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3996e5290a
This patch teaches the DAGCombiner how to fold shuffles according to the following new rules: 1. shuffle(shuffle(x, y), undef) -> x 2. shuffle(shuffle(x, y), undef) -> y 3. shuffle(shuffle(x, y), undef) -> shuffle(x, undef) 4. shuffle(shuffle(x, y), undef) -> shuffle(y, undef) The backend avoids to combine shuffles according to rules 3. and 4. if the resulting shuffle does not have a legal mask. This is to avoid introducing illegal shuffles that are potentially expanded into a sub-optimal sequence of target specific dag nodes during vector legalization. Added test case combine-vec-shuffle-2.ll to verify that we correctly triggers the new rules when combining shuffles. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212748 91177308-0d34-0410-b5e6-96231b3b80d8
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