[Libre-soc-dev] new svp64 page

Jacob Lifshay programmerjake at gmail.com
Thu Dec 10 03:46:13 GMT 2020

On Wed, Dec 9, 2020 at 6:45 PM Luke Kenneth Casson Leighton
<lkcl at lkcl.net> wrote:
> these may potentially also be used for FP to introduce rounding modes
> rather than require an SPR as at present.  something to think about.

The vast majority of code uses Round to nearest Ties to even for FP or
Truncate for FP -> int, however PowerISA provides a quite full suite
of rounding instructions:
frin: round to nearest integer (FP -> FP)
frip: ceil (FP -> FP)
friz: trunc (FP -> FP)
frim: floor (FP -> FP)

There are also instructions for conversion from (FP -> Int) with
either round to nearest or truncate.

I don't think we will have any issues with needing to fiddle with SPRs
for any common operations, since we don't need to change them except
in very rare circumstances (rare enough that LLVM didn't actually
support them at all on any ISA until the last year or two, and I think
they're still experimental).


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