[llvm] [AMDGPU] Add mark last scratch load pass (PR #75512)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Why limit this to spill slots, and not general stack accesses?

Because LLVM already provides liveness info for spill slots, which we rely on here.

It might be nice to do it for general memory accesses in future but that would require some new analysis to find last uses.

https://github.com/llvm/llvm-project/pull/75512