1 files changed, 28 insertions, 3 deletions
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index 3f466495c5e..2dea4c45cbd 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -202,10 +202,35 @@ dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
     // shall be unset in the root value (OuterShift). If ShAmtsDiff is negative,
     // we'll need to also produce a mask to unset ShAmtsDiff high bits.
     // So, does *any* channel need a mask? (is ShiftShAmt u>= MaskShAmt ?)
-    if (!match(ShAmtsDiff, m_NonNegative()))
-      return nullptr; // FIXME.
+    if (!match(ShAmtsDiff, m_NonNegative())) {
+      // This sub-fold (with mask) is invalid for 'ashr' "masking" instruction.
+      if (match(Masked, m_AShr(m_Value(), m_Value())))
+        return nullptr;
+      // For a mask we need to get rid of old masking instruction.
+      if (!Masked->hasOneUse())
+        return nullptr; // Else we can't perform the fold.
+      Type *Ty = X->getType();
+      unsigned BitWidth = Ty->getScalarSizeInBits();
+      // We should produce compute the mask in wider type, and truncate later!
+      // Get type twice as wide element-wise (same number of elements!).
+      Type *ExtendedScalarTy = Type::getIntNTy(Ty->getContext(), 2 * BitWidth);
+      Type *ExtendedTy =
+          Ty->isVectorTy()
+              ? VectorType::get(ExtendedScalarTy, Ty->getVectorNumElements())
+              : ExtendedScalarTy;
+      auto *ExtendedNumHighBitsToClear = ConstantExpr::getZExt(
+          ConstantExpr::getAdd(
+              ConstantExpr::getNeg(ShAmtsDiff),
+              ConstantInt::get(Ty, BitWidth, /*isSigned=*/false)),
+          ExtendedTy);
+      // And compute the mask as usual: (-1 l>> (ShAmtsDiff))
+      auto *ExtendedAllOnes = ConstantExpr::getAllOnesValue(ExtendedTy);
+      auto *ExtendedMask =
+          ConstantExpr::getLShr(ExtendedAllOnes, ExtendedNumHighBitsToClear);
+      NewMask = ConstantExpr::getTrunc(ExtendedMask, Ty);
+    } else
+      NewMask = nullptr; // No mask needed.
     // All good, we can do this fold.
-    NewMask = nullptr; // No mask needed.
   } else
     return nullptr; // Don't know anything about this pattern.