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

[Sean Silva]

On Tue, Jun 17, 2014 at 10:11 PM, Rafael Avila de Espindola <
rafael.espindola at gmail.com> wrote:

> If we require the host to have sse2 or other IEEE 754 conformant
>  implementation, would it be possible to use hardware float?
>

I don't think that IEEE 754 actually guarantees bit-exact results in all
cases.

-- Sean Silva


>
> Sent from my iPhone
>
> > On Jun 17, 2014, at 21:34, "Duncan P. N. Exon Smith" <
> dexonsmith at apple.com> wrote:
> >
> > I'm currently working on stripping usage of `UnsignedFloat` out of
> > `-block-freq`.  Before I finish...
> >
> > I propose adding a soft-float class to llvm/Support/ or llvm/Analysis/.
> >
> >  - Portable (unlike hard-floats).
> >      - Well-defined for all platforms and safe to use in code.
> >
> >  - Easy to use and reason about (unlike `APFloat`).
> >      - Uses operators.
> >      - No special numbers.
> >      - Every operation well-defined (even divide-by-zero).
> >      - No rounding modes.
> >      - Digits represented simply as a 32-bit or 64-bit integer.
> >
> >  - Lowers barrier to entry for writing passes with weight-like logic
> >    (and other uses of numbers).
> >      - Mapping to `uint64_t` is often a hard problem in itself.
> >      - First iteration can focus on the pass logic; second iteration
> >        can remove the floats.
> >
> >  - Already written and (mostly) tested.
> >      - There's currently one in `-block-freq` called `UnsignedFloat`.
> >        Tests are out of tree, though.
> >
> > IIUC, the consensus here is:  hard-floats are worse than soft-floats,
> > and both are worse integers.  However, we have passes in the tree (e.g.,
> > spill placement) that use hard-floats anyway.
> >
> > With a small amount of effort, I can finish testing `UnsignedFloat`
> > (and/or `SignedFloat`) and we can remove hard-floats entirely by using
> > these classes as drop-in replacements.  The long-term answer (for most
> > passes) is mapping to `uint64_t`, but this gets rid of undefined
> > behaviour *now*, and provides a simple and practical alternative to
> > hard-floats going forward.
> >
> > Thoughts?
> >
> > -- dpnes
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