[llvm] [PowerPC][MachineLICM] Aggressively hoist CP loads with PC-Rel (PR #133313)

[Nemanja Ivanovic via llvm-commits]

nemanjai wrote:

> If this specific addressing mode for vector loads is so bad you never want to use it, then... don't use it?

I'm not sure what you mean here. PC-Relative addressing is certainly an addressing mode that we would like to use. It has advantages of not having to maintain a TOC pointer, etc. However, it can be a bit of a performance problem in loops so we would like to hoist out constant pool loads as much as possible.

> Why not just compute the address of the constant pool entry as a separate instruction?

I don't follow how you're suggesting we do this. The constant pool will be in a section that the linker will lay out and it will replace the pc-relative relocations with actual values at link time. How can we compute the address at compile time?

https://github.com/llvm/llvm-project/pull/133313