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

[Rafael Avila de Espindola]

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

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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