[ValueTracking] Out of range shifts might be undef

If it is possible for the RHS of a shift operation to be greater than or equal
to the bit-width, then the result might be undef, and we can't report any known
bits.

In some cases, this was allowing a transformation in instcombine which widened
an undef value from i1 to i32, increasing the range of values that a function
could return.

Differential revision: https://reviews.llvm.org/D30781

llvm-svn: 297724

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 1761dac269d..a3d4198edcb 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -848,6 +848,14 @@ static void computeKnownBitsFromShiftOperator(
 
   computeKnownBits(I->getOperand(1), KnownZero, KnownOne, Depth + 1, Q);
 
+  // If the shift amount could be greater than or equal to the bit-width of the LHS, the
+  // value could be undef, so we don't know anything about it.
+  if ((~KnownZero).uge(BitWidth)) {
+    KnownZero.clearAllBits();
+    KnownOne.clearAllBits();
+    return;
+  }
+
   // Note: We cannot use KnownZero.getLimitedValue() here, because if
   // BitWidth > 64 and any upper bits are known, we'll end up returning the
   // limit value (which implies all bits are known).