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This patch teaches how to fold a shuffle according to rule: shuffle (shuffle (x, undef, M0), undef, M1) -> shuffle(x, undef, M2) We do this only if the resulting mask M2 is legal; this is to avoid introducing illegal shuffles that are potentially expanded into a sub-optimal sequence of target specific dag nodes. This patch has the advantage of being target independent, since it works on ISD nodes. Therefore, all targets (not only x86) can take advantage of this rule. The idea behind this patch is that most shuffle pairs can be safely combined before we run the legalizer on vector operations. This allows us to combine/simplify dag nodes earlier in the process and not only immediately before instruction selection stage. That said. This patch is not meant to replace any existing target specific combine rules; backends might still introduce new shuffles during legalization stage. Also, this rule is very simple and avoids to aggressively optimize shuffles. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212539 91177308-0d34-0410-b5e6-96231b3b80d8
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