[Libre-soc-isa] [Bug 559] analyse implications of automatic detection of changing VL loop direction

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=559

--- Comment #7 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #6)
> (In reply to Jacob Lifshay from comment #5)
> > (In reply to Luke Kenneth Casson Leighton from comment #4)
> > > i'm inclined to close this one as invalid.
> > 
> > I'd instead close it as completed, since we did analyze the implications of
> > automatic detection of changing VL loop direction as the title says.
> 
> good point :)
> 
> still for due diligence there are a few things left to cover.
> 
> VSLIDE is the usual instruction which moves registers inside a vector up and
> down.  SV you simply leave the elements in place and issue an instruction
> that starts at a different offset.

we still might need vslide since registers != elements since elements don't
have to be 64-bit.

I guess twin predication with a dest_mask of src_mask << slide_by will work,
but it'll be 2/3 instructions rather than 1. Also, some implementations may be
able to implement vslide more efficiently.

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