Index: llvm/lib/Target/AArch64/AArch64InstrAtomics.td
===================================================================
--- llvm/lib/Target/AArch64/AArch64InstrAtomics.td
+++ llvm/lib/Target/AArch64/AArch64InstrAtomics.td
@@ -271,22 +271,46 @@
 def stxr_1 : PatFrag<(ops node:$val, node:$ptr),
                      (int_aarch64_stxr node:$val, node:$ptr), [{
   return cast<MemIntrinsicSDNode>(N)->getMemoryVT() == MVT::i8;
-}]>;
+}]> {
+  let GISelPredicateCode = [{
+    if (!MI.hasOneMemOperand())
+      return false;
+    return (*MI.memoperands_begin())->getSize() == 1;
+  }];
+}
 
 def stxr_2 : PatFrag<(ops node:$val, node:$ptr),
                      (int_aarch64_stxr node:$val, node:$ptr), [{
   return cast<MemIntrinsicSDNode>(N)->getMemoryVT() == MVT::i16;
-}]>;
+}]> {
+  let GISelPredicateCode = [{
+    if (!MI.hasOneMemOperand())
+      return false;
+    return (*MI.memoperands_begin())->getSize() == 2;
+  }];
+}
 
 def stxr_4 : PatFrag<(ops node:$val, node:$ptr),
                      (int_aarch64_stxr node:$val, node:$ptr), [{
   return cast<MemIntrinsicSDNode>(N)->getMemoryVT() == MVT::i32;
-}]>;
+}]> {
+  let GISelPredicateCode = [{
+    if (!MI.hasOneMemOperand())
+      return false;
+    return (*MI.memoperands_begin())->getSize() == 4;
+  }];
+}
 
 def stxr_8 : PatFrag<(ops node:$val, node:$ptr),
                      (int_aarch64_stxr node:$val, node:$ptr), [{
   return cast<MemIntrinsicSDNode>(N)->getMemoryVT() == MVT::i64;
-}]>;
+}]> {
+  let GISelPredicateCode = [{
+    if (!MI.hasOneMemOperand())
+      return false;
+    return (*MI.memoperands_begin())->getSize() == 8;
+  }];
+}
 
 
 def : Pat<(stxr_1 GPR64:$val, GPR64sp:$addr),
Index: llvm/test/CodeGen/AArch64/GlobalISel/select-stx.mir
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/AArch64/GlobalISel/select-stx.mir
@@ -0,0 +1,120 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=aarch64-- -run-pass=instruction-select -verify-machineinstrs %s -o - | FileCheck %s
+
+--- |
+  define void @test_store_i8(i32, i8 %val, i8* %addr) { ret void }
+  define void @test_store_i16(i32, i16 %val, i16* %addr) { ret void }
+  define void @test_store_i32(i32, i32 %val, i32* %addr) { ret void }
+  define void @test_store_i64(i32, i64 %val, i64* %addr) { ret void }
+...
+---
+name:            test_store_i8
+alignment:       2
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+machineFunctionInfo: {}
+body:             |
+  bb.0:
+    liveins: $w0, $w1, $x2
+
+    ; CHECK-LABEL: name: test_store_i8
+    ; CHECK: liveins: $w0, $w1, $x2
+    ; CHECK: [[COPY:%[0-9]+]]:gpr32all = COPY $w1
+    ; CHECK: [[COPY1:%[0-9]+]]:gpr64sp = COPY $x2
+    ; CHECK: [[SUBREG_TO_REG:%[0-9]+]]:gpr64all = SUBREG_TO_REG 0, [[COPY]], %subreg.sub_32
+    ; CHECK: [[COPY2:%[0-9]+]]:gpr32 = COPY [[SUBREG_TO_REG]].sub_32
+    ; CHECK: early-clobber %5:gpr32 = STXRB [[COPY2]], [[COPY1]] :: (volatile store 1 into %ir.addr)
+    ; CHECK: $w0 = COPY %5
+    ; CHECK: RET_ReallyLR implicit $w0
+    %3:gpr(s32) = COPY $w1
+    %2:gpr(p0) = COPY $x2
+    %6:gpr(s64) = G_CONSTANT i64 255
+    %7:gpr(s64) = G_ANYEXT %3(s32)
+    %4:gpr(s64) = G_AND %7, %6
+    %5:gpr(s32) = G_INTRINSIC_W_SIDE_EFFECTS intrinsic(@llvm.aarch64.stxr), %4(s64), %2(p0) :: (volatile store 1 into %ir.addr)
+    $w0 = COPY %5(s32)
+    RET_ReallyLR implicit $w0
+
+...
+---
+name:            test_store_i16
+alignment:       2
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+machineFunctionInfo: {}
+body:             |
+  bb.0:
+    liveins: $w0, $w1, $x2
+
+    ; CHECK-LABEL: name: test_store_i16
+    ; CHECK: liveins: $w0, $w1, $x2
+    ; CHECK: [[COPY:%[0-9]+]]:gpr32all = COPY $w1
+    ; CHECK: [[COPY1:%[0-9]+]]:gpr64sp = COPY $x2
+    ; CHECK: [[SUBREG_TO_REG:%[0-9]+]]:gpr64all = SUBREG_TO_REG 0, [[COPY]], %subreg.sub_32
+    ; CHECK: [[COPY2:%[0-9]+]]:gpr32 = COPY [[SUBREG_TO_REG]].sub_32
+    ; CHECK: early-clobber %5:gpr32 = STXRH [[COPY2]], [[COPY1]] :: (volatile store 2 into %ir.addr)
+    ; CHECK: $w0 = COPY %5
+    ; CHECK: RET_ReallyLR implicit $w0
+    %3:gpr(s32) = COPY $w1
+    %2:gpr(p0) = COPY $x2
+    %6:gpr(s64) = G_CONSTANT i64 65535
+    %7:gpr(s64) = G_ANYEXT %3(s32)
+    %4:gpr(s64) = G_AND %7, %6
+    %5:gpr(s32) = G_INTRINSIC_W_SIDE_EFFECTS intrinsic(@llvm.aarch64.stxr), %4(s64), %2(p0) :: (volatile store 2 into %ir.addr)
+    $w0 = COPY %5(s32)
+    RET_ReallyLR implicit $w0
+
+...
+---
+name:            test_store_i32
+alignment:       2
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+machineFunctionInfo: {}
+body:             |
+  bb.0:
+    liveins: $w0, $w1, $x2
+
+    ; CHECK-LABEL: name: test_store_i32
+    ; CHECK: liveins: $w0, $w1, $x2
+    ; CHECK: [[COPY:%[0-9]+]]:gpr32 = COPY $w1
+    ; CHECK: [[COPY1:%[0-9]+]]:gpr64sp = COPY $x2
+    ; CHECK: early-clobber %3:gpr32 = STXRW [[COPY]], [[COPY1]] :: (volatile store 4 into %ir.addr)
+    ; CHECK: $w0 = COPY %3
+    ; CHECK: RET_ReallyLR implicit $w0
+    %1:gpr(s32) = COPY $w1
+    %2:gpr(p0) = COPY $x2
+    %3:gpr(s64) = G_ZEXT %1(s32)
+    %4:gpr(s32) = G_INTRINSIC_W_SIDE_EFFECTS intrinsic(@llvm.aarch64.stxr), %3(s64), %2(p0) :: (volatile store 4 into %ir.addr)
+    $w0 = COPY %4(s32)
+    RET_ReallyLR implicit $w0
+
+...
+---
+name:            test_store_i64
+alignment:       2
+legalized:       true
+regBankSelected: true
+tracksRegLiveness: true
+machineFunctionInfo: {}
+body:             |
+  bb.0:
+    liveins: $w0, $x1, $x2
+
+    ; CHECK-LABEL: name: test_store_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 = STXRX [[COPY]], [[COPY1]] :: (volatile store 8 into %ir.addr)
+    ; 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.stxr), %1(s64), %2(p0) :: (volatile store 8 into %ir.addr)
+    $w0 = COPY %3(s32)
+    RET_ReallyLR implicit $w0
+
+...
Index: llvm/test/CodeGen/AArch64/arm64-ldxr-stxr.ll
===================================================================
--- llvm/test/CodeGen/AArch64/arm64-ldxr-stxr.ll
+++ llvm/test/CodeGen/AArch64/arm64-ldxr-stxr.ll
@@ -91,39 +91,57 @@
 declare i64 @llvm.aarch64.ldxr.p0i32(i32*) nounwind
 declare i64 @llvm.aarch64.ldxr.p0i64(i64*) nounwind
 
