[llvm] MachineBlockPlacement: Add tolerance to comparisons (PR #67197)

[via llvm-commits]

WenleiHe wrote:

> I'm not sure I can sensible measure it with our production workloads as BOLT will reorder all the basic blocks code again anyway for us

We can do prod measurement with BOLT turned off. I don't expect meaningful perf movement, but it's indeed a good sanity check. 

> I could also see this being useful to reduce day-to-day churn as sampling PGO data changed, though that likely needs tuning and tighter values for the BlockFreqCompBits constant added here.

Yes. It needs to be 1) off by default initially, 2) tunable for the rounding. And just to be clear, I'm mostly thinking about rounding within BFI, not this change as is. 

https://github.com/llvm/llvm-project/pull/67197