Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -6201,13 +6201,21 @@
       }
     }
 
-    // sext(setcc x, y, cc) -> (select (setcc x, y, cc), -1, 0)
+    // sext(setcc x, y, cc) -> (select (setcc x, y, cc), T, 0)
+    // Here, T can be 1 or -1, depending on the type of the setcc and
+    // getBooleanContents().
     unsigned ElementWidth = VT.getScalarType().getSizeInBits();
+    unsigned SetCCWidth = N0.getValueType().getScalarType().getSizeInBits();
+
     SDLoc DL(N);
-    SDValue NegOne =
-      DAG.getConstant(APInt::getAllOnesValue(ElementWidth), DL, VT);
+    APInt TrueInt =
+        ((SetCCWidth != 1) && (TLI.getBooleanContents(VT) ==
+                               TargetLowering::ZeroOrOneBooleanContent))
+            ? APInt(ElementWidth, 1)
+            : APInt::getAllOnesValue(ElementWidth);
+    SDValue TrueVal = DAG.getConstant(TrueInt, DL, VT);
     if (SDValue SCC = SimplifySelectCC(
-            DL, N0.getOperand(0), N0.getOperand(1), NegOne,
+            DL, N0.getOperand(0), N0.getOperand(1), TrueVal,
             DAG.getConstant(0, DL, VT),
             cast<CondCodeSDNode>(N0.getOperand(2))->get(), true))
       return SCC;
@@ -6221,7 +6229,7 @@
         SDValue SetCC = DAG.getSetCC(DL, SetCCVT,
                                      N0.getOperand(0), N0.getOperand(1), CC);
         return DAG.getSelect(DL, VT, SetCC,
-                             NegOne, DAG.getConstant(0, DL, VT));
+                             TrueVal, DAG.getConstant(0, DL, VT));
       }
     }
   }
Index: test/CodeGen/X86/pr28504.ll
===================================================================
--- test/CodeGen/X86/pr28504.ll
+++ test/CodeGen/X86/pr28504.ll
@@ -0,0 +1,45 @@
+; RUN: llc < %s | FileCheck %s
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-LABEL: main:
+; CHECK: movb  $1, %al
+; CHECK: xorl  %ecx, %ecx
+; CHECK: cmpb  $1, %al
+; CHECK: sete  %cl
+define i32 @main() {
+bb:
+  %tmp1 = alloca i8, align 1
+  %tmp2 = alloca i32, align 4
+  %tmp3 = alloca i16, align 2
+  store i8 1, i8* %tmp1, align 1
+  %tmp4 = load i8, i8* %tmp1, align 1
+  %tmp5 = sext i8 %tmp4 to i32
+  %tmp6 = load i8, i8* %tmp1, align 1
+  %tmp7 = zext i8 %tmp6 to i32         
+  %tmp8 = sub nsw i32 %tmp5, %tmp7     
+  %tmp9 = load i8, i8* %tmp1, align 1
+  %tmp10 = sext i8 %tmp9 to i32        
+  %tmp11 = icmp eq i32 %tmp10, 1       
+  %tmp12 = zext i1 %tmp11 to i32       
+  %tmp13 = add nsw i32 %tmp8, %tmp12   
+  %tmp14 = trunc i32 %tmp13 to i8      
+  %tmp15 = sext i8 %tmp14 to i16
+  %tmp16 = sext i16 %tmp15 to i32      
+  store i32 %tmp16, i32* %tmp2, align 4  
+  %tmp17 = load i8, i8* %tmp1, align 1   
+  %tmp18 = sext i8 %tmp17 to i16
+  store i16 %tmp18, i16* %tmp3, align 2
+  %tmp19 = load i32, i32* %tmp2, align 4
+  %tmp20 = icmp ne i32 %tmp19, 1
+  br i1 %tmp20, label %bb21, label %bb22
+
+bb21:                                             ; preds = %bb
+  call void @abort()
+  unreachable
+
+bb22:                                             ; preds = %bb
+  ret i32 0
+}
+
+declare void @abort()