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diff --git a/llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombineInternal.h b/llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombineInternal.h
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+//===- AggressiveInstCombineInternal.h --------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the instruction pattern combiner classes.
+// Currently, it handles pattern expressions for:
+//  * Truncate instruction
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIB_TRANSFORMS_AGGRESSIVEINSTCOMBINE_COMBINEINTERNAL_H
+#define LLVM_LIB_TRANSFORMS_AGGRESSIVEINSTCOMBINE_COMBINEINTERNAL_H
+
+#include "llvm/ADT/MapVector.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/BasicAliasAnalysis.h"
+#include "llvm/Analysis/ConstantFolding.h"
+#include "llvm/Analysis/GlobalsModRef.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/IR/DataLayout.h"
+#include "llvm/Pass.h"
+using namespace llvm;
+
+//===----------------------------------------------------------------------===//
+// TruncInstCombine - looks for expression dags dominated by trunc instructions
+// and for each eligible dag, it will create a reduced bit-width expression and
+// replace the old expression with this new one and remove the old one.
+// Eligible expression dag is such that:
+//   1. Contains only supported instructions.
+//   2. Supported leaves: ZExtInst, SExtInst, TruncInst and Constant value.
+//   3. Can be evaluated into type with reduced legal bit-width (or Trunc type).
+//   4. All instructions in the dag must not have users outside the dag.
+//      Only exception is for {ZExt, SExt}Inst with operand type equal to the
+//      new reduced type chosen in (3).
+//
+// The motivation for this optimization is that evaluating and expression using
+// smaller bit-width is preferable, especially for vectorization where we can
+// fit more values in one vectorized instruction. In addition, this optimization
+// may decrease the number of cast instructions, but will not increase it.
+//===----------------------------------------------------------------------===//
+
+namespace llvm {
+  class DataLayout;
+  class DominatorTree;
+  class TargetLibraryInfo;
+
+class TruncInstCombine {
+  TargetLibraryInfo &TLI;
+  const DataLayout &DL;
+  const DominatorTree &DT;
+
+  /// List of all TruncInst instructions to be processed.
+  SmallVector<TruncInst *, 4> Worklist;
+
+  /// Current processed TruncInst instruction.
+  TruncInst *CurrentTruncInst;
+
+  /// Information per each instruction in the expression dag.
+  struct Info {
+    /// Number of LSBs that are needed to generate a valid expression.
+    unsigned ValidBitWidth = 0;
+    /// Minimum number of LSBs needed to generate the ValidBitWidth.
+    unsigned MinBitWidth = 0;
+    /// The reduced value generated to replace the old instruction.
+    Value *NewValue = nullptr;
+  };
+  /// An ordered map representing expression dag post-dominated by current
+  /// processed TruncInst. It maps each instruction in the dag to its Info
+  /// structure. The map is ordered such that each instruction appears before
+  /// all other instructions in the dag that uses it.
+  MapVector<Instruction *, Info> InstInfoMap;
+
+public:
+  TruncInstCombine(TargetLibraryInfo &TLI, const DataLayout &DL,
+                   const DominatorTree &DT)
+      : TLI(TLI), DL(DL), DT(DT), CurrentTruncInst(nullptr) {}
+
+  /// Perform TruncInst pattern optimization on given function.
+  bool run(Function &F);
+
+private:
+  /// Build expression dag dominated by the /p CurrentTruncInst and append it to
+  /// the InstInfoMap container.
+  ///
+  /// \return true only if succeed to generate an eligible sub expression dag.
+  bool buildTruncExpressionDag();
+
+  /// Calculate the minimal allowed bit-width of the chain ending with the
+  /// currently visited truncate's operand.
+  ///
+  /// \return minimum number of bits to which the chain ending with the
+  /// truncate's operand can be shrunk to.
+  unsigned getMinBitWidth();
+
+  /// Build an expression dag dominated by the current processed TruncInst and
+  /// Check if it is eligible to be reduced to a smaller type.
+  ///
+  /// \return the scalar version of the new type to be used for the reduced
+  ///         expression dag, or nullptr if the expression dag is not eligible
+  ///         to be reduced.
+  Type *getBestTruncatedType();
+
+  /// Given a \p V value and a \p SclTy scalar type return the generated reduced
+  /// value of \p V based on the type \p SclTy.
+  ///
+  /// \param V value to be reduced.
+  /// \param SclTy scalar version of new type to reduce to.
+  /// \return the new reduced value.
+  Value *getReducedOperand(Value *V, Type *SclTy);
+
+  /// Create a new expression dag using the reduced /p SclTy type and replace
+  /// the old expression dag with it. Also erase all instructions in the old
+  /// dag, except those that are still needed outside the dag.
+  ///
+  /// \param SclTy scalar version of new type to reduce expression dag into.
+  void ReduceExpressionDag(Type *SclTy);
+};
+} // end namespace llvm.
+
+#endif