Try manually assigning registers #68

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1. pelletier

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   on Jan 11, 2022
2. mmcloughlin commented on Apr 24, 2022

   

   mmcloughlin

   on Apr 24, 2022

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   Could you elaborate on the problem you've seen with the avo allocator, and consider filing an issue on the avo side?
3. pelletier commented on Apr 26, 2022

   

   pelletier

   on Apr 26, 2022

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   This issue comes from @chriso saying there may be dependencies being created between YMM registers that could be avoiding by manually picking registers: #58 (comment).

   I haven't looked closely at what dependencies exist today though, so I don't know if there is an actual problem. I'd be happy to open an issue on avo if I have something concrete! Did a quick scan of the register allocation, which looks like:

   Y0:  incompleteMaskY
   Y1:  continuation4BytesY
   Y2:  continuation3BytesY
   Y3:  nibble1Y
   Y4:  nibble2Y
   Y5:  nibble3Y
   Y6:  nibbleMaskY
   Y7:  msbMaskY
   Y8:  previousBlockY		vp		off3	continuationBitsY
   Y9:  errorY
   Y10: incompletePreviousBlockY	previousY	off2	lowPrev			highCurr	tmpY
   Y11: currentBlockY		tmp2
   Y12: zeroes			blend		highPrev
   Y13: shuffle
   Y14: tmp1
   // Y15?
   

   I'm curious why Y15 wasn't used. At first glance I'm a little surprised to have registers like Y10 that are used for many things, but Y13 and Y14 that are only used on a short section of the loop.
4. mmcloughlin commented on Apr 26, 2022

   

   mmcloughlin

   on Apr 26, 2022

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   Ah, okay, so it's not failing to find an allocation, but you think a different allocation would improve performance.

   Regarding Y15, I think the algorithm as written will use the minimum number of registers it can, so that should be equal to the max number of live variables at any point. In your case that must be 15, so the highest used is Y14.

   I can't remember the precise implementation, but it's picking an allocation from a pool of available registers. Perhaps changing this logic to LRU would approximate what you're looking for, and minimize dependencies. There's likely a ton of research on this that I should read up on. The original allocator was intended to be simple.