[clang] [llvm] [AMDGPU] Implement variadic functions by IR lowering (PR #93362)

[Jon Chesterfield via cfe-commits]

JonChesterfield wrote:

Joseph reports "memory error" from a libc test when running with this patch. This is unfortunate. I haven't reproduced that yet (I don't mean libc passes, I mean libc fails with or without this patch). The blast radius for "memory error" on amdgpu is wide but there is very little amdgpu specific code in this patch so it's either something handling addrspacecast incorrectly or an unlucky interaction with something outside of this patch.

My plan is to spin up a separate patch which is the non-amdgpu part of this and hope someone signs off on it - the development overhead of juggling lots of branches is significantly compromising time to solution here. Bringing up x64 / aarch64 / nvptx or similar will, if I'm lucky, uncover a bug in this pass which is causing the libc test failure.

For debugging amdgpu, I'll add more tests around addrspace cast and hope to see a bug in the IR, try to get libc to pass and, in extremis, try to build rocm from source in case the debugger helps.

https://github.com/llvm/llvm-project/pull/93362