[Libre-soc-isa] [Bug 650] write rfc for OpenPower fpr <-> gpr moves/conversions

bugzilla-daemon at libre-soc.org bugzilla-daemon at libre-soc.org
Thu Jun 3 22:50:51 BST 2021


--- Comment #18 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #16)
> (In reply to Luke Kenneth Casson Leighton from comment #15)
> > nuts, hit send too soon.
> > 
> > https://git.libre-soc.org/?p=libreriscv.git;a=commitdiff;
> > h=caa35bce868af3880119aa4cacbd520b23eaae34
> > 
> > what we think is obvious, is not.  the list is "note-form", i.e.
> > it's a map - a reminder - to *you* - of what is obvious *to you*.
> it's intended to be a short summary/motivating explanation. The full
> semantics are in the instruction descriptions.

they need words as well... sigh

> > 
> > unfortunately, with no explicit context, the indented bullet-list
> > makes no sense (to me, who does not know the context), and
> > consequently it's *definitely* not going to make sense to someone
> > who has even less context.
> > 
> > in particular, the "standard Integer -> FP conversion": which standard
> > is that? it needs to be explicitly listed.
> The standard used by nearly all programming languages and cpus: IEEE 754.

okaay, now i have context.  so i am adding this:

 index c497f6f7..5d33ef43 100644
--- a/openpower/sv/int_fp_mv.mdwn
+++ b/openpower/sv/int_fp_mv.mdwn
@@ -34,8 +34,7 @@ If we're adding new Integer <-> FP conversion instructions,
we may
 as well take this opportunity to modernise the instructions and make them
 well suited for common/important conversion sequences:

-* standard Integer -> FP conversion (**TODO, which standard?** can it
-  be described in words? how does it differ from the other "standards"?)
+* standard Integer -> FP IEEE754 conversion 
 * standard OpenPower FP -> Integer conversion (saturation with NaN
   converted to minimum valid integer)
 * Rust FP -> Integer conversion (saturation with NaN converted to 0)
@@ -54,9 +53,9 @@ the feature being proposed.

 ## standard Integer -> FP conversion

-TODO, explain this further
-- rounding mode read from FPSCR
+This conversion is outlined in the IEEE754 specification.  It is used
+by nearly all programming languages and CPUs.  In the case of OpenPOWER,
+the rounding mode is read from FPSCR

 # standard OpenPower FP -> Integer conversion

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