diff --git a/llvm/lib/Target/AArch64/AArch64ISelDAGToDAG.cpp b/llvm/lib/Target/AArch64/AArch64ISelDAGToDAG.cpp
--- a/llvm/lib/Target/AArch64/AArch64ISelDAGToDAG.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelDAGToDAG.cpp
@@ -2748,15 +2748,15 @@
     SDValue Add1 = ShiftAmt->getOperand(1);
     uint64_t Add0Imm;
     uint64_t Add1Imm;
-    // If we are shifting by X+/-N where N == 0 mod Size, then just shift by X
-    // to avoid the ADD/SUB.
-    if (isIntImmediate(Add1, Add1Imm) && (Add1Imm % Size == 0))
+    if (isIntImmediate(Add1, Add1Imm) && (Add1Imm % Size == 0)) {
+      // If we are shifting by X+/-N where N == 0 mod Size, then just shift by X
+      // to avoid the ADD/SUB.
       NewShiftAmt = Add0;
-    // If we are shifting by N-X where N == 0 mod Size, then just shift by -X to
-    // generate a NEG instead of a SUB of a constant.
-    else if (ShiftAmt->getOpcode() == ISD::SUB &&
-             isIntImmediate(Add0, Add0Imm) && Add0Imm != 0 &&
-             (Add0Imm % Size == 0)) {
+    } else if (ShiftAmt->getOpcode() == ISD::SUB &&
+               isIntImmediate(Add0, Add0Imm) && Add0Imm != 0 &&
+               (Add0Imm % Size == 0)) {
+      // If we are shifting by N-X where N == 0 mod Size, then just shift by -X
+      // to generate a NEG instead of a SUB from a constant.
       unsigned NegOpc;
       unsigned ZeroReg;
       EVT SubVT = ShiftAmt->getValueType(0);
@@ -2773,6 +2773,26 @@
       MachineSDNode *Neg =
           CurDAG->getMachineNode(NegOpc, DL, SubVT, Zero, Add1);
       NewShiftAmt = SDValue(Neg, 0);
+    } else if (ShiftAmt->getOpcode() == ISD::SUB &&
+               isIntImmediate(Add0, Add0Imm) && (Add0Imm % Size == Size - 1)) {
+      // If we are shifting by N-X where N == -1 mod Size, then just shift by ~X
+      // to generate a NOT instead of a SUB from a constant.
+      unsigned NotOpc;
+      unsigned ZeroReg;
+      EVT SubVT = ShiftAmt->getValueType(0);
+      if (SubVT == MVT::i32) {
+        NotOpc = AArch64::EONWrr;
+        ZeroReg = AArch64::WZR;
+      } else {
+        assert(SubVT == MVT::i64);
+        NotOpc = AArch64::EONXrr;
+        ZeroReg = AArch64::XZR;
+      }
+      SDValue Zero =
+          CurDAG->getCopyFromReg(CurDAG->getEntryNode(), DL, ZeroReg, SubVT);
+      MachineSDNode *Not =
+          CurDAG->getMachineNode(NotOpc, DL, SubVT, Add1, Zero);
+      NewShiftAmt = SDValue(Not, 0);
     } else
       return false;
   } else {
diff --git a/llvm/test/CodeGen/AArch64/shift-amount-mod.ll b/llvm/test/CodeGen/AArch64/shift-amount-mod.ll
--- a/llvm/test/CodeGen/AArch64/shift-amount-mod.ll
+++ b/llvm/test/CodeGen/AArch64/shift-amount-mod.ll
@@ -318,8 +318,7 @@
 define i32 @reg32_shl_by_complemented(i32 %val, i32 %shamt) nounwind {
 ; CHECK-LABEL: reg32_shl_by_complemented:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    mov w8, #31
-; CHECK-NEXT:    sub w8, w8, w1
+; CHECK-NEXT:    eon w8, w1, wzr
 ; CHECK-NEXT:    lsl w0, w0, w8
 ; CHECK-NEXT:    ret
   %negshamt = sub i32 31, %shamt
@@ -330,8 +329,7 @@
 ; CHECK-LABEL: load32_shl_by_complemented:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    ldr w8, [x0]
-; CHECK-NEXT:    mov w9, #31
-; CHECK-NEXT:    sub w9, w9, w1
+; CHECK-NEXT:    eon w9, w1, wzr
 ; CHECK-NEXT:    lsl w0, w8, w9
 ; CHECK-NEXT:    ret
   %val = load i32, i32* %valptr
@@ -342,8 +340,7 @@
 define void @store32_shl_by_complemented(i32 %val, i32* %dstptr, i32 %shamt) nounwind {
 ; CHECK-LABEL: store32_shl_by_complemented:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    mov w8, #31
-; CHECK-NEXT:    sub w8, w8, w2
+; CHECK-NEXT:    eon w8, w2, wzr
