diff --git a/llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp b/llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp
--- a/llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp
+++ b/llvm/lib/Target/ARM/ARMExpandPseudoInsts.cpp
@@ -1566,6 +1566,15 @@
   Register DesiredReg = MI.getOperand(3).getReg();
   Register NewReg = MI.getOperand(4).getReg();
 
+  if (IsThumb) {
+    assert(STI->hasV8MBaselineOps() &&
+           "CMP_SWAP not expected to be custom expanded for Thumb1");
+    assert((UxtOp == 0 || UxtOp == ARM::tUXTB || UxtOp == ARM::tUXTH) &&
+           "Use of t2UXTB/t2UXTH requires at least ARMv8-M.baseline");
+    assert(ARM::tGPRRegClass.contains(DesiredReg) &&
+           "DesiredReg used for UXT op must be tGPR");
+  }
+
   MachineFunction *MF = MBB.getParent();
   auto LoadCmpBB = MF->CreateMachineBasicBlock(MBB.getBasicBlock());
   auto StoreBB = MF->CreateMachineBasicBlock(MBB.getBasicBlock());
@@ -2779,17 +2788,26 @@
     case ARM::VTBX3Pseudo: ExpandVTBL(MBBI, ARM::VTBX3, true); return true;
     case ARM::VTBX4Pseudo: ExpandVTBL(MBBI, ARM::VTBX4, true); return true;
 
+    case ARM::tCMP_SWAP_8:
+      assert(STI->isThumb());
+      return ExpandCMP_SWAP(MBB, MBBI, ARM::t2LDREXB, ARM::t2STREXB, ARM::tUXTB,
+                            NextMBBI);
+    case ARM::tCMP_SWAP_16:
+      assert(STI->isThumb());
+      return ExpandCMP_SWAP(MBB, MBBI, ARM::t2LDREXH, ARM::t2STREXH, ARM::tUXTH,
+                            NextMBBI);
+
     case ARM::CMP_SWAP_8:
       if (STI->isThumb())
         return ExpandCMP_SWAP(MBB, MBBI, ARM::t2LDREXB, ARM::t2STREXB,
-                              ARM::tUXTB, NextMBBI);
+                              ARM::t2UXTB, NextMBBI);
       else
         return ExpandCMP_SWAP(MBB, MBBI, ARM::LDREXB, ARM::STREXB,
                               ARM::UXTB, NextMBBI);
     case ARM::CMP_SWAP_16:
       if (STI->isThumb())
         return ExpandCMP_SWAP(MBB, MBBI, ARM::t2LDREXH, ARM::t2STREXH,
-                              ARM::tUXTH, NextMBBI);
+                              ARM::t2UXTH, NextMBBI);
       else
         return ExpandCMP_SWAP(MBB, MBBI, ARM::LDREXH, ARM::STREXH,
                               ARM::UXTH, NextMBBI);
diff --git a/llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp b/llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp
--- a/llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp
+++ b/llvm/lib/Target/ARM/ARMISelDAGToDAG.cpp
@@ -3299,9 +3299,9 @@
   unsigned Opcode;
   EVT MemTy = cast<MemSDNode>(N)->getMemoryVT();
   if (MemTy == MVT::i8)
-    Opcode = ARM::CMP_SWAP_8;
+    Opcode = Subtarget->isThumb() ? ARM::tCMP_SWAP_8 : ARM::CMP_SWAP_8;
   else if (MemTy == MVT::i16)
-    Opcode = ARM::CMP_SWAP_16;
+    Opcode = Subtarget->isThumb() ? ARM::tCMP_SWAP_16 : ARM::CMP_SWAP_16;
   else if (MemTy == MVT::i32)
     Opcode = ARM::CMP_SWAP_32;
   else
diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -19120,6 +19120,14 @@
   if (AI->isFloatingPointOperation())
     return AtomicExpansionKind::CmpXChg;
 
+  // At -O0, fast-regalloc cannot cope with the live vregs necessary to
+  // implement atomicrmw without spilling. If the target address is also on the
+  // stack and close enough to the spill slot, this can lead to a situation
+  // where the monitor always gets cleared and the atomic operation can never
+  // succeed. So at -O0 lower this operation to a CAS loop.
