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Sanjay Patel
8209d78723
As noted in the code comment, transforming this in the other direction might require a separate transform here in CGP given the block-at-a-time DAG constraint. Besides that theoretical motivation, there are 2 practical motivations for the subtract-of-cmps form: 1. The codegen for both x86 and PPC is better for this IR (though PPC could be better still). There is discussion about canonicalizing IR to the select form ( http://lists.llvm.org/pipermail/llvm-dev/2017-July/114885.html ), so we probably need to add DAG transforms for those patterns anyway, but this improves the memcmp output without waiting for that step. 2. If we allow vector-sized chunks for the load and compare, x86 is better prepared to convert that to optimal code when using subtract-of-cmps, so another prerequisite patch is avoided if we choose to enable that. Differential Revision: https://reviews.llvm.org/D34904 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309597 91177308-0d34-0410-b5e6-96231b3b80d8