[ValueTracking] don't crash when assumptions conflict (PR36270)

The last assume in the test says that %B12 is 0. 
The first assume says that %and1 is less than %B12. 
Therefore, %and1 is unsigned less than 0...does not compute.

That means this line:
Known.Zero.setHighBits(RHSKnown.countMinLeadingZeros() + 1);
...tries to set more bits than exist.

Differential Revision: https://reviews.llvm.org/D43052

llvm-svn: 324610

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 5a8bd4b..ada31e0 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -816,6 +816,14 @@ static void computeKnownBitsFromAssume(const Value *V, KnownBits &Known,
       KnownBits RHSKnown(BitWidth);
       computeKnownBits(A, RHSKnown, Depth+1, Query(Q, I));
 
+      // If the RHS is known zero, then this assumption must be wrong (nothing
+      // is unsigned less than zero). Signal a conflict and get out of here.
+      if (RHSKnown.isZero()) {
+        Known.Zero.setAllBits();
+        Known.One.setAllBits();
+        break;
+      }
+
       // Whatever high bits in c are zero are known to be zero (if c is a power
       // of 2, then one more).
       if (isKnownToBeAPowerOfTwo(A, false, Depth + 1, Query(Q, I)))