forked from OSchip/llvm-project
d05eae7a7b
This restores support for selecting the SLLW/SRLW/SRAW instructions, which was removed in rL348067 as the previous patterns made some unsafe assumptions. Also see the related llvm-dev discussion <http://lists.llvm.org/pipermail/llvm-dev/2018-December/128497.html> Ultimately I didn't introduce a custom SelectionDAG node, but instead added a DAG combine that inserts an AssertZext i5 on the shift amount for an i32 variable-length shift and also added an ANY_EXTEND DAG-combine which will instead produce a SIGN_EXTEND for an i32 variable-length shift, increasing the opportunity to safely select SLLW/SRLW/SRAW. There are obviously different ways of addressing this (a number discussed in the llvm-dev thread), so I'd welcome further feedback and comments. Note that there are now some cases in test/CodeGen/RISCV/rv64i-exhaustive-w-insts.ll where sraw/srlw/sllw is selected even though sra/srl/sll could be used without any extra instructions. Given both are semantically equivalent, there doesn't seem a good reason to prefer one vs the other. Given that would require more logic to still select sra/srl/sll in those cases, I've left it preferring the *w variants. Differential Revision: https://reviews.llvm.org/D56264 llvm-svn: 350992 |
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README.txt
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