[clang] [compiler-rt] [llvm] [Profile] Refactor profile correlation. (PR #70856)

[Zequan Wu via llvm-commits]

ZequanWu wrote:

> If you omit the data and names sections from the object files, then __llvm_profile_end_data() - __llvm_profile_begin_data() and __llvm_profile_end_names() - __llvm_profile_begin_names() are both 0 and there's no need for [special casing this in the runtime](https://github.com/llvm/llvm-project/blob/24b11ba24da3e65f718391ccc85d4d22a081e893/compiler-rt/lib/profile/InstrProfilingBuffer.c#L59).

>From my experiments, I found that for debug info correlation `__llvm_profile_end_data()` - `__llvm_profile_begin_data()` and `__llvm_profile_end_names()` - `__llvm_profile_begin_names()` are both 0 at runtime because `{__start_/__stop_}{__llvm_prf_names/data}` symbols are null if there is no `__llvm_prf_names/data`. 

But for binary correlation, even though `__llvm_prf_names/data` don't have `SHF_ALLOC` flag so they are not loaded into memory, `__llvm_profile_end_data()` - `__llvm_profile_begin_data()` and `__llvm_profile_end_names()` - `__llvm_profile_begin_names()` still produces the size of name and data sections, that's why we need a bit in the global profile version variable to indicate they are empty at runtime. I feel like this is a bug in lld as __start_/__stop_ symbols for non allocated sections should be null. Even if we have a way to fix that, we still need the mode bit at llvm-profile merging step to indicate if the raw profile need debug info/binary to correlate. However, we can get ride of the runtime checking `__llvm_profile_has_correlation` to allow code compiled w/wo debug-info linked together works well.

https://github.com/llvm/llvm-project/pull/70856