1 files changed, 19 insertions, 1 deletions
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index f38dc436722..e49e6cec65c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3494,7 +3494,8 @@ foldShiftIntoShiftInAnotherHandOfAndInICmp(ICmpInst &I, const SimplifyQuery SQ,
   Instruction *NarrowestShift = XShift;
 
   Type *WidestTy = WidestShift->getType();
-  assert(NarrowestShift->getType() == I.getOperand(0)->getType() &&
+  Type *NarrowestTy = NarrowestShift->getType();
+  assert(NarrowestTy == I.getOperand(0)->getType() &&
          "We did not look past any shifts while matching XShift though.");
   bool HadTrunc = WidestTy != I.getOperand(0)->getType();
 
@@ -3533,6 +3534,23 @@ foldShiftIntoShiftInAnotherHandOfAndInICmp(ICmpInst &I, const SimplifyQuery SQ,
   if (XShAmt->getType() != YShAmt->getType())
     return nullptr;
 
+  // As input, we have the following pattern:
+  //   icmp eq/ne (and ((x shift Q), (y oppositeshift K))), 0
+  // We want to rewrite that as:
+  //   icmp eq/ne (and (x shift (Q+K)), y), 0  iff (Q+K) u< bitwidth(x)
+  // While we know that originally (Q+K) would not overflow
+  // (because  2 * (N-1) u<= iN -1), we have looked past extensions of
+  // shift amounts. so it may now overflow in smaller bitwidth.
+  // To ensure that does not happen, we need to ensure that the total maximal
+  // shift amount is still representable in that smaller bit width.
+  unsigned MaximalPossibleTotalShiftAmount =
+      (WidestTy->getScalarSizeInBits() - 1) +
+      (NarrowestTy->getScalarSizeInBits() - 1);
+  APInt MaximalRepresentableShiftAmount =
+      APInt::getAllOnesValue(XShAmt->getType()->getScalarSizeInBits());
+  if (MaximalRepresentableShiftAmount.ult(MaximalPossibleTotalShiftAmount))
+    return nullptr;
+
   // Can we fold (XShAmt+YShAmt) ?
   auto *NewShAmt = dyn_cast_or_null<Constant>(
       SimplifyAddInst(XShAmt, YShAmt, /*isNSW=*/false,