diff --git a/llvm/lib/Transforms/AggressiveInstCombine/TruncInstCombine.cpp b/llvm/lib/Transforms/AggressiveInstCombine/TruncInstCombine.cpp
--- a/llvm/lib/Transforms/AggressiveInstCombine/TruncInstCombine.cpp
+++ b/llvm/lib/Transforms/AggressiveInstCombine/TruncInstCombine.cpp
@@ -29,10 +29,12 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Instruction.h"
+#include "llvm/Support/KnownBits.h"
 
 using namespace llvm;
 
@@ -61,6 +63,7 @@
   case Instruction::And:
   case Instruction::Or:
   case Instruction::Xor:
+  case Instruction::Shl:
     Ops.push_back(I->getOperand(0));
     Ops.push_back(I->getOperand(1));
     break;
@@ -98,8 +101,21 @@
       // Worklist and the Stack, and add it to the instruction info map.
       Worklist.pop_back();
       Stack.pop_back();
+
       // Insert I to the Info map.
-      InstInfoMap.insert(std::make_pair(I, Info()));
+      // Initialize MinBitWidth for `shl` instruction with the minimum number
+      // that is greater than shift amount (i.e. shift amount + 1).
+      // Also normalize MinBitWidth not to be greater than source bitwidth.
+      unsigned MinBitWidth;
+      if (I->getOpcode() == Instruction::Shl) {
+        KnownBits KnownRHS = computeKnownBits(I->getOperand(1), DL);
+        const unsigned SrcBitWidth = KnownRHS.getBitWidth();
+        MinBitWidth = KnownRHS.getMaxValue().getZExtValue();
+        if (MinBitWidth != std::numeric_limits<unsigned>::max())
+          MinBitWidth++;
+        MinBitWidth = std::min(MinBitWidth, SrcBitWidth);
+      }
+      InstInfoMap.insert(std::make_pair(I, Info{0, MinBitWidth}));
       continue;
     }
 
@@ -127,6 +143,7 @@
     case Instruction::And:
     case Instruction::Or:
     case Instruction::Xor:
+    case Instruction::Shl:
     case Instruction::Select: {
       SmallVector<Value *, 2> Operands;
       getRelevantOperands(I, Operands);
@@ -137,7 +154,7 @@
       // TODO: Can handle more cases here:
       // 1. shufflevector, extractelement, insertelement
       // 2. udiv, urem
-      // 3. shl, lshr, ashr
+      // 3. lshr, ashr
       // 4. phi node(and loop handling)
       // ...
       return false;
@@ -356,10 +373,14 @@
     case Instruction::Mul:
     case Instruction::And:
     case Instruction::Or:
-    case Instruction::Xor: {
+    case Instruction::Xor:
+    case Instruction::Shl: {
       Value *LHS = getReducedOperand(I->getOperand(0), SclTy);
       Value *RHS = getReducedOperand(I->getOperand(1), SclTy);
       Res = Builder.CreateBinOp((Instruction::BinaryOps)Opc, LHS, RHS);
+      // `shl nsw` is more poisonous if bitwidth is smaller.
+      if (Opc == Instruction::Shl)
+        cast<Instruction>(Res)->setHasNoUnsignedWrap(I->hasNoUnsignedWrap());
       break;
     }
     case Instruction::Select: {
diff --git a/llvm/test/Transforms/AggressiveInstCombine/trunc_shifts.ll b/llvm/test/Transforms/AggressiveInstCombine/trunc_shifts.ll
--- a/llvm/test/Transforms/AggressiveInstCombine/trunc_shifts.ll
+++ b/llvm/test/Transforms/AggressiveInstCombine/trunc_shifts.ll
@@ -3,10 +3,9 @@
 
 define i16 @shl_1_commute(i8 %x) {
 ; CHECK-LABEL: @shl_1_commute(
-; CHECK-NEXT:    [[ZEXT:%.*]] = zext i8 [[X:%.*]] to i32
-; CHECK-NEXT:    [[SHL:%.*]] = shl i32 [[ZEXT]], 1
-; CHECK-NEXT:    [[TRUNC:%.*]] = trunc i32 [[SHL]] to i16
-; CHECK-NEXT:    ret i16 [[TRUNC]]
+; CHECK-NEXT:    [[ZEXT:%.*]] = zext i8 [[X:%.*]] to i16
+; CHECK-NEXT:    [[SHL:%.*]] = shl i16 [[ZEXT]], 1
+; CHECK-NEXT:    ret i16 [[SHL]]
 ;
   %zext = zext i8 %x to i32
   %shl = shl i32 %zext, 1
@@ -16,10 +15,9 @@
 
 define i16 @shl_15_commute(i8 %x) {
 ; CHECK-LABEL: @shl_15_commute(
-; CHECK-NEXT:    [[ZEXT:%.*]] = zext i8 [[X:%.*]] to i32
-; CHECK-NEXT:    [[SHL:%.*]] = shl i32 [[ZEXT]], 15
-; CHECK-NEXT:    [[TRUNC:%.*]] = trunc i32 [[SHL]] to i16
-; CHECK-NEXT:    ret i16 [[TRUNC]]
+; CHECK-NEXT:    [[ZEXT:%.*]] = zext i8 [[X:%.*]] to i16
+; CHECK-NEXT:    [[SHL:%.*]] = shl i16 [[ZEXT]], 15
+; CHECK-NEXT:    ret i16 [[SHL]]
 ;
   %zext = zext i8 %x to i32
   %shl = shl i32 %zext, 15
@@ -29,10 +27,9 @@
 
