[PATCH] D12799: [ValueTracking] Teach isKnownNonZero a new trick

[Philip Reames via llvm-commits]

reames added a subscriber: reames.

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Comment at: lib/Analysis/ValueTracking.cpp:1837
@@ +1836,3 @@
+        APInt LoBits = APInt::getLowBitsSet(BitWidth, Shift->getLimitedValue());
+        if ((KnownZero & LoBits).eq(LoBits))
+          return KnownOne.getBoolValue() || isKnownNonZero(X, DL, Depth, Q);
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This really seems like two distinct cases and the code would be clearer if organized as such.

case 1 - There's a known one bit somewhere in the portion not shifted out.

case 2 - All bits shifted out are known-zero and X is known non-zero.  

I think what you have is correct, I just found the flow of the code confusing on first read.  

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Comment at: unittests/Analysis/ValueTrackingTest.cpp:203
@@ +202,3 @@
+
+TEST_F(KnownValueTest, NonZeroShiftRight) {
+  parseAssembly(
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I'd probably just phrase this as a normal IR test for inst combine or inst simplify.  What's here is correct, just needlessly complicated.


Repository:
  rL LLVM

http://reviews.llvm.org/D12799