[llvm] r225034 - InstCombine: try to transform A-B < 0 into A < B

Chandler Carruth chandlerc at google.com
Wed Jan 14 18:01:39 PST 2015

On Wed, Jan 14, 2015 at 5:53 PM, Kevin Qin <kevinqindev at gmail.com> wrote:

> Hi Michael and David,
> I also see some performance regressions on aarch64 target introduced by
> this commit. No further invesitgation on what's the root cause, but I guess
> the one Michael shared may cause regression on many targets, including
> aarch64.
> I argee with Michael's suggestion that revert this commit firstly, and
> make more performance test over it.

It would be good to understand whether the change David made early today
would have addressed these issues or not.

(Either way, it has made me suspect that we may want A - B < 0 to be the
canonical form, and lower it cleverly in the backend.)
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