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authorSanjay Patel <spatel@rotateright.com>2017-07-31 18:08:24 +0000
committerSanjay Patel <spatel@rotateright.com>2017-07-31 18:08:24 +0000
commitfea731a4aa6aabf270fbb9ba6401ca8826c55a9b (patch)
tree93d0d7e6532ef8f3b297548da5097f879e878312 /lldb/packages/Python/lldbsuite/test/python_api/debugger/TestDebuggerAPI.py
parent70d35e102ef8dbba10e2db84ea2dcbe95bbbfd38 (diff)
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[CGP] use subtract or subtract-of-cmps for result of memcmp expansion
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 llvm-svn: 309597
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