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On PPC64, we end up with lots of i32 -> i64 zero extensions, not only from all of the usual places, but also from the ABI, which specifies that values passed are zero extended. Almost all 32-bit PPC instructions in PPC64 mode are defined to do *something* to the higher-order bits, and for some instructions, that action clears those bits (thus providing a zero-extended result). This is especially common after rotate-and-mask instructions. Adding an additional instruction to zero-extend the results of these instructions is unnecessary. This PPCISelDAGToDAG peephole optimization examines these zero-extensions, and looks back through their operands to see if all instructions will implicitly zero extend their results. If so, we convert these instructions to their 64-bit variants (which is an internal change only, the actual encoding of these instructions is the same as the original 32-bit ones) and remove the unnecessary zero-extension (changing where the INSERT_SUBREG instructions are to make everything internally consistent). git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224169 91177308-0d34-0410-b5e6-96231b3b80d8
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