Index: llvm/lib/Transforms/AggressiveInstCombine/TruncInstCombine.cpp
===================================================================
--- llvm/lib/Transforms/AggressiveInstCombine/TruncInstCombine.cpp
+++ llvm/lib/Transforms/AggressiveInstCombine/TruncInstCombine.cpp
@@ -56,6 +56,10 @@
     Ops.push_back(I->getOperand(0));
     Ops.push_back(I->getOperand(1));
     break;
+  case Instruction::Select:
+    Ops.push_back(I->getOperand(1));
+    Ops.push_back(I->getOperand(2));
+    break;
   default:
     llvm_unreachable("Unreachable!");
   }
@@ -114,7 +118,8 @@
     case Instruction::Mul:
     case Instruction::And:
     case Instruction::Or:
-    case Instruction::Xor: {
+    case Instruction::Xor:
+    case Instruction::Select: {
       SmallVector<Value *, 2> Operands;
       getRelevantOperands(I, Operands);
       for (Value *Operand : Operands)
@@ -123,7 +128,7 @@
     }
     default:
       // TODO: Can handle more cases here:
-      // 1. select, shufflevector, extractelement, insertelement
+      // 1. shufflevector, extractelement, insertelement
       // 2. udiv, urem
       // 3. shl, lshr, ashr
       // 4. phi node(and loop handling)
@@ -351,6 +356,13 @@
       Res = Builder.CreateBinOp((Instruction::BinaryOps)Opc, LHS, RHS);
       break;
     }
+    case Instruction::Select: {
+      Value *Op0 = I->getOperand(0);
+      Value *LHS = getReducedOperand(I->getOperand(1), SclTy);
+      Value *RHS = getReducedOperand(I->getOperand(2), SclTy);
+      Res = Builder.CreateSelect(Op0, LHS, RHS);
+      break;
+    }
     default:
       llvm_unreachable("Unhandled instruction");
     }
Index: llvm/test/Transforms/AggressiveInstCombine/trunc_select.ll
===================================================================
--- llvm/test/Transforms/AggressiveInstCombine/trunc_select.ll
+++ llvm/test/Transforms/AggressiveInstCombine/trunc_select.ll
@@ -7,11 +7,8 @@
 define dso_local i16 @select_i16(i16 %a, i16 %b, i1 %cond) {
 ; CHECK-LABEL: @select_i16(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CONV0:%.*]] = sext i16 [[A:%.*]] to i32
-; CHECK-NEXT:    [[CONV1:%.*]] = sext i16 [[B:%.*]] to i32
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i32 [[CONV0]], i32 [[CONV1]]
-; CHECK-NEXT:    [[CONV4:%.*]] = trunc i32 [[SEL]] to i16
-; CHECK-NEXT:    ret i16 [[CONV4]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i16 [[A:%.*]], i16 [[B:%.*]]
+; CHECK-NEXT:    ret i16 [[SEL]]
 ;
 entry:
   %conv0 = sext i16 %a to i32
@@ -24,11 +21,8 @@
 define dso_local i8 @select_i8(i8 %a, i8 %b, i1 %cond) {
 ; CHECK-LABEL: @select_i8(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CONV0:%.*]] = sext i8 [[A:%.*]] to i32
-; CHECK-NEXT:    [[CONV1:%.*]] = sext i8 [[B:%.*]] to i32
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i32 [[CONV0]], i32 [[CONV1]]
-; CHECK-NEXT:    [[CONV4:%.*]] = trunc i32 [[SEL]] to i8
-; CHECK-NEXT:    ret i8 [[CONV4]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i8 [[A:%.*]], i8 [[B:%.*]]
+; CHECK-NEXT:    ret i8 [[SEL]]
 ;
 entry:
   %conv0 = sext i8 %a to i32
@@ -41,11 +35,10 @@
 define dso_local i16 @select_i8Ops_trunc_i16(i8 %a, i8 %b, i1 %cond) {
 ; CHECK-LABEL: @select_i8Ops_trunc_i16(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CONV0:%.*]] = sext i8 [[A:%.*]] to i32
-; CHECK-NEXT:    [[CONV1:%.*]] = sext i8 [[B:%.*]] to i32
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i32 [[CONV0]], i32 [[CONV1]]
-; CHECK-NEXT:    [[CONV4:%.*]] = trunc i32 [[SEL]] to i16
-; CHECK-NEXT:    ret i16 [[CONV4]]
+; CHECK-NEXT:    [[CONV0:%.*]] = sext i8 [[A:%.*]] to i16
+; CHECK-NEXT:    [[CONV1:%.*]] = sext i8 [[B:%.*]] to i16