 ; CHECK-NEXT:    lsl w8, w0, w8
 ; CHECK-NEXT:    str w8, [x1]
 ; CHECK-NEXT:    ret
@@ -356,8 +353,7 @@
 ; CHECK-LABEL: modify32_shl_by_complemented:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    ldr w8, [x0]
-; CHECK-NEXT:    mov w9, #31
-; CHECK-NEXT:    sub w9, w9, w1
+; CHECK-NEXT:    eon w9, w1, wzr
 ; CHECK-NEXT:    lsl w8, w8, w9
 ; CHECK-NEXT:    str w8, [x0]
 ; CHECK-NEXT:    ret
@@ -371,8 +367,7 @@
 define i64 @reg64_shl_by_complemented(i64 %val, i64 %shamt) nounwind {
 ; CHECK-LABEL: reg64_shl_by_complemented:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    mov w8, #63
-; CHECK-NEXT:    sub x8, x8, x1
+; CHECK-NEXT:    eon x8, x1, xzr
 ; CHECK-NEXT:    lsl x0, x0, x8
 ; CHECK-NEXT:    ret
   %negshamt = sub i64 63, %shamt
@@ -383,8 +378,7 @@
 ; CHECK-LABEL: load64_shl_by_complemented:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    ldr x8, [x0]
-; CHECK-NEXT:    mov w9, #63
-; CHECK-NEXT:    sub x9, x9, x1
+; CHECK-NEXT:    eon x9, x1, xzr
 ; CHECK-NEXT:    lsl x0, x8, x9
 ; CHECK-NEXT:    ret
   %val = load i64, i64* %valptr
@@ -395,8 +389,7 @@
 define void @store64_shl_by_complemented(i64 %val, i64* %dstptr, i64 %shamt) nounwind {
 ; CHECK-LABEL: store64_shl_by_complemented:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    mov w8, #63
-; CHECK-NEXT:    sub x8, x8, x2
+; CHECK-NEXT:    eon x8, x2, xzr
 ; CHECK-NEXT:    lsl x8, x0, x8
 ; CHECK-NEXT:    str x8, [x1]
 ; CHECK-NEXT:    ret
@@ -409,8 +402,7 @@
 ; CHECK-LABEL: modify64_shl_by_complemented:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    ldr x8, [x0]
-; CHECK-NEXT:    mov w9, #63
-; CHECK-NEXT:    sub x9, x9, x1
+; CHECK-NEXT:    eon x9, x1, xzr
 ; CHECK-NEXT:    lsl x8, x8, x9
 ; CHECK-NEXT:    str x8, [x0]
 ; CHECK-NEXT:    ret
@@ -427,8 +419,7 @@
 define i32 @reg32_lshr_by_complemented(i32 %val, i32 %shamt) nounwind {
 ; CHECK-LABEL: reg32_lshr_by_complemented:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    mov w8, #31
-; CHECK-NEXT:    sub w8, w8, w1
+; CHECK-NEXT:    eon w8, w1, wzr
 ; CHECK-NEXT:    lsr w0, w0, w8
 ; CHECK-NEXT:    ret
   %negshamt = sub i32 31, %shamt
@@ -439,8 +430,7 @@
 ; CHECK-LABEL: load32_lshr_by_complemented:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    ldr w8, [x0]
-; CHECK-NEXT:    mov w9, #31
-; CHECK-NEXT:    sub w9, w9, w1
+; CHECK-NEXT:    eon w9, w1, wzr
 ; CHECK-NEXT:    lsr w0, w8, w9
 ; CHECK-NEXT:    ret
   %val = load i32, i32* %valptr
@@ -451,8 +441,7 @@
 define void @store32_lshr_by_complemented(i32 %val, i32* %dstptr, i32 %shamt) nounwind {
 ; CHECK-LABEL: store32_lshr_by_complemented:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    mov w8, #31
-; CHECK-NEXT:    sub w8, w8, w2
+; CHECK-NEXT:    eon w8, w2, wzr
 ; CHECK-NEXT:    lsr w8, w0, w8
 ; CHECK-NEXT:    str w8, [x1]
 ; CHECK-NEXT:    ret
@@ -465,8 +454,7 @@
 ; CHECK-LABEL: modify32_lshr_by_complemented:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    ldr w8, [x0]
-; CHECK-NEXT:    mov w9, #31
-; CHECK-NEXT:    sub w9, w9, w1
+; CHECK-NEXT:    eon w9, w1, wzr
 ; CHECK-NEXT:    lsr w8, w8, w9
 ; CHECK-NEXT:    str w8, [x0]
 ; CHECK-NEXT:    ret
@@ -480,8 +468,7 @@
 define i64 @reg64_lshr_by_complemented(i64 %val, i64 %shamt) nounwind {
 ; CHECK-LABEL: reg64_lshr_by_complemented:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    mov w8, #63
-; CHECK-NEXT:    sub x8, x8, x1
+; CHECK-NEXT:    eon x8, x1, xzr
 ; CHECK-NEXT:    lsr x0, x0, x8
 ; CHECK-NEXT:    ret
   %negshamt = sub i64 63, %shamt
@@ -492,8 +479,7 @@
 ; CHECK-LABEL: load64_lshr_by_complemented:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    ldr x8, [x0]
