[llvm] [aarch64][win] Add support for import call optimization (equivalent to MSVC /d2ImportCallOptimization) (PR #121516)

[Daniel Paoliello via llvm-commits]

dpaoliello wrote:

> Is the only thing we gain here, the performance benefits of a direct branch, which is easier for the branch predictor, compared to an indirect branch? While that obviously is better, this feels like a whole lot of extra work and structures, for something that feels like relatively small gains. Is there something else to be gained in relation to this as well that I'm missing (and/or that isn't mentioned yet)? Or is the gains from better branch prediction much bigger than what I'm thinking here?

I don't have the exact numbers, but we did see significant performance improvement within the Windows kernel for some components (which were especially chatty with other .sys files) when MSVC added this optimization. I'll see if I can get one of my colleagues to provide more details.

https://github.com/llvm/llvm-project/pull/121516