[PATCH] D48853: [SCEV] Add zext(C + x + ...) -> D + zext(C-D + x + ...)<nuw> transform

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev added inline comments.


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Comment at: lib/Analysis/ScalarEvolution.cpp:1816
+        const DataLayout &DL = getDataLayout();
+        KnownBits Known = computeKnownBits(OpV, DL, 0, &AC, nullptr, &DT);
+        MinValue = Known.One.ugt(MinValue) ? Known.One : MinValue;
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I don't understand why we need this. `computeKnownBits` is used to deduce ranges of SCEVUnknown. All other SCEV nodes are supposed to propagate range information (e.g. range of sum is a range from sum of min to sum of max, and so on). Thus, in theory, we should be able to identify the range of any SCEV correctly, unless we have some missing logic in range calculation.

What is `OpV` in the example you're trying to improve, and why SCEV was unable to deduce its range via `getUnsignedRange(getSCEV(OpV))`?


Repository:
  rL LLVM

https://reviews.llvm.org/D48853