[PATCH] D16836: [CodeGenPrepare] Don't transform select instructions into branches when both of operands are cheap

[Benjamin Kramer via llvm-commits]

bkramer added a comment.

In http://reviews.llvm.org/D16836#345074, @spatel wrote:

> I would be very interested to know the perf results of completely removing the heuristic-based load transform. I'm not sure what LLVM policy is for this type of situation. I've cc'd @bkramer in case it is possible to know which app/benchmark was improved by r156234. Clearly, it was driven by an improvement for OoO x86-64.


Back then this had significant effects on the LLVM test suite on 2012-class x86_64 hardware. The heuristic is extremely dumb though because LLVM was/is lacking infrastructure to do it properly.

1. Introducing branches at the SelectionDAG level is extremely messy.
2. No profile info available (not even heuristically) on selects.
3. No info on the cost of an instruction. This is somewhat solved by TTI but I'm not sure if it handles loads.

If I were to implement it properly I would start by adding branch weights to selects, we currently lose a lot of profile data in SimplifyCFG. Then we can drive the transform by transforming biased selects into branches first, maybe with a TTI-based version of the current heuristic as a fall back if no info is there.

If you don't see any regressions from disabling the load-based heuristic feel free to throw it out. Both the hardware and LLVM have changed since 2012 so maybe the problems I was papering over with the load-based heuristic aren't there anymore.


http://reviews.llvm.org/D16836