+  if (getTargetMachine().getOptLevel() == CodeGenOpt::None)
+    return AtomicExpansionKind::CmpXChg;
+
   unsigned Size = AI->getType()->getPrimitiveSizeInBits();
   bool hasAtomicRMW = !Subtarget->isThumb() || Subtarget->hasV8MBaselineOps();
   return (Size <= (Subtarget->isMClass() ? 32U : 64U) && hasAtomicRMW)
diff --git a/llvm/lib/Target/ARM/ARMInstrThumb.td b/llvm/lib/Target/ARM/ARMInstrThumb.td
--- a/llvm/lib/Target/ARM/ARMInstrThumb.td
+++ b/llvm/lib/Target/ARM/ARMInstrThumb.td
@@ -1766,3 +1766,21 @@
 def tLDRConstPool
   : tAsmPseudo<"ldr${p} $Rt, $immediate",
                (ins tGPR:$Rt, const_pool_asm_imm:$immediate, pred:$p)>;
+
+//===----------------------------------
+// Atomic cmpxchg for -O0
+//===----------------------------------
+
+// See ARMInstrInfo.td. These two thumb specific pseudos are required to
+// restrict the register class for the UXTB/UXTH ops used in the expansion.
+
+let Constraints = "@earlyclobber $Rd,@earlyclobber $temp",
+    mayLoad = 1, mayStore = 1 in {
+def tCMP_SWAP_8 : PseudoInst<(outs GPR:$Rd, GPR:$temp),
+                             (ins GPR:$addr, tGPR:$desired, GPR:$new),
+                             NoItinerary, []>, Sched<[]>;
+
+def tCMP_SWAP_16 : PseudoInst<(outs GPR:$Rd, GPR:$temp),
+                              (ins GPR:$addr, tGPR:$desired, GPR:$new),
+                              NoItinerary, []>, Sched<[]>;
+}
diff --git a/llvm/test/CodeGen/ARM/atomicrmw_exclusive_monitor_ints.ll b/llvm/test/CodeGen/ARM/atomicrmw_exclusive_monitor_ints.ll
--- a/llvm/test/CodeGen/ARM/atomicrmw_exclusive_monitor_ints.ll
+++ b/llvm/test/CodeGen/ARM/atomicrmw_exclusive_monitor_ints.ll
@@ -16,7 +16,7 @@
 define i8 @test_xchg_i8() {
 ; COMMON-LABEL: test_xchg_i8:
 ; EXPAND32: ldrexb
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexb
 ; THUMB1: bl __sync_lock_test_and_set_1
 entry:
@@ -26,7 +26,7 @@
 define i8 @test_add_i8() {
 ; COMMON-LABEL: test_add_i8:
 ; EXPAND32: ldrexb
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexb
 ; THUMB1: bl __sync_fetch_and_add_1
 entry:
@@ -36,7 +36,7 @@
 define i8 @test_sub_i8() {
 ; COMMON-LABEL: test_sub_i8:
 ; EXPAND32: ldrexb
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexb
 ; THUMB1: bl __sync_fetch_and_sub_1
 entry:
@@ -46,7 +46,7 @@
 define i8 @test_and_i8() {
 ; COMMON-LABEL: test_and_i8:
 ; EXPAND32: ldrexb
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexb
 ; THUMB1: bl __sync_fetch_and_and_1
 entry:
@@ -56,7 +56,7 @@
 define i8 @test_nand_i8() {
 ; COMMON-LABEL: test_nand_i8:
 ; EXPAND32: ldrexb
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexb
 ; THUMB1: bl __sync_fetch_and_nand_1
 entry:
@@ -66,7 +66,7 @@
 define i8 @test_or_i8() {
 ; COMMON-LABEL: test_or_i8:
 ; EXPAND32: ldrexb
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexb
