[LLVMdev] [RFC] Add a simple soft-float class

[Duncan P. N. Exon Smith]

> On 2014 Jun 18, at 09:29, Sean Silva <chisophugis at gmail.com> wrote:
> 
> Why not just use APFloat? (maybe even APInt would be sufficient for the desired semantics?)

APFloat's API is awkward to use when doing math -- it's designed for
compiling and optimizing code that uses floats, not for use as a simple
number representation.

It is possible to wrap APFloat though.

> Is it purely a performance concern?

Largely (assuming the other option is wrapping APFloat).

Also, wrapping APFloat to give similar semantics is a fair bit a work.

UnsignedFloat interoperates well with integers -- it's easy to compose out
of (and decompose into) exponent and digit parts.  Again, this is API that
could be added as a wrapper around APFloat.

> Because it seems like you (and Andy) are supporting this as a suboptimal but convenient replacement for a proper integer-based representation.

In a way, but I think of it as a suboptimal but safe replacement for
hardware floats.  We currently use hardware floats as a suboptimal but
convenient replacement for proper integer-based representations -- I'd like
to swap in a soft-float instead.

For some problems, proper integer-based representations are really hard,
and can resemble a poor man's soft-float even when done right.

> On the other hand, if it's just a performance optimization over APFloat/APInt, then this soft-float thing becomes quick hack + optimization which is a combination that gives me a pretty bad gut feeling (but may still be warranted, of course).

It's a performance optimization over: a heavily wrapped APFloat to give it
simple semantics.  Not sure if that gives you a better feeling...