-; CHECK-NEXT:    mov w9, #63
-; CHECK-NEXT:    sub x9, x9, x1
+; CHECK-NEXT:    eon x9, x1, xzr
 ; CHECK-NEXT:    lsr x0, x8, x9
 ; CHECK-NEXT:    ret
   %val = load i64, i64* %valptr
@@ -504,8 +490,7 @@
 define void @store64_lshr_by_complemented(i64 %val, i64* %dstptr, i64 %shamt) nounwind {
 ; CHECK-LABEL: store64_lshr_by_complemented:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    mov w8, #63
-; CHECK-NEXT:    sub x8, x8, x2
+; CHECK-NEXT:    eon x8, x2, xzr
 ; CHECK-NEXT:    lsr x8, x0, x8
 ; CHECK-NEXT:    str x8, [x1]
 ; CHECK-NEXT:    ret
@@ -518,8 +503,7 @@
 ; CHECK-LABEL: modify64_lshr_by_complemented:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    ldr x8, [x0]
-; CHECK-NEXT:    mov w9, #63
-; CHECK-NEXT:    sub x9, x9, x1
+; CHECK-NEXT:    eon x9, x1, xzr
 ; CHECK-NEXT:    lsr x8, x8, x9
 ; CHECK-NEXT:    str x8, [x0]
 ; CHECK-NEXT:    ret
@@ -536,8 +520,7 @@
 define i32 @reg32_ashr_by_complemented(i32 %val, i32 %shamt) nounwind {
 ; CHECK-LABEL: reg32_ashr_by_complemented:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    mov w8, #31
-; CHECK-NEXT:    sub w8, w8, w1
+; CHECK-NEXT:    eon w8, w1, wzr
 ; CHECK-NEXT:    asr w0, w0, w8
 ; CHECK-NEXT:    ret
   %negshamt = sub i32 31, %shamt
@@ -548,8 +531,7 @@
 ; CHECK-LABEL: load32_ashr_by_complemented:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    ldr w8, [x0]
-; CHECK-NEXT:    mov w9, #31
-; CHECK-NEXT:    sub w9, w9, w1
+; CHECK-NEXT:    eon w9, w1, wzr
 ; CHECK-NEXT:    asr w0, w8, w9
 ; CHECK-NEXT:    ret
   %val = load i32, i32* %valptr
@@ -560,8 +542,7 @@
 define void @store32_ashr_by_complemented(i32 %val, i32* %dstptr, i32 %shamt) nounwind {
 ; CHECK-LABEL: store32_ashr_by_complemented:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    mov w8, #31
-; CHECK-NEXT:    sub w8, w8, w2
+; CHECK-NEXT:    eon w8, w2, wzr
 ; CHECK-NEXT:    asr w8, w0, w8
 ; CHECK-NEXT:    str w8, [x1]
 ; CHECK-NEXT:    ret
@@ -574,8 +555,7 @@
 ; CHECK-LABEL: modify32_ashr_by_complemented:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    ldr w8, [x0]
-; CHECK-NEXT:    mov w9, #31
-; CHECK-NEXT:    sub w9, w9, w1
+; CHECK-NEXT:    eon w9, w1, wzr
 ; CHECK-NEXT:    asr w8, w8, w9
 ; CHECK-NEXT:    str w8, [x0]
 ; CHECK-NEXT:    ret
@@ -589,8 +569,7 @@
 define i64 @reg64_ashr_by_complemented(i64 %val, i64 %shamt) nounwind {
 ; CHECK-LABEL: reg64_ashr_by_complemented:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    mov w8, #63
-; CHECK-NEXT:    sub x8, x8, x1
+; CHECK-NEXT:    eon x8, x1, xzr
 ; CHECK-NEXT:    asr x0, x0, x8
 ; CHECK-NEXT:    ret
   %negshamt = sub i64 63, %shamt
@@ -601,8 +580,7 @@
 ; CHECK-LABEL: load64_ashr_by_complemented:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    ldr x8, [x0]
-; CHECK-NEXT:    mov w9, #63
-; CHECK-NEXT:    sub x9, x9, x1
+; CHECK-NEXT:    eon x9, x1, xzr
 ; CHECK-NEXT:    asr x0, x8, x9
 ; CHECK-NEXT:    ret
   %val = load i64, i64* %valptr
@@ -613,8 +591,7 @@
 define void @store64_ashr_by_complemented(i64 %val, i64* %dstptr, i64 %shamt) nounwind {
 ; CHECK-LABEL: store64_ashr_by_complemented:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    mov w8, #63
-; CHECK-NEXT:    sub x8, x8, x2
+; CHECK-NEXT:    eon x8, x2, xzr
 ; CHECK-NEXT:    asr x8, x0, x8
 ; CHECK-NEXT:    str x8, [x1]
 ; CHECK-NEXT:    ret
@@ -627,8 +604,7 @@
 ; CHECK-LABEL: modify64_ashr_by_complemented:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    ldr x8, [x0]
-; CHECK-NEXT:    mov w9, #63
-; CHECK-NEXT:    sub x9, x9, x1
+; CHECK-NEXT:    eon x9, x1, xzr
 ; CHECK-NEXT:    asr x8, x8, x9
 ; CHECK-NEXT:    str x8, [x0]
 ; CHECK-NEXT:    ret