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Mirror of https://github.com/RPCS3/llvm-mirror
a3c6b40b1a
The motivation for filling out these select-of-constants cases goes back to D24480, where we discussed removing an IR fold from add(zext) --> select. And that goes back to: https://reviews.llvm.org/rL75531 https://reviews.llvm.org/rL159230 The idea is that we should always canonicalize patterns like this to a select-of-constants in IR because that's the smallest IR and the best for value tracking. Note that we currently do the opposite in some cases (like the cases in *this* patch). Ie, the proposed folds in this patch already exist in InstCombine today: https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/InstCombine/InstCombineSelect.cpp#L1151 As this patch shows, most targets generate better machine code for simple ext/add/not ops rather than a select of constants. So the follow-up steps to make this less of a patchwork of special-case folds and missing IR canonicalization: 1. Have DAGCombiner convert any select of constants into ext/add/not ops. 2 Have InstCombine canonicalize in the other direction (create more selects). Differential Revision: https://reviews.llvm.org/D30180 llvm-svn: 296137 |
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