcontroller programming - it is almost always appropriate. Most of
the time, floating point data is a convenient way of handling quantities
like voltage, current, speed, etc., that have limited practical range
and limited true accuracy. You rarely do large numbers of floating
point calculations on each bit of data (thus errors have limited
build-up), ranges tend to be "friendly" to computer floating point
operations, and small levels of inaccuracy are acceptable - but you
usually need the results as fast as possible on hardware that has only
limited support for the floating point calculations.
But like any floating point work,
you need to understand the limitations and the suitability for the task
at hand.
Exactly. So we violently agree :-)
Yes indeed. Floating point is often subtly misunderstood.
I have no idea what kind of "Numeric" the OP wants, so he has to figure
out what kind of re-arrangements and transforms fit his needs. And he
might want to support different kinds of "number" with different properties.
A compiler is not a computer algebra system, and the two things are only
very vaguely related.
