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

[Fangrui Song via cfe-commits]

MaskRay wrote:

I am nervous with new `if (hasCorrelation())` conditions as well.


If you use the same name for a non-SHF_ALLOC section and a SHF_ALLOC section, the linked output has the SHF_ALLOC flag and you cannot tell what components were non-SHF_ALLOC.
```
.section aa,"", at progbits,unique,1
.quad 0

.section aa,"a", at progbits,unique,2
.quad 0
```

Usually, the better idea is to use different section names. It may require some trial and error to see how much complexity this scheme will bring.

> I feel like this is a bug in lld as _start/_stop symbols for non allocated sections should be null. 

It isn't. This is "undefined behavior, no diagnostic required".

For
```
extern char __start___llvm_covfun __attribute__((weak));
extern char __stop___llvm_covfun __attribute__((weak));

int main() {
  printf("%p, %p\n", &__start___llvm_covfun, &__stop___llvm_covfun);
  return 0;
}
```

The program is ill-formed when `__llvm_covfun` does not have the SHF_ALLOC flag.
`__start_`/`__stop_` symbols were not designed to be used with non-SHF_ALLOC output sections.
It's invalid to reference a non-SHF_ALLOC section from SHF_ALLOC code. The regular undefined weak rule (handwavy "Unresolved weak symbols have a zero value." in the specification) does not necessarily apply.

I noticed that GNU ld defines `__start_`/`__stop_` as well and its linked a.out prints non-zero addresses, just like lld.


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