x/crypto/chacha20poly1305: 15% performance regression on certain cpus about go

[randall77]

That processor is 13 years old now.
We could make the check dynamic if not building with GOAMD64=v2 or better. The challenge is how do we test that code? I suspect we don't have any processors without PSHUFB support in our trybot fleet.
GODEBUG=cpu.X=off for the right value of X might be enough, not sure.

[mauri870]

There is more discussion going on at #63872 to discuss how to test GOAMD64=v1 features in the builder fleet.

[rfjakob]

I suspect we don't have any processors without PSHUFB support in our trybot fleet.

The CPU in question does have PSHUFB. Looking at https://en.wikipedia.org/wiki/X86-64#Microarchitecture_levels, this makes it a x86-64-v2 CPU.

So the question is more about testing on x86-64-v2. I guess the trybot fleet is x86-64-v3 and later.