[PATCH] D150587: [KnownBits] Make shl/lshr/ashr implementations optimal

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n added inline comments.


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Comment at: llvm/lib/Support/KnownBits.cpp:243
   APInt MinShiftAmount = RHS.getMinValue();
-  if (MinShiftAmount.ult(BitWidth)) {
-    MinLeadingZeros += MinShiftAmount.getZExtValue();
-    MinLeadingZeros = std::min(MinLeadingZeros, BitWidth);
-  }
+  if (MinShiftAmount.uge(BitWidth))
+    // Always poison. Return unknown because we don't like returning conflict.
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Not for this patch, but in future installments, maybe a third argument to specify behavior here? In the X86 backend, for example, we remove masks on `shiftamt` b.c its known processor will just modulo `shiftamt` by `bitwidth`. Likewise some targets just return zero. If we could specify the behavior, it may be usable in `TargetLowering::computeKnownBitsForTargetNode`.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D150587/new/

https://reviews.llvm.org/D150587