 ; THUMB1: bl __sync_fetch_and_or_1
 entry:
@@ -76,7 +76,7 @@
 define i8 @test_xor_i8() {
 ; COMMON-LABEL: test_xor_i8:
 ; EXPAND32: ldrexb
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexb
 ; THUMB1: bl __sync_fetch_and_xor_1
 entry:
@@ -86,7 +86,7 @@
 define i8 @test_max_i8() {
 ; COMMON-LABEL: test_max_i8:
 ; EXPAND32: ldrexb
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexb
 ; THUMB1: bl __sync_fetch_and_max_1
 entry:
@@ -96,7 +96,7 @@
 define i8 @test_min_i8() {
 ; COMMON-LABEL: test_min_i8:
 ; EXPAND32: ldrexb
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexb
 ; THUMB1: bl __sync_fetch_and_min_1
 entry:
@@ -106,7 +106,7 @@
 define i8 @test_umax_i8() {
 ; COMMON-LABEL: test_umax_i8:
 ; EXPAND32: ldrexb
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexb
 ; THUMB1: bl __sync_fetch_and_umax_1
 entry:
@@ -116,7 +116,7 @@
 define i8 @test_umin_i8() {
 ; COMMON-LABEL: test_umin_i8:
 ; EXPAND32: ldrexb
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexb
 ; THUMB1: bl __sync_fetch_and_umin_1
 entry:
@@ -128,7 +128,7 @@
 define i16 @test_xchg_i16() {
 ; COMMON-LABEL: test_xchg_i16:
 ; EXPAND32: ldrexh
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexh
 ; THUMB1: bl __sync_lock_test_and_set_2
 entry:
@@ -138,7 +138,7 @@
 define i16 @test_add_i16() {
 ; COMMON-LABEL: test_add_i16:
 ; EXPAND32: ldrexh
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexh
 ; THUMB1: bl __sync_fetch_and_add_2
 entry:
@@ -148,7 +148,7 @@
 define i16 @test_sub_i16() {
 ; COMMON-LABEL: test_sub_i16:
 ; EXPAND32: ldrexh
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexh
 ; THUMB1: bl __sync_fetch_and_sub_2
 entry:
@@ -158,7 +158,7 @@
 define i16 @test_and_i16() {
 ; COMMON-LABEL: test_and_i16:
 ; EXPAND32: ldrexh
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexh
 ; THUMB1: bl __sync_fetch_and_and_2
 entry:
@@ -168,7 +168,7 @@
 define i16 @test_nand_i16() {
 ; COMMON-LABEL: test_nand_i16:
 ; EXPAND32: ldrexh
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexh
 ; THUMB1: bl __sync_fetch_and_nand_2
 entry:
@@ -178,7 +178,7 @@
 define i16 @test_or_i16() {
 ; COMMON-LABEL: test_or_i16:
 ; EXPAND32: ldrexh
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexh
 ; THUMB1: bl __sync_fetch_and_or_2
 entry:
@@ -188,7 +188,7 @@
 define i16 @test_xor_i16() {
 ; COMMON-LABEL: test_xor_i16:
 ; EXPAND32: ldrexh
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexh
 ; THUMB1: bl __sync_fetch_and_xor_2
 entry:
@@ -198,7 +198,7 @@
 define i16 @test_max_i16() {
 ; COMMON-LABEL: test_max_i16:
 ; EXPAND32: ldrexh
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexh
 ; THUMB1: bl __sync_fetch_and_max_2
 entry:
@@ -208,7 +208,7 @@
 define i16 @test_min_i16() {
 ; COMMON-LABEL: test_min_i16:
 ; EXPAND32: ldrexh
