[PATCH] [PGO] Hoist hot case statement from switches

[Chad Rosier]

Hi bruno, bob.wilson, chandlerc, hfinkel, grosbach,

This patch identifies hot cases, based on profile information, and inserts the necessary conditional logic to jump to the hottest case statement prior to getting into the switching logic.

A few comments:
1. I'm still working on getting performance numbers, so feel free to grab the patch and test yourself.
2. The 80% "hot" threshold is entirely arbitrary and likely needs tuning.  By default, we don't consider switches with fewer than 4 cases, so 80% seems reasonable, IMHO.
3. I would like to add additional tests, so suggestions are welcome.  Also, I'm not sure how to verify profile information is being propagated correctly; the logic looks correct, but I'd like to have a test in place to ensure no future regressions.

The ideal solution would be a balanced binary tree, but this seems to be a reasonable first step.

 Chad

http://reviews.llvm.org/D5786

Files:
  lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
  lib/CodeGen/SelectionDAG/SelectionDAGBuilder.h
  test/CodeGen/AArch64/switch-pgo.ll
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