Try to clarify semantics of fptrunc

[Sean Silva via llvm-commits]

+destination type, ``ty2``, then the results are undefined. If the cast
produces
+an inexact result how rounding is performed (e.g. truncation, also known as
+round to zero) is undefined.

probably want a comma between `result` and `how`

Other than that, this looks fine. I'd like a second opinion from Dan Gohman
though since he has recently defined a target's floating point lowering.

-- Sean Silva

On Fri, Aug 28, 2015 at 11:51 AM, Dan Liew <dan at su-root.co.uk> wrote:

> Hi,
>
> This patch follows on from an e-mail I sent to llvm-dev a while ago [1].
>
> The issue I see is that the documented semantics of ``fptrunc`` are
> misleading
>
> * ``the value cannot fit within the destination type`` is ambiguous.
>   It could mean overflow or an inexact result that requires rounding or
>   it could mean both.
>
> * Using "truncation" in the semantics is very misleading given that it
>   doesn't necessarily truncate (i.e. round to zero). For example on
>   x86_64 with SSE2 this is currently mapped to cvtsd2ss instruction
>   who's rounding behaviour is dependent on the MXCSR register which
>   is usually set to round to nearest even by default.
>
> This patch tries to address this.
>
>
> [1] http://lists.llvm.org/pipermail/llvm-dev/2015-August/089504.html
>
> Thanks,
> Dan
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150828/18a35ffc/attachment.html>