Secondly, no mainstream compiler actually compiles code to the PCMPISTRI instruction. Presumably it was meant to be used directly as assembly. I'm not sure why you are bringing in this obscure instruction into the discussion of superoptimizers.
I personally have a paranoid fantasy where NSA/GCHQ introduced this instruction to speed up password cracking. :D
In any practical vectorized tight inner-loop, the block you're trying to optimize is inherently going to be large. Superoptimization is exponential in the size of the block being optimized, which limits its utility. That was my entire point: it becomes unacceptably expensive way too quickly to get used in compilers. (Some of the code I'm looking at right now has 100s of instructions in a single basic block, definitely not atypical for a compiler).
Of course I could be wrong. If you do know of any open-source compiler which compiles code to PCMPISTRI, let me know.
Superoptimization is not exactly exponential, it is NP-hard.
Of course, in the paranoid conspiracy I was referring to in my previous comment I predict the NSA/GCHQ is also using this directly via assembly. xD