[PATCH] D11616: DAGCombiner: Improve DAGCombiner select normalization

[Matthias Braun]

MatzeB created this revision.
MatzeB added reviewers: resistor, charlieturner7c5, ab, chandlerc.
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* Optimize SELECTs first before folding them into SELECT_CC:
This lets us catch a few more cases to optimize.

* Use CSE opportunities before looking at target preferences:
The current code normalizes select(C0, x, select(C1, x, y)) towards
select(C0|C1, x, y) if the targets prefers that form. This patch adds an
additional rule that if the select(C1, x, y) part already exists in the
function then we want to normalize into the other direction because the
effects of reusing the existing value are bigger than transforming into
the target preferred form.

This addresses regressions following r238793, see also:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150727/290272.html

Repository:
  rL LLVM

http://reviews.llvm.org/D11616

Files:
  lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  test/CodeGen/AArch64/dag-combine-select.ll

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