[AMDGPU] Use implicit operand to preserve liveness of COPY (#164911)

When lowering spills / restores, we may end up partially lowering the
spill via copies and the remaining portion with loads/stores. In this
partial lowering case,the implicit-def operands added to the restore
load clobber the preceding copies -- telling MachineCopyPropagation to
delete them. By also attaching an implicit operand to the load, the
COPYs have an artificial use and thus will not be deleted - this is the
same strategy taken in https://github.com/llvm/llvm-project/pull/115285

I'm not sure that we need implicit-def operands on any load restore, but
I guess it may make sense if it needs to be split into multiple loads
and some have been optimized out as containing undef elements.

These implicit / implicit-def operands continue to cause correctness
issues. A previous / ongoing long term plan to remove them is being
addressed via:


https://discourse.llvm.org/t/llvm-codegen-rfc-add-mo-lanemask-type-and-a-new-copy-lanemask-instruction/88021
https://github.com/llvm/llvm-project/pull/151123
https://github.com/llvm/llvm-project/pull/151124

0 files changed, 0 insertions, 0 deletions