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

[Sean Silva]

On Fri, Jul 10, 2015 at 9:00 PM, escha <escha at apple.com> wrote:

>
>
> 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. 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.
>

Totally reasonable. Glad that's on the radar.

-- Sean Silva


>
> —escha
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150710/95da195f/attachment.html>