Re: [PATCH], Add PowerPC ISA 3.0 IEEE 128-bit floating point round to odd built-in functions

[Segher Boessenkool <segher@kernel.crashing.org>]

On Fri, Sep 29, 2017 at 08:42:45PM +0000, Joseph Myers wrote:
> 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.)

Thanks.

So this seems to be the same as Mike's patch does.  I was hoping for a
nifty trick, not another huge project :-)  Oh well.


Segher