Re: Issue #365 vsetvl{i} x0, x0 instruction forms


David Horner
 

I wholeheartedly agree with resolving on the mailing list.
This should be the rule not exception.


On 2020-07-21 11:58 p.m., Krste Asanovic wrote:
I want to bring this to group's attention as I think I've convinced
myself that Guy's suggestion is the correct path to follow, i.e.,

vsetvli x0, x0, imm

will raise vill if the new SEW'/LMUL' ratio is not the same as the old
SEW/LMUL implying vl might change. Similarly for vsetvl version.
My considerations for allowing vl to change were

a) having a compelling reason to change PoR.
     vsetvl[i]is extremely important to RVV success.
     It deserves deep scrutiny.
     Challenging each and every change,
    as well as proposing any plausible enhancement
    are equally important to get this feature,
    more so than others, right(tm).

b) tracking assemblers and compilers could present warnings.
     Part of my support was my bias towards encouraging vl tracking support.
     Tracking vl in code has substantial benefits beyond a replacement for this x0,x0 behaviour.
     I believe RVV success and adoption will be substantially hampered without it.I believe that ultimately IOT machines will benefit from RVV
         if we continue to emphasize  simplicity  the design.
     It is however specific to RVV.
     So marginal hardware support that appears to mitigate a need for vl tracking gets a check in the negative column.

c) A perceived simplicity of PoR for minimal designs.
     I am biased toward ensuring simple machines can efficiently support for RVV.

     Initial uptake is likely to be in the application/HPC domain, but
     I believe that ultimately IOT machines will benefit from RVV
         if we continue to emphasize  simplicity  the design.

d) Setting vill is excellent as a means to avoid trap behaviour.
     however it requires explicit check after vtype setting ops.
     Opportunistic approaches will rely on the subsequent fault.
     This situation is theoretically impossible to statically backward trace.
     A given RVV data instruction could be branched to from anywhere,
         conditional execution could have executed any vsetvl instruction
         with virtually any rs1 value.
     This biases me away from setting vill, in the x0,x0 case setting vl avoid vill set.
     However, in practice branching into a loop will be errant behaviour and
        RVV data instructions will be paired with a vsetvli instruction.
     My paranoia causes me this too heavily at times. (.... reweighing risks)


e)  in the x0,x0 formulation, vsetvli cannot determine from immediate parsing alone vill state.
     we have strived to ensure the immediate format will meet virtually all in loop use cases.
     Ideally, vsetvl is reserved for context switch (and custom) situations.
     I considered x0,x0 a punt to vsetvl (potentially slow) path to allow for the immediate form optimization
     (i.e. no vill setting considerations after parse) .
     However, reweighing the benefit of retaining vl and requiring a late setting of vill.
     Given vill setting can always be performed on a slow path
     with little real impact to normal code ....  reweighing risks.


Apart from the debugging motivation that Guy presented,
see my point d.
I would add
that this definition effectively removes any read or write of vl from
the instruction, possibly removing hazards and simplifying dependency
tracking and relieving an OoO machine from providing a new rename
register for vl (might still need for vtype).
this does not talk to my point c.

I could not find any non-esoteric use for the vl-trimming behavior of
the current PoR for larger SEW/LMUL,
I've found coders and compiler writers collectively more ingenious than I,
 not only more eyes in free software but a spectrum of inner-eye perceptions and mindsets.

So although relevant to the discussion, in the negative it is not compelling as a benefit.
so given these benefits I move we
adopt the "sets vill for non-iso SEW/LMUL" meaning.
The circuit has
to calculate (vsew_new-vlmul_new)!=(vsew_old-vlmul_old) to determine
vill, but now never needs to read
I disagree with this behaviour. increasing VLMAX does not invalidate current vl and should not" raise an exception" even indirectly.
If we are needing a warning , let assembler/compilers do it note b above.

I also disagree that we always set vill if VLMAX reduced but vl is still < newVLMAX.
Only if the ratio changes do we need to read vl, so in the frequent case I agree vl read can be avoided.
To avoid a vl read
or write vl.
My principle is hardware should not attempt to debug or correct software.
Although hardware developers may believe a specific validation/verification facility will be useful to programmers (SEW/LMUL in-variance checking)
such "policy" should not be imposed but rather a means to electively support such a policy be provided.
Setting vill when original vl cannot be maintained is valid, enforcing an invariance policy is not.
...
As a general optimization guide, software should endeavor to use this
form instead of passing in AVL to avoid the vl update when not
necessary.
I agree.
This is what was envisioned by providing x0,x0.
Further, this encoding implies an intent which makes code clearer.
Someone doing tricks needs to add a comment.

I'm leaning to accepting the proposal as I amended.

I hope this is one we can resolve on the mailing list to save time in
the next meeting.
as do I.

Krste


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