[llvm] r241927 - ComputeKnownBits: be a bit smarter about ADDs

[escha]

> 
> What's up with this giant immediate we're now generating instead of an imm8? Seems worse.

It’s a known issue: https://llvm.org/bugs/show_bug.cgi?id=24085 <https://llvm.org/bugs/show_bug.cgi?id=24085>. It’s likely a backend problem; computeKnownBits decides that the constant can be shrunk (which we already do in a lot of places, this just adds one more), so sometimes enhancing computeKnownBits can cause de-optimizations if the backend doesn’t know how to optimize the constant appropriately.

The reviewers on Phab decided we should commit it and file a bug report about the deoptimization, since better KnownBits tracking *should* always be better.

—escha

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150710/c8d57fbb/attachment.html>