Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2135,7 +2135,8 @@
 
 Instruction *InstCombiner::visitSwitchInst(SwitchInst &SI) {
   Value *Cond = SI.getCondition();
-  unsigned BitWidth = cast<IntegerType>(Cond->getType())->getBitWidth();
+  Type *Ty = Cond->getType();
+  unsigned BitWidth = cast<IntegerType>(Ty)->getBitWidth();
   APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
   computeKnownBits(Cond, KnownZero, KnownOne, 0, &SI);
   unsigned LeadingKnownZeros = KnownZero.countLeadingOnes();
@@ -2151,22 +2152,21 @@
 
   unsigned NewWidth = BitWidth - std::max(LeadingKnownZeros, LeadingKnownOnes);
 
-  // Truncate the condition operand if the new type is equal to or larger than
-  // the largest legal integer type. We need to be conservative here since
-  // x86 generates redundant zero-extension instructions if the operand is
-  // truncated to i8 or i16.
+  // Truncate the condition operand if the new type is better (smaller/legal)
+  // than the old type.
   bool TruncCond = false;
-  if (NewWidth > 0 && BitWidth > NewWidth &&
-      NewWidth >= DL.getLargestLegalIntTypeSize()) {
-    TruncCond = true;
-    IntegerType *Ty = IntegerType::get(SI.getContext(), NewWidth);
-    Builder->SetInsertPoint(&SI);
-    Value *NewCond = Builder->CreateTrunc(SI.getCondition(), Ty, "trunc");
-    SI.setCondition(NewCond);
-
-    for (auto &C : SI.cases())
-      static_cast<SwitchInst::CaseIt *>(&C)->setValue(ConstantInt::get(
-          SI.getContext(), C.getCaseValue()->getValue().trunc(NewWidth)));
+  if (NewWidth > 0 && BitWidth > NewWidth) {
+    Type *NewType = Type::getIntNTy(SI.getContext(), NewWidth);
+    if (ShouldChangeType(Ty, NewType)) {
+      TruncCond = true;
+      Builder->SetInsertPoint(&SI);
+      Value *NewCond = Builder->CreateTrunc(Cond, NewType, "trunc");
+      SI.setCondition(NewCond);
+
+      for (auto &C : SI.cases())
+        static_cast<SwitchInst::CaseIt *>(&C)->setValue(ConstantInt::get(
+            SI.getContext(), C.getCaseValue()->getValue().trunc(NewWidth)));
+    }
   }
 
   if (Instruction *I = dyn_cast<Instruction>(Cond)) {
Index: test/Transforms/InstCombine/narrow-switch.ll
===================================================================
--- test/Transforms/InstCombine/narrow-switch.ll
+++ test/Transforms/InstCombine/narrow-switch.ll
@@ -1,6 +1,7 @@
-; RUN: opt < %s -instcombine -S | FileCheck %s
+; Vary legal integer types in data layout.
 
-target datalayout = "e-m:o-p:32:32-f64:32:64-v64:32:64-v128:32:128-a:0:32-n32-S32"
+; RUN: opt < %s -instcombine -S -default-data-layout=n32 | FileCheck %s
+; RUN: opt < %s -instcombine -S -default-data-layout=n32:64 | FileCheck %s --check-prefix=CHECK64
 
 ; CHECK-LABEL: define i32 @positive1
 ; CHECK: switch i32
@@ -8,6 +9,14 @@
 ; CHECK: i32 100, label
 ; CHECK: i32 1001, label
 
+; It doesn't matter if 64-bit integers are legal; 32-bit integers are legal too, so shrink.
+;
+; CHECK64-LABEL: @positive1
+; CHECK64: switch i32
+; CHECK64: i32 10, label
+; CHECK64: i32 100, label
+; CHECK64: i32 1001, label
+
 define i32 @positive1(i64 %a) {
 entry:
   %and = and i64 %a, 4294967295
@@ -96,11 +105,20 @@
 ; truncation to generate the sub constant expressions that leads
 ; to the recomputed condition.
 ;
+; If both 64-bit and 59-bit integers are illegal, it's ok to shrink.
+;
 ; CHECK-LABEL: @trunc64to59
 ; CHECK: switch i59
 ; CHECK: i59 0, label
 ; CHECK: i59 18717182647723699, label
 
+; If 64-bit integers are legal, we should not shrink to an illegal type.
+;
+; CHECK64-LABEL: @trunc64to59
+; CHECK64: switch i64
+; CHECK64: i64 0, label
+; CHECK64: i64 18717182647723699, label
+
 define void @trunc64to59(i64 %a) {
 entry:
   %tmp0 = and i64 %a, 15
@@ -121,3 +139,4 @@
 sw.default:
   ret void
 }
+
Index: test/Transforms/InstCombine/pr21651.ll
===================================================================
--- test/Transforms/InstCombine/pr21651.ll
+++ test/Transforms/InstCombine/pr21651.ll
@@ -15,9 +15,9 @@
 }
 
 ; CHECK-LABEL: define void @PR21651(
-; CHECK:   switch i2 0, label %out [
-; CHECK:     i2 0, label %out
-; CHECK:     i2 1, label %out
+; CHECK:   switch i1 false, label %out [
+; CHECK:     i1 false, label %out
+; CHECK:     i1 true, label %out
 ; CHECK:   ]
 ; CHECK: out:                                              ; preds = %0, %0, %0
 ; CHECK:   ret void