[llvm] [X86] Lower vXi8 multiplies by constant using PMADDUBSW on SSSE3+ targets (PR #95403)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> @phoebewang Do you have any thoughts on non-constants? I could add a IsPMADDUBSWSlow tuning tag for SandyBridge, but it'd be annoying if we had to set it for any of the generic x86-64-v levels as well.



> @phoebewang Do you have any thoughts on non-constants? I could add a IsPMADDUBSWSlow tuning tag for SandyBridge, but it'd be annoying if we had to set it for any of the generic x86-64-v levels as well.

I see the cycles is still slightly better on SandyBridge. I'm ok to start without a tuning tag. We can revisit it if we notice notable performance drop.

https://github.com/llvm/llvm-project/pull/95403