[llvm] [SLP]Reduce number of alternate instruction, where possible (PR #123360)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> I'm not clear whether this patch makes it easier or harder to remove the main-alt opcode kludge in the future and allow general splitting - copyable elements, more than 2 subnodes, non-matching instruction types (e.g. binop and intrinsic).

1. Main/alt opcode cannot be removed completely. X86 has instruction addsub, which can be combined effectively, plus to small alt/main combinations (<2) lead to inefficient vectorization.
2. Copyable elements vectorization will require some other stuff (possibly like adding pseudo-instructions for correct scheduling). This is the work I plan for the next patch. 

> 
> I guess I'm asking - is there a larger plan for this or is it focused on improving existing main-alt opcode performance?

Currently, this is mostly all we can do about main/alternate vectorization.



https://github.com/llvm/llvm-project/pull/123360