[clang-tools-extra] Bfi precision (PR #66285)

[Matthias Braun via cfe-commits]

MatzeB wrote:

I think I traced the clang regression down to `InlineCostCallAnalyzer::isColdCallSite` returning `true` in more instances. It previously did not do that because of what feels more like an accidental loss of precision:

The example I analyzed starts out as a single-block function with a call resulting in this BFI:
```
 - entry: float = 1.0, int = 8
 ```
 
 It seems that when we inline calls we just scale their BFI values to the callsite. And given that the entry block only had a factor of `int = 8` we have basically no precision left on the low-end. So after inlining a bit more the BFI in my example looks like this:
 ```
  - entry: float = 1.0, int = 8
 - if.then.i: float = 0.0, int = 0
 - if.else.i: float = 0.0, int = 7
 - if.end15.sink.split.i: float = 0.0, int = 0
 - if.end15.i: float = 0.0, int = 8
 - if.then19.i: float = 0.0, int = 5
 - _ZN4llvm12DenseMapBaseINS_8DenseMapIPNS_10StructTypeEPNS_12StructLayoutENS_12DenseMapInfoIS3_vEENS_6detail12DenseMapPairIS3_S5_EEEES3_S5_S7_SA_E20InsertIntoBucketImplIS3_EEPSA_RKS3_RKT_SE_.exit: float = 0.0, int = 8
 ```
 Note that some blocks frequencies got scaled down to `0`. But more importantly no matter what we do it is impossible to have any code marked as "cold" in this situation. Because the cold code threshold is computed as 2% of the entry block frequency. With an entry block value of `int = 8` the 2% cold-threshold is integer `0` and it is effectively impossible for any block to be below that threshold.
 
 
 In comparison with the new conversion scheme we end up with:
 ```
  - entry: float = 1.0, int = 18014398509481984
 - if.then.i: float = 0.0, int = 9002696048640
 - if.else.i: float = 0.0, int = 18005395813433344
 - if.end15.sink.split.i: float = 0.0, int = 18000892999733
 - if.end15.i: float = 0.0, int = 18014398509481984
 - if.then19.i: float = 0.0, int = 11258999068426240
 - _ZN4llvm12DenseMapBaseINS_8DenseMapIPNS_10StructTypeEPNS_12StructLayoutENS_12DenseMapInfoIS3_vEENS_6detail12DenseMapPairIS3_S5_EEEES3_S5_S7_SA_E20InsertIntoBucketImplIS3_EEPSA_RKS3_RKT_SE_.exit: float = 0.0, int = 18014398509481984
 ```
for the same situation and blocks like `if.end15.sink.split.i` end up being classified as cold code now.
 

https://github.com/llvm/llvm-project/pull/66285