Index: llvm/trunk/lib/Target/AArch64/AArch64InstructionSelector.cpp
===================================================================
--- llvm/trunk/lib/Target/AArch64/AArch64InstructionSelector.cpp
+++ llvm/trunk/lib/Target/AArch64/AArch64InstructionSelector.cpp
@@ -96,6 +96,8 @@
   bool selectConcatVectors(MachineInstr &I, MachineRegisterInfo &MRI) const;
   bool selectSplitVectorUnmerge(MachineInstr &I,
                                 MachineRegisterInfo &MRI) const;
+  bool selectIntrinsicWithSideEffects(MachineInstr &I,
+                                      MachineRegisterInfo &MRI) const;
 
   unsigned emitConstantPoolEntry(Constant *CPVal, MachineFunction &MF) const;
   MachineInstr *emitLoadFromConstantPool(Constant *CPVal,
@@ -1735,14 +1737,7 @@
     return STI.isTargetDarwin() ? selectVaStartDarwin(I, MF, MRI)
                                 : selectVaStartAAPCS(I, MF, MRI);
   case TargetOpcode::G_INTRINSIC_W_SIDE_EFFECTS:
-    if (!I.getOperand(0).isIntrinsicID())
-      return false;
-    if (I.getOperand(0).getIntrinsicID() != Intrinsic::trap)
-      return false;
-    BuildMI(MBB, I, I.getDebugLoc(), TII.get(AArch64::BRK))
-      .addImm(1);
-    I.eraseFromParent();
-    return true;
+    return selectIntrinsicWithSideEffects(I, MRI);
   case TargetOpcode::G_IMPLICIT_DEF: {
     I.setDesc(TII.get(TargetOpcode::IMPLICIT_DEF));
     const LLT DstTy = MRI.getType(I.getOperand(0).getReg());
@@ -2705,6 +2700,71 @@
   return true;
 }
 
+/// Helper function to emit the correct opcode for a llvm.aarch64.stlxr
+/// intrinsic.
+static unsigned getStlxrOpcode(unsigned NumBytesToStore) {
+  switch (NumBytesToStore) {
+  // TODO: 1, 2, and 4 byte stores.
+  case 8:
+    return AArch64::STLXRX;
+  default:
+    LLVM_DEBUG(dbgs() << "Unexpected number of bytes to store! ("
+                      << NumBytesToStore << ")\n");
+    break;
+  }
+  return 0;
+}
+
+bool AArch64InstructionSelector::selectIntrinsicWithSideEffects(
+    MachineInstr &I, MachineRegisterInfo &MRI) const {
+  // Find the intrinsic ID.
+  auto IntrinOp = find_if(I.operands(), [&](const MachineOperand &Op) {
+    return Op.isIntrinsicID();
+  });
+  if (IntrinOp == I.operands_end())
+    return false;
+  unsigned IntrinID = IntrinOp->getIntrinsicID();
+  MachineIRBuilder MIRBuilder(I);
+
+  // Select the instruction.
+  switch (IntrinID) {
+  default:
+    return false;
+  case Intrinsic::trap:
+    MIRBuilder.buildInstr(AArch64::BRK, {}, {}).addImm(1);
+    break;
+  case Intrinsic::aarch64_stlxr:
+    unsigned StatReg = I.getOperand(0).getReg();
+    assert(RBI.getSizeInBits(StatReg, MRI, TRI) == 32 &&
+           "Status register must be 32 bits!");
+    unsigned SrcReg = I.getOperand(2).getReg();
+
+    if (RBI.getSizeInBits(SrcReg, MRI, TRI) != 64) {
+      LLVM_DEBUG(dbgs() << "Only support 64-bit sources right now.\n");
+      return false;
+    }
+
+    unsigned PtrReg = I.getOperand(3).getReg();
+    assert(MRI.getType(PtrReg).isPointer() && "Expected pointer operand");
+
+    // Expect only one memory operand.
+    if (!I.hasOneMemOperand())
+      return false;
+
+    const MachineMemOperand *MemOp = *I.memoperands_begin();
+    unsigned NumBytesToStore = MemOp->getSize();
+    unsigned Opc = getStlxrOpcode(NumBytesToStore);
+    if (!Opc)
+      return false;
+
+    auto StoreMI = MIRBuilder.buildInstr(Opc, {StatReg}, {SrcReg, PtrReg});
+    constrainSelectedInstRegOperands(*StoreMI, TII, TRI, RBI);
+  }
+
+  I.eraseFromParent();
+  return true;
+}
+
 /// SelectArithImmed - Select an immediate value that can be represented as
 /// a 12-bit value shifted left by either 0 or 12.  If so, return true with
 /// Val set to the 12-bit value and Shift set to the shifter operand.
Index: llvm/trunk/test/CodeGen/AArch64/GlobalISel/select-stlxr-intrin.mir
===================================================================
--- llvm/trunk/test/CodeGen/AArch64/GlobalISel/select-stlxr-intrin.mir
+++ llvm/trunk/test/CodeGen/AArch64/GlobalISel/select-stlxr-intrin.mir
@@ -0,0 +1,32 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=aarch64-unknown-unknown -run-pass=instruction-select -verify-machineinstrs %s -o - | FileCheck %s
+
+--- |
+  define i32 @test_store_release_i64(i32 %a, i64* %addr) {
+    ret i32 %a
+  }
+...
+---
+name:            test_store_release_i64
+alignment:       2
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $w0, $x1, $x2
+
+    ; CHECK-LABEL: name: test_store_release_i64
+    ; CHECK: liveins: $w0, $x1, $x2
+    ; CHECK: [[COPY:%[0-9]+]]:gpr64 = COPY $x1
+    ; CHECK: [[COPY1:%[0-9]+]]:gpr64sp = COPY $x2
+    ; CHECK: early-clobber %2:gpr32 = STLXRX [[COPY]], [[COPY1]]
+    ; CHECK: $w0 = COPY %2
+    ; CHECK: RET_ReallyLR implicit $w0
+    %1:gpr(s64) = COPY $x1
+    %2:gpr(p0) = COPY $x2
+    %3:gpr(s32) = G_INTRINSIC_W_SIDE_EFFECTS intrinsic(@llvm.aarch64.stlxr), %1(s64), %2(p0) :: (volatile store 8 into %ir.addr)
+    $w0 = COPY %3(s32)
+    RET_ReallyLR implicit $w0
+
+...
Index: llvm/trunk/test/CodeGen/AArch64/arm64-ldxr-stxr.ll
===================================================================
--- llvm/trunk/test/CodeGen/AArch64/arm64-ldxr-stxr.ll
+++ llvm/trunk/test/CodeGen/AArch64/arm64-ldxr-stxr.ll
@@ -1,4 +1,5 @@
 ; RUN: llc < %s -mtriple=arm64-linux-gnu | FileCheck %s
+; RUN: llc < %s -global-isel -global-isel-abort=2 -pass-remarks-missed=gisel* -mtriple=arm64-linux-gnu 2>&1 | FileCheck %s --check-prefixes=GISEL,FALLBACK
 
 %0 = type { i64, i64 }
 
@@ -257,9 +258,12 @@
   ret i32 %res
 }
 
+; FALLBACK-NOT: remark:{{.*}}test_store_release_i64
 define i32 @test_store_release_i64(i32, i64 %val, i64* %addr) {
 ; CHECK-LABEL: test_store_release_i64:
 ; CHECK: stlxr w0, x1, [x2]
+; GISEL-LABEL: test_store_release_i64:
+; GISEL: stlxr w0, x1, [x2]
   %res = call i32 @llvm.aarch64.stlxr.p0i64(i64 %val, i64* %addr)
   ret i32 %res
 }