[llvm] [InstCombine] Try optimizing with knownbits which determined from Cond (PR #91762)

[Nikita Popov via llvm-commits]

nikic wrote:

This all looks quite convoluted to me. I think the high-level approach here should basically be https://github.com/llvm/llvm-project/compare/main...nikic:perf/select-known-bits, which can be further generalized via SimplifyDemandedBits for non-constant pattern. Main challenge is compile-time overhead, doing this is not free: http://llvm-compile-time-tracker.com/compare.php?from=534f8569a3c9fccfd5cbc5f632b63ad0cf711098&to=3e43eb32001514cddbda56dc22fadc40d557e025&stat=instructions:u

The basic idea is to reuse existing KnownBits infra and inject the select condition into it.

https://github.com/llvm/llvm-project/pull/91762