math/big: optimize amd64 asm shlVU and shrVU for shift==0 case

[josharian]

When shift == 0, shlVU and shrVU reduce to a memcopy. When z.ptr == x.ptr, it further reduces to a no-op. The pure Go implementation has these optimizations, as of https://go-review.googlesource.com/c/go/+/164967. The arm64 implementation has one of them (see #31084 (comment)). We should add both to the amd64 implementation.

cc @griesemer

[gopherbot]

Change https://golang.org/cl/170257 mentions this issue: math/big: optimize amd64 asm shlVU and shrVU for shift==0 case

[nightlyone]

Can this be done in the wrappers/callers instead so the per-arch assembler as well as the generic can just assume that this optimization has been applied?

This also allows SSA to see where these conditions might be constant either now or in the future.

[josharian]

Good question. As of this moment there aren’t any pure go wrappers for these functions—they all go straight to the assembly implementations. Now that we have mid-stack inlining, it might make sense to change that, and do optimizations like this in the wrappers, so they can skip the call entirely. Want to experiment and send a CL for 1.16 if appropriate?