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexh
 ; THUMB1: bl __sync_fetch_and_min_2
 entry:
@@ -218,7 +218,7 @@
 define i16 @test_umax_i16() {
 ; COMMON-LABEL: test_umax_i16:
 ; EXPAND32: ldrexh
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexh
 ; THUMB1: bl __sync_fetch_and_umax_2
 entry:
@@ -228,7 +228,7 @@
 define i16 @test_umin_i16() {
 ; COMMON-LABEL: test_umin_i16:
 ; EXPAND32: ldrexh
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strexh
 ; THUMB1: bl __sync_fetch_and_umin_2
 entry:
@@ -240,7 +240,7 @@
 define i32 @test_xchg_i32() {
 ; COMMON-LABEL: test_xchg_i32:
 ; EXPAND32: ldrex
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strex
 ; THUMB1: bl __sync_lock_test_and_set_4
 entry:
@@ -250,7 +250,7 @@
 define i32 @test_add_i32() {
 ; COMMON-LABEL: test_add_i32:
 ; EXPAND32: ldrex
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strex
 ; THUMB1: bl __sync_fetch_and_add_4
 entry:
@@ -260,7 +260,7 @@
 define i32 @test_sub_i32() {
 ; COMMON-LABEL: test_sub_i32:
 ; EXPAND32: ldrex
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strex
 ; THUMB1: bl __sync_fetch_and_sub_4
 entry:
@@ -270,7 +270,7 @@
 define i32 @test_and_i32() {
 ; COMMON-LABEL: test_and_i32:
 ; EXPAND32: ldrex
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strex
 ; THUMB1: bl __sync_fetch_and_and_4
 entry:
@@ -280,7 +280,7 @@
 define i32 @test_nand_i32() {
 ; COMMON-LABEL: test_nand_i32:
 ; EXPAND32: ldrex
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strex
 ; THUMB1: bl __sync_fetch_and_nand_4
 entry:
@@ -290,7 +290,7 @@
 define i32 @test_or_i32() {
 ; COMMON-LABEL: test_or_i32:
 ; EXPAND32: ldrex
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strex
 ; THUMB1: bl __sync_fetch_and_or_4
 entry:
@@ -300,7 +300,7 @@
 define i32 @test_xor_i32() {
 ; COMMON-LABEL: test_xor_i32:
 ; EXPAND32: ldrex
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strex
 ; THUMB1: bl __sync_fetch_and_xor_4
 entry:
@@ -310,7 +310,7 @@
 define i32 @test_max_i32() {
 ; COMMON-LABEL: test_max_i32:
 ; EXPAND32: ldrex
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strex
 ; THUMB1: bl __sync_fetch_and_max_4
 entry:
@@ -320,7 +320,7 @@
 define i32 @test_min_i32() {
 ; COMMON-LABEL: test_min_i32:
 ; EXPAND32: ldrex
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strex
 
 ; THUMB1: bl __sync_fetch_and_min_4
@@ -331,7 +331,7 @@
 define i32 @test_umax_i32() {
 ; COMMON-LABEL: test_umax_i32:
 ; EXPAND32: ldrex
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strex
 ; THUMB1: bl __sync_fetch_and_umax_4
 entry:
@@ -341,7 +341,7 @@
 define i32 @test_umin_i32() {
 ; COMMON-LABEL: test_umin_i32:
 ; EXPAND32: ldrex
-; EXPAND32: str
+; EXPAND32-NOT: str
 ; EXPAND32: strex
 ; THUMB1: bl __sync_fetch_and_umin_4
 entry:
@@ -352,10 +352,10 @@
