This pass attempts to optimize three broad categories of memcpy:
1. Self-copies: These we can discard out-of-hand.
2. Vector copies: It doesn't matter what the vector size is or if the
source and destination have different vector types, it's still easy
enough to emit a load/store pair.
3. Tightly packed copies: In the case where a type is tightly packed
(no padding bits), we can replace the memcpy with a copy_deref
instruction which the optimizer is far better at handling.
This has proven capable of getting rid of many of the memcpy instances
in some rather gnarly OpenCL C kernels I've been looking at, even after
coming out of LLVM's optimizer.
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6871>