 define i16 @shl_16_not_commute(i8 %x) {
 ; CHECK-LABEL: @shl_16_not_commute(
-; CHECK-NEXT:    [[ZEXT:%.*]] = zext i8 [[X:%.*]] to i32
-; CHECK-NEXT:    [[SHL:%.*]] = shl i32 [[ZEXT]], 15
-; CHECK-NEXT:    [[TRUNC:%.*]] = trunc i32 [[SHL]] to i16
-; CHECK-NEXT:    ret i16 [[TRUNC]]
+; CHECK-NEXT:    [[ZEXT:%.*]] = zext i8 [[X:%.*]] to i16
+; CHECK-NEXT:    [[SHL:%.*]] = shl i16 [[ZEXT]], 15
+; CHECK-NEXT:    ret i16 [[SHL]]
 ;
   %zext = zext i8 %x to i32
   %shl = shl i32 %zext, 15
@@ -55,11 +52,10 @@
 
 define i16 @shl_var_commute(i8 %x) {
 ; CHECK-LABEL: @shl_var_commute(
-; CHECK-NEXT:    [[ZEXT:%.*]] = zext i8 [[X:%.*]] to i32
-; CHECK-NEXT:    [[AND:%.*]] = and i32 [[ZEXT]], 15
-; CHECK-NEXT:    [[SHL:%.*]] = shl i32 [[ZEXT]], [[AND]]
-; CHECK-NEXT:    [[TRUNC:%.*]] = trunc i32 [[SHL]] to i16
-; CHECK-NEXT:    ret i16 [[TRUNC]]
+; CHECK-NEXT:    [[ZEXT:%.*]] = zext i8 [[X:%.*]] to i16
+; CHECK-NEXT:    [[AND:%.*]] = and i16 [[ZEXT]], 15
+; CHECK-NEXT:    [[SHL:%.*]] = shl i16 [[ZEXT]], [[AND]]
+; CHECK-NEXT:    ret i16 [[SHL]]
 ;
   %zext = zext i8 %x to i32
   %and = and i32 %zext, 15
@@ -70,10 +66,9 @@
 
 define <2 x i16> @shl_vector_commute(<2 x i8> %x) {
 ; CHECK-LABEL: @shl_vector_commute(
-; CHECK-NEXT:    [[Z:%.*]] = zext <2 x i8> [[X:%.*]] to <2 x i32>
-; CHECK-NEXT:    [[S:%.*]] = shl <2 x i32> [[Z]], <i32 4, i32 10>
-; CHECK-NEXT:    [[T:%.*]] = trunc <2 x i32> [[S]] to <2 x i16>
-; CHECK-NEXT:    ret <2 x i16> [[T]]
+; CHECK-NEXT:    [[Z:%.*]] = zext <2 x i8> [[X:%.*]] to <2 x i16>
+; CHECK-NEXT:    [[S:%.*]] = shl <2 x i16> [[Z]], <i16 4, i16 10>
+; CHECK-NEXT:    ret <2 x i16> [[S]]
 ;
   %z = zext <2 x i8> %x to <2 x i32>
   %s = shl <2 x i32> %z, <i32 4, i32 10>
@@ -287,10 +282,9 @@
 
 define i16 @shl_nuw(i8 %x, i8 %sh1) {
 ; CHECK-LABEL: @shl_nuw(
-; CHECK-NEXT:    [[Z:%.*]] = zext i8 [[X:%.*]] to i32
-; CHECK-NEXT:    [[S:%.*]] = shl nuw i32 [[Z]], 8
-; CHECK-NEXT:    [[T:%.*]] = trunc i32 [[S]] to i16
-; CHECK-NEXT:    ret i16 [[T]]
+; CHECK-NEXT:    [[Z:%.*]] = zext i8 [[X:%.*]] to i16
+; CHECK-NEXT:    [[S:%.*]] = shl nuw i16 [[Z]], 8
+; CHECK-NEXT:    ret i16 [[S]]
 ;
   %z = zext i8 %x to i32
   %s = shl nuw i32 %z, 8
@@ -300,10 +294,9 @@
 
 define i16 @shl_nsw(i8 %x, i8 %sh1) {
 ; CHECK-LABEL: @shl_nsw(
-; CHECK-NEXT:    [[Z:%.*]] = zext i8 [[X:%.*]] to i32
-; CHECK-NEXT:    [[S:%.*]] = shl nsw i32 [[Z]], 8
-; CHECK-NEXT:    [[T:%.*]] = trunc i32 [[S]] to i16
-; CHECK-NEXT:    ret i16 [[T]]
+; CHECK-NEXT:    [[Z:%.*]] = zext i8 [[X:%.*]] to i16
+; CHECK-NEXT:    [[S:%.*]] = shl i16 [[Z]], 8
+; CHECK-NEXT:    ret i16 [[S]]
 ;
   %z = zext i8 %x to i32
   %s = shl nsw i32 %z, 8