Enhancement: Allow reading less than "max" counters

[travisdowns]

Currently the PFC_START/END macros have a fixed behavior of reading exactly the first 7 counters (3 fixed, 4 programmable).

Sometimes you only want to read 1 or 2 counters, or sometimes you want to read up to 11 counters (e.g., because I have hyperthreading off, I actually have all 8 counters available to a single thread).

Probably with some macro-programming tricks we can generate the appropriate code to read different numbers of counters. Not particularly urgent (if necessary one can always add the specific variant desired via copy-paste).

[obilaniu]

Yes, this might be valuable... The reason I always read 3+4 counters in a specific order and with no dynamic customization was to maximize the predictability of the PFC_START/PFC_END macros and minimize the number of counters that they bump (e.g. branch mispredict), so that pfcRemoveBias() would work well (and work well it does, in general).

I recently refactored those macros, so if it were somehow possible to generate the inline asm start/end/biasremove blobs you needed, that would be great. Could also consider JITting that code at the price of a small call/ret overhead.

[travisdowns]

Indeed, but creating the more specific versions shouldn't cause any additional variance, since you'd also generate the specific version of pfcRemoveBias in the same way. This all happening at compile-time, so it shouldn't change the predictability at all (since the macros are purely a compile time construct and get injected directly at each call site in their entirety).