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

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=650

--- Comment #37 from Jacob Lifshay <programmerjake at gmail.com> ---
Also:
> Effectively, fmvtgs is a macro-fusion of frsp fmvtg and therefore has the exact same exception and flags behaviour of frsp

that's not correct since imho we want the semantics of stfs not frsp. stfs
expects the input to already be f32 but in f64 form, and stores that f32 to
memory. it never sets flags. frsp converts f64 to f32, then back to f64. fmvtg
moves a f64 to a gpr. fusing frsp and fmvtg would produce a f64 in the gpr.

the f64 -> f32 conversion in fp stores is described in the spec v3.1 section
4.6.3. it isn't how frsp does it, it's designed to be simpler because it
doesn't need to round correctly since the input should already be a valid f32
value as a f64.

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