 define i64 @test_xchg_i64() {
 ; COMMON-LABEL: test_xchg_i64:
 ; EXPAND64: ldrexd
-; EXPAND64: str
+; EXPAND64-NOT: str
 ; EXPAND64: strexd
 ; THUMB1: bl __sync_lock_test_and_set_8
-; BASELINE64: bl __sync_lock_test_and_set_8
+; BASELINE64: bl __sync_val_compare_and_swap_8
 entry:
   %0 = atomicrmw xchg i64* @atomic_i64, i64 1 monotonic
   ret i64 %0
@@ -363,10 +363,10 @@
 define i64 @test_add_i64() {
 ; COMMON-LABEL: test_add_i64:
 ; EXPAND64: ldrexd
-; EXPAND64: str
+; EXPAND64-NOT: str
 ; EXPAND64: strexd
 ; THUMB1: bl __sync_fetch_and_add_8
-; BASELINE64: bl __sync_fetch_and_add_8
+; BASELINE64: bl __sync_val_compare_and_swap_8
 entry:
   %0 = atomicrmw add i64* @atomic_i64, i64 1 monotonic
   ret i64 %0
@@ -374,10 +374,10 @@
 define i64 @test_sub_i64() {
 ; COMMON-LABEL: test_sub_i64:
 ; EXPAND64: ldrexd
-; EXPAND64: str
+; EXPAND64-NOT: str
 ; EXPAND64: strexd
 ; THUMB1: bl __sync_fetch_and_sub_8
-; BASELINE64: bl __sync_fetch_and_sub_8
+; BASELINE64: bl __sync_val_compare_and_swap_8
 entry:
   %0 = atomicrmw sub i64* @atomic_i64, i64 1 monotonic
   ret i64 %0
@@ -385,10 +385,10 @@
 define i64 @test_and_i64() {
 ; COMMON-LABEL: test_and_i64:
 ; EXPAND64: ldrexd
-; EXPAND64: str
+; EXPAND64-NOT: str
 ; EXPAND64: strexd
 ; THUMB1: bl __sync_fetch_and_and_8
-; BASELINE64: bl __sync_fetch_and_and_8
+; BASELINE64: bl __sync_val_compare_and_swap_8
 entry:
   %0 = atomicrmw and i64* @atomic_i64, i64 1 monotonic
   ret i64 %0
@@ -396,10 +396,10 @@
 define i64 @test_nand_i64() {
 ; COMMON-LABEL: test_nand_i64:
 ; EXPAND64: ldrexd
-; EXPAND64: str
+; EXPAND64-NOT: str
 ; EXPAND64: strexd
 ; THUMB1: bl __sync_fetch_and_nand_8
-; BASELINE64: bl __sync_fetch_and_nand_8
+; BASELINE64: bl __sync_val_compare_and_swap_8
 entry:
   %0 = atomicrmw nand i64* @atomic_i64, i64 1 monotonic
   ret i64 %0
@@ -407,10 +407,10 @@
 define i64 @test_or_i64() {
 ; COMMON-LABEL: test_or_i64:
 ; EXPAND64: ldrexd
-; EXPAND64: str
+; EXPAND64-NOT: str
 ; EXPAND64: strexd
 ; THUMB1: bl __sync_fetch_and_or_8
-; BASELINE64: bl __sync_fetch_and_or_8
+; BASELINE64: bl __sync_val_compare_and_swap_8
 entry:
   %0 = atomicrmw or i64* @atomic_i64, i64 1 monotonic
   ret i64 %0
@@ -418,10 +418,10 @@
 define i64 @test_xor_i64() {
 ; COMMON-LABEL: test_xor_i64:
 ; EXPAND64: ldrexd
-; EXPAND64: str
+; EXPAND64-NOT: str
 ; EXPAND64: strexd
 ; THUMB1: bl __sync_fetch_and_xor_8
-; BASELINE64: bl __sync_fetch_and_xor_8
+; BASELINE64: bl __sync_val_compare_and_swap_8
 entry:
   %0 = atomicrmw xor i64* @atomic_i64, i64 1 monotonic
   ret i64 %0
@@ -430,10 +430,10 @@
 define i64 @test_max_i64() {
 ; COMMON-LABEL: test_max_i64:
 ; EXPAND64: ldrexd
-; EXPAND64: str
+; EXPAND64-NOT: str
 ; EXPAND64: strexd
 ; THUMB1: bl __sync_fetch_and_max_8
-; BASELINE64: bl __sync_fetch_and_max_8
+