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i16 [[CONV0]], i16 [[CONV1]]
+; CHECK-NEXT:    ret i16 [[SEL]]
 ;
 entry:
   %conv0 = sext i8 %a to i32
@@ -59,10 +52,8 @@
 define dso_local i16 @select_i16_const(i16 %a, i1 %cond) {
 ; CHECK-LABEL: @select_i16_const(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CONV:%.*]] = sext i16 [[A:%.*]] to i32
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i32 109, i32 [[CONV]]
-; CHECK-NEXT:    [[CONV4:%.*]] = trunc i32 [[SEL]] to i16
-; CHECK-NEXT:    ret i16 [[CONV4]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i16 109, i16 [[A:%.*]]
+; CHECK-NEXT:    ret i16 [[SEL]]
 ;
 entry:
   %conv = sext i16 %a to i32
@@ -75,10 +66,8 @@
 define dso_local i16 @select_i16_bigConst(i16 %a, i1 %cond) {
 ; CHECK-LABEL: @select_i16_bigConst(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CONV:%.*]] = sext i16 [[A:%.*]] to i32
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i32 3080196, i32 [[CONV]]
-; CHECK-NEXT:    [[CONV4:%.*]] = trunc i32 [[SEL]] to i16
-; CHECK-NEXT:    ret i16 [[CONV4]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i16 4, i16 [[A:%.*]]
+; CHECK-NEXT:    ret i16 [[SEL]]
 ;
 entry:
   %conv = sext i16 %a to i32
@@ -90,10 +79,8 @@
 define dso_local i8 @select_i8_const(i8 %a, i1 %cond) {
 ; CHECK-LABEL: @select_i8_const(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CONV:%.*]] = sext i8 [[A:%.*]] to i32
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i32 109, i32 [[CONV]]
-; CHECK-NEXT:    [[CONV4:%.*]] = trunc i32 [[SEL]] to i8
-; CHECK-NEXT:    ret i8 [[CONV4]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i8 109, i8 [[A:%.*]]
+; CHECK-NEXT:    ret i8 [[SEL]]
 ;
 entry:
   %conv = sext i8 %a to i32
@@ -106,10 +93,8 @@
 define dso_local i8 @select_i8_bigConst(i8 %a, i1 %cond) {
 ; CHECK-LABEL: @select_i8_bigConst(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CONV:%.*]] = sext i8 [[A:%.*]] to i32
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i32 20228, i32 [[CONV]]
-; CHECK-NEXT:    [[CONV4:%.*]] = trunc i32 [[SEL]] to i8
-; CHECK-NEXT:    ret i8 [[CONV4]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i8 4, i8 [[A:%.*]]
+; CHECK-NEXT:    ret i8 [[SEL]]
 ;
 entry:
   %conv = sext i8 %a to i32
@@ -121,11 +106,10 @@
 define dso_local i16 @select_sext(i8 %a, i1 %cond) {
 ; CHECK-LABEL: @select_sext(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CONV:%.*]] = sext i8 [[A:%.*]] to i32
-; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 0, [[CONV]]
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i32 [[SUB]], i32 [[CONV]]
-; CHECK-NEXT:    [[CONV4:%.*]] = trunc i32 [[SEL]] to i16
-; CHECK-NEXT:    ret i16 [[CONV4]]
+; CHECK-NEXT:    [[CONV:%.*]] = sext i8 [[A:%.*]] to i16
+; CHECK-NEXT:    [[SUB:%.*]] = sub i16 0, [[CONV]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i16 [[SUB]], i16 [[CONV]]
+; CHECK-NEXT:    ret i16 [[SEL]]
 ;
 entry:
   %conv = sext i8 %a to i32
@@ -138,11 +122,10 @@
 define dso_local i16 @select_zext(i8 %a, i1 %cond) {
 ; CHECK-LABEL: @select_zext(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[CONV:%.*]] = zext i8 [[A:%.*]] to i32
-; CHECK-NEXT:    [[SUB:%.*]] = sub nsw i32 0, [[CONV]]
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i32 [[SUB]], i32 [[CONV]]
-; CHECK-NEXT:    [[CONV4:%.*]] = trunc i32 [[SEL]] to i16
-; CHECK-NEXT:    ret i16 [[CONV4]]
+; CHECK-NEXT:    [[CONV:%.*]] = zext i8 [[A:%.*]] to i16
+; CHECK-NEXT:    [[SUB:%.*]] = sub i16 0, [[CONV]]
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], i16 [[SUB]], i16 [[CONV]]
+; CHECK-NEXT:    ret i16 [[SEL]]
 ;
 entry:
   %conv = zext i8 %a to i32