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Re: [PATCH], Add PowerPC ISA 3.0 IEEE 128-bit floating point round to odd built-in functions
On Fri, 29 Sep 2017, Joseph Myers wrote:
> On Fri, 29 Sep 2017, Segher Boessenkool wrote:
>
> > How do other ports deal with this? Insns with a specific rounding mode?
> > Have a separate unspec for every operation? Not very nice either :-(
>
> Well, ideally you'd have a machine-independent representation of constant
> rounding modes that could be used with the TS 18661-1 FENV_ROUND pragma,
> respectively FENV_DEC_ROUND for decimal floating point (as the standard
> machine-independent way of accessing such a facility at the C language
> level - you'd then need to extend it to handle round-to-odd, but given the
> basic facility, accepting additional rounding mode names with it should be
> easy). But I don't know what that would look like in either GIMPLE or
> RTL, and I'd certainly expect it to be a large project (quite likely
> depending on other large projects to handle dynamic rounding modes
> properly through optimizers). So you probably can't do much better than
> lots of unspecs and machine-specific built-in functions at present.
(But the answer to your question seems to be that AVX512 uses something
involving UNSPEC_EMBEDDED_ROUNDING.)
--
Joseph S. Myers
joseph@codesourcery.com