[clang] [llvm] [AArch64] Implement the atomic store with hint intrinsic (PR #198316)

[Kerry McLaughlin via cfe-commits]

kmclaughlin-arm wrote:

> Things to try:
> 
>     * Change the patch to insert the new diagnostics somewhere else in the file.
> 
>     * Diff the assembly to see where there are non-trivial changes.

Thank you for the suggestions, @efriedma.

I tried moving the diagnostics to the end of DiagnosticSemaKinds.td and comparing the assembly before & after, but could not find any reason for the difference still.

It looks like the way I was attempting to reproduce the compile-time regression locally was not giving accurate results. After some discussion with @c-rhodes, we started investigating the changes to AArch64InstrAtomics.td again and found that lowering the AddedComplexity value I used on the atomic hint patterns improves the compile time.
I've created a PR for this which improves the CTMark geomean by -0.12% on aarch64-O0-g: https://github.com/llvm/llvm-project/pull/230478

https://github.com/llvm/llvm-project/pull/198316