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

[Sanjoy Das via llvm-commits]

sanjoy requested changes to this revision.
sanjoy added a comment.
This revision now requires changes to proceed.

Comments inline.


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Comment at: lib/Analysis/ValueTracking.cpp:803
@@ +802,3 @@
+      KnownZero.clearAllBits();
+      KnownOne.clearAllBits();
+    }
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This looks excessively pessimistic -- why can't we `KnownOne &= ~KnownZero` or `KnownZero &= ~KnownOne` instead?  Both seem better than what we have here.


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Comment at: lib/Analysis/ValueTracking.cpp:1078
@@ -1069,1 +1077,3 @@
              APInt::getLowBitsSet(BitWidth, ShiftAmt); // Low bits known 0.
+    // If this shift has "nsw" keyword, then the result is either a poison 
+    // value or has the same sign bit as the first operand.
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Indent is off.


https://reviews.llvm.org/D23296