Simon Pilgrim 89b89650f3 [SelectionDAG] Attempt to split BITREVERSE vector legalization into BSWAP and BITREVERSE stages ...

For BITREVERSE, bit shifting/masking every bit in a vector element is a very lengthy procedure.

If the input vector type is a whole multiple of bytes wide then we can split this into a BSWAP shuffle stage (to reverse at the byte level) and then a BITREVERSE stage applied to each byte. Most vector capable targets can efficiently BSWAP using shuffles resulting in a considerable reduction in instructions.

With this patch targets would only need to implement a target specific vXi8 BITREVERSE implementation to efficiently reverse most legal vector types.

Differential Revision: http://reviews.llvm.org/D19978

llvm-svn: 269290

2016-05-12 13:09:49 +00:00

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