[clang] [compiler-rt] [clang-tools-extra] [PGO] Exposing PGO's Counter Reset and File Dumping APIs (PR #76471)

[Qiongsi Wu via cfe-commits]

qiongsiwu wrote:

I realized one problem during testing IRPGO (thanks again for the suggestion @minglotus-6 !). 

A functions control flow may change between `-fprofile-generate` and `-fprofile-use` when we make use of definitions in the new header. For example, one may have the following code:

```c
void main() {
   ...
  if (__llvm_profile_dump())
    return error;
  
  cleanup();
}
```

During `-fprofile-generate`, `__llvm_profile_dump` is a declared name and main's control flow includes a branch to `return error;`. During `-fprofile-use`, `__llvm_profile_dump()` is replaced by `(0)` and the frontend eliminates the `if` statement and the branch to `return error`. Such control flow change can lead to PGO warnings (hash mismatch). 

I think it may be OK to keep the PR this way because the new macros can potentially cause control flow changes directly as well. The documentation is updated (https://github.com/llvm/llvm-project/pull/76471/files#diff-7389be311daf0b9b476c876bef04245fa3c0ad9337ce865682174bd77d53b648R2908) to advise against using these APIs in a program's hot regions and warn about possible impact on control flow. 

Do you all think this is reasonable? 

https://github.com/llvm/llvm-project/pull/76471