Index: llvm/trunk/lib/Analysis/InstructionSimplify.cpp
===================================================================
--- llvm/trunk/lib/Analysis/InstructionSimplify.cpp
+++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp
@@ -561,6 +561,10 @@
       match(Op0, m_Xor(m_Value(Y), m_SignMask())))
     return Y;
 
+  // add nuw %x, -1  ->  -1, because %x can only be 0.
+  if (isNUW && match(Op1, m_AllOnes()))
+    return Op1; // Which is -1.
+
   /// i1 add -> xor.
   if (MaxRecurse && Op0->getType()->isIntOrIntVectorTy(1))
     if (Value *V = SimplifyXorInst(Op0, Op1, Q, MaxRecurse-1))
Index: llvm/trunk/test/Transforms/InstCombine/set-lowbits-mask-canonicalize.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/set-lowbits-mask-canonicalize.ll
+++ llvm/trunk/test/Transforms/InstCombine/set-lowbits-mask-canonicalize.ll
@@ -39,7 +39,7 @@
 
 define i32 @shl_add_nuw(i32 %NBits) {
 ; CHECK-LABEL: @shl_add_nuw(
-; CHECK-NEXT:    ret i32 0
+; CHECK-NEXT:    ret i32 -1
 ;
   %setbit = shl i32 1, %NBits
   %ret = add nuw i32 %setbit, -1
@@ -48,7 +48,7 @@
 
 define i32 @shl_add_nsw_nuw(i32 %NBits) {
 ; CHECK-LABEL: @shl_add_nsw_nuw(
-; CHECK-NEXT:    ret i32 0
+; CHECK-NEXT:    ret i32 -1
 ;
   %setbit = shl i32 1, %NBits
   %ret = add nuw nsw i32 %setbit, -1
@@ -81,7 +81,7 @@
 
 define i32 @shl_nsw_add_nuw(i32 %NBits) {
 ; CHECK-LABEL: @shl_nsw_add_nuw(
-; CHECK-NEXT:    ret i32 0
+; CHECK-NEXT:    ret i32 -1
 ;
   %setbit = shl nsw i32 1, %NBits
   %ret = add nuw i32 %setbit, -1
@@ -90,7 +90,7 @@
 
 define i32 @shl_nsw_add_nsw_nuw(i32 %NBits) {
 ; CHECK-LABEL: @shl_nsw_add_nsw_nuw(
-; CHECK-NEXT:    ret i32 0
+; CHECK-NEXT:    ret i32 -1
 ;
   %setbit = shl nsw i32 1, %NBits
   %ret = add nuw nsw i32 %setbit, -1
@@ -123,7 +123,7 @@
 
 define i32 @shl_nuw_add_nuw(i32 %NBits) {
 ; CHECK-LABEL: @shl_nuw_add_nuw(
-; CHECK-NEXT:    ret i32 0
+; CHECK-NEXT:    ret i32 -1
 ;
   %setbit = shl nuw i32 1, %NBits
   %ret = add nuw i32 %setbit, -1
@@ -132,7 +132,7 @@
 
 define i32 @shl_nuw_add_nsw_nuw(i32 %NBits) {
 ; CHECK-LABEL: @shl_nuw_add_nsw_nuw(
-; CHECK-NEXT:    ret i32 0
+; CHECK-NEXT:    ret i32 -1
 ;
   %setbit = shl nuw i32 1, %NBits
   %ret = add nuw nsw i32 %setbit, -1
@@ -165,7 +165,7 @@
 
 define i32 @shl_nsw_nuw_add_nuw(i32 %NBits) {
 ; CHECK-LABEL: @shl_nsw_nuw_add_nuw(
-; CHECK-NEXT:    ret i32 0
+; CHECK-NEXT:    ret i32 -1
 ;
   %setbit = shl nuw nsw i32 1, %NBits
   %ret = add nuw i32 %setbit, -1
@@ -174,7 +174,7 @@
 
 define i32 @shl_nsw_nuw_add_nsw_nuw(i32 %NBits) {
 ; CHECK-LABEL: @shl_nsw_nuw_add_nsw_nuw(
-; CHECK-NEXT:    ret i32 0
+; CHECK-NEXT:    ret i32 -1
 ;
   %setbit = shl nuw nsw i32 1, %NBits
   %ret = add nuw nsw i32 %setbit, -1
Index: llvm/trunk/test/Transforms/InstSimplify/constantfold-add-nuw-allones-to-allones.ll
===================================================================
--- llvm/trunk/test/Transforms/InstSimplify/constantfold-add-nuw-allones-to-allones.ll
+++ llvm/trunk/test/Transforms/InstSimplify/constantfold-add-nuw-allones-to-allones.ll
@@ -7,8 +7,7 @@
 
 define i8 @add_nuw (i8 %x) {
 ; CHECK-LABEL: @add_nuw(
-; CHECK-NEXT:    [[RET:%.*]] = add nuw i8 [[X:%.*]], -1
-; CHECK-NEXT:    ret i8 [[RET]]
+; CHECK-NEXT:    ret i8 -1
 ;
   %ret = add nuw i8 %x, -1
   ; nuw here means that %x can only be 0
@@ -17,8 +16,7 @@
 
 define i8 @add_nuw_nsw (i8 %x) {
 ; CHECK-LABEL: @add_nuw_nsw(
-; CHECK-NEXT:    [[RET:%.*]] = add nuw nsw i8 [[X:%.*]], -1
-; CHECK-NEXT:    ret i8 [[RET]]
+; CHECK-NEXT:    ret i8 -1
 ;
   %ret = add nuw nsw i8 %x, -1
   ; nuw here means that %x can only be 0
@@ -27,8 +25,7 @@
 
 define i8 @add_nuw_commute (i8 %x) {
 ; CHECK-LABEL: @add_nuw_commute(
-; CHECK-NEXT:    [[RET:%.*]] = add nuw i8 -1, [[X:%.*]]
-; CHECK-NEXT:    ret i8 [[RET]]
+; CHECK-NEXT:    ret i8 -1
 ;
   %ret = add nuw i8 -1, %x ; swapped
   ; nuw here means that %x can only be 0
@@ -60,8 +57,7 @@
 
 define <2 x i8> @add_vec(<2 x i8> %x) {
 ; CHECK-LABEL: @add_vec(
-; CHECK-NEXT:    [[RET:%.*]] = add nuw <2 x i8> [[X:%.*]], <i8 -1, i8 -1>
-; CHECK-NEXT:    ret <2 x i8> [[RET]]
+; CHECK-NEXT:    ret <2 x i8> <i8 -1, i8 -1>
 ;
   %ret = add nuw <2 x i8> %x, <i8 -1, i8 -1>
   ret <2 x i8> %ret
@@ -69,8 +65,7 @@
 
 define <3 x i8> @add_vec_undef(<3 x i8> %x) {
 ; CHECK-LABEL: @add_vec_undef(
-; CHECK-NEXT:    [[RET:%.*]] = add nuw <3 x i8> [[X:%.*]], <i8 -1, i8 undef, i8 -1>
-; CHECK-NEXT:    ret <3 x i8> [[RET]]
+; CHECK-NEXT:    ret <3 x i8> <i8 -1, i8 undef, i8 -1>
 ;
   %ret = add nuw <3 x i8> %x, <i8 -1, i8 undef, i8 -1>
   ret <3 x i8> %ret