[Libre-soc-isa] [Bug 213] SimpleV Standard writeup needed

bugzilla-daemon at libre-soc.org bugzilla-daemon at libre-soc.org
Mon Oct 19 06:00:32 BST 2020


--- Comment #56 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #55)
> (In reply to Jacob Lifshay from comment #53)
> > (In reply to Luke Kenneth Casson Leighton from comment #52)
> > > ahh actually, a single scalar intreg as a predicate mask is dead simple. 
> > > it's one read.  that's it.
> > 
> > That's true ... if you completely ignore the need to generate masks.

I meant that masks should be able to have some bits calculated and used by
succeeding masked vector instructions before other bits have finished
calculating -- this is critical for vector chaining where a vector compare,
optional scalar bitwise logic ops, then vector instructions using the computed
mask are all chained together. Probably only `and`, `andn`, and `or` need to
have special handling allowing chaining through them, everything else can just
wait for all bits of the mask to be available before executing the operation.

Chaining is as described in comment #53

> briefly (it's late here), so i'll just do this one and the rest tomorrow.
> i'm not [ignoring it]: i'm assuming that integer scalar
> operations (and, xor etc) on those integer scalar registers would
> be sufficient to cover the role of generating the masks because the
> masks *are* the (one) scalar int reg, the one scalar int reg *is* the
> mask.
> i.e. once computed (generated) using integer scalar operations that
> int reg (mask) goes straight (ok after DM hazard clearance) into the
> bit-level subdivision needed to turn it into a vector mask.
> what am i missing?  did you mean something different by "need to generate
> mask"?
> i interpret "generate mask" to be "operations such as bitwise ANDing" and
> for that, clearly, straight scalar 64-bit AND is perfectly sufficient.

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