[PATCH] D93264: [CSSPGO] Introducing distribution factor for pseudo probe.

[Hongtao Yu via Phabricator via cfe-commits]

hoy added inline comments.


================
Comment at: llvm/include/llvm/IR/PseudoProbe.h:41
   //  [18:3] - probe id
-  //  [25:19] - reserved
+  //  [25:19] - probe distribution factor
   //  [28:26] - probe type, see PseudoProbeType
----------------
hoy wrote:
> wmi wrote:
> > The bits in discriminator is a scare resource. Have you considered using less bits to represent probe distribution factor? I guess it is possible that using a little more coarse grain distribution factor won't affect performance.
> That's a good point. We are using seven bits to represent [0, 100] so that integral numbers can be distinguished. Yes, we could use fewer bits to represent, say 4 bits to represent only even numbers. We could also not use any bits here but instead use the distribution factor of the outer block probes when the competition of those bits are high. I can do an experiment to see how well that works.
On a second thought, using the distribution factor of block probes for call probe may not work well since a callsite may be surrounded by more than one block probes. 

We could use also fewer bits like 6 bits to encode even numbers in the range [0, 100], or 5 bits to encoding multiples of 3 in [0, 100]. I did a profile quality measurement with the even number encoding. It's OK overall except for two SPEC benchmarks. I guess it's a trade-off we'll have to take when there's a competition on those bits. 


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https://reviews.llvm.org/D93264