Index: llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
===================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -834,6 +834,12 @@
 
   // Compute the KnownZero/KnownOne bits to simplify things downstream.
   computeKnownBits(I, KnownZero, KnownOne, Depth, CxtI);
+
+  // If this user is only demanding bits that we know, return the known
+  // constant.
+  if ((DemandedMask & (KnownZero|KnownOne)) == DemandedMask)
+    return Constant::getIntegerValue(ITy, KnownOne);
+
   return nullptr;
 }
 
Index: llvm/trunk/test/Transforms/InstCombine/and2.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/and2.ll
+++ llvm/trunk/test/Transforms/InstCombine/and2.ll
@@ -171,7 +171,7 @@
 define i32 @test14(i32 %a, i32 %b) {
 ; CHECK-LABEL: @test14(
 ; CHECK-NEXT:    [[X:%.*]] = shl i32 [[A:%.*]], 8
-; CHECK-NEXT:    [[Y:%.*]] = sub i32 [[X]], [[B:%.*]]
+; CHECK-NEXT:    [[Y:%.*]] = sub i32 0, [[B:%.*]]
 ; CHECK-NEXT:    [[Z:%.*]] = and i32 [[Y]], 128
 ; CHECK-NEXT:    [[W:%.*]] = mul i32 [[Z]], [[X]]
 ; CHECK-NEXT:    ret i32 [[W]]