[PATCH] D23296: [ValueTracking] Improve ValueTracking on left shift with nsw flag

[David Majnemer via llvm-commits]

majnemer requested changes to this revision.
This revision now requires changes to proceed.

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Comment at: lib/Analysis/ValueTracking.cpp:798-802
@@ -797,1 +797,7 @@
     KnownOne  = KOF(KnownOne, ShiftAmt);
+    // If there is conflict between KnownZero and KnownOne, this must be an 
+    // overflowing left shift, so the shift result is undefined. We can return
+    // arbitrary values.
+    if ((KnownZero & KnownOne) != 0) 
+      KnownOne &= ~KnownZero;
+    
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This goes against the ethos of ComputeKnownValues.  Conflicts should return "I don't know" so that we can transform stuff into undef.


https://reviews.llvm.org/D23296