+; FALLBACK-NOT: remark:{{.*}}test_store_i8
 define i32 @test_store_i8(i32, i8 %val, i8* %addr) {
 ; CHECK-LABEL: test_store_i8:
 ; CHECK-NOT: uxtb
 ; CHECK-NOT: and
 ; CHECK: stxrb w0, w1, [x2]
+; GISEL-LABEL: test_store_i8:
+; GISEL-NOT: uxtb
+; GISEL-NOT: and
+; GISEL: stxrb w0, w1, [x2]
   %extval = zext i8 %val to i64
   %res = call i32 @llvm.aarch64.stxr.p0i8(i64 %extval, i8* %addr)
   ret i32 %res
 }
 
+; FALLBACK-NOT: remark:{{.*}}test_store_i16
 define i32 @test_store_i16(i32, i16 %val, i16* %addr) {
 ; CHECK-LABEL: test_store_i16:
 ; CHECK-NOT: uxth
 ; CHECK-NOT: and
 ; CHECK: stxrh w0, w1, [x2]
+; GISEL-LABEL: test_store_i16:
+; GISEL-NOT: uxth
+; GISEL-NOT: and
+; GISEL: stxrh w0, w1, [x2]
   %extval = zext i16 %val to i64
   %res = call i32 @llvm.aarch64.stxr.p0i16(i64 %extval, i16* %addr)
   ret i32 %res
 }
 
+; FALLBACK-NOT: remark:{{.*}}test_store_i32
 define i32 @test_store_i32(i32, i32 %val, i32* %addr) {
 ; CHECK-LABEL: test_store_i32:
 ; CHECK-NOT: uxtw
 ; CHECK-NOT: and
 ; CHECK: stxr w0, w1, [x2]
+; GISEL-LABEL: test_store_i32:
+; GISEL-NOT: uxtw
+; GISEL-NOT: and
+; GISEL: stxr w0, w1, [x2]
   %extval = zext i32 %val to i64
   %res = call i32 @llvm.aarch64.stxr.p0i32(i64 %extval, i32* %addr)
   ret i32 %res
 }
 
+; FALLBACK-NOT: remark:{{.*}}test_store_i64
 define i32 @test_store_i64(i32, i64 %val, i64* %addr) {
 ; CHECK-LABEL: test_store_i64:
 ; CHECK: stxr w0, x1, [x2]
+; GISEL-LABEL: test_store_i64:
+; GISEL: stxr w0, x1, [x2]
   %res = call i32 @llvm.aarch64.stxr.p0i64(i64 %val, i64* %addr)
   ret i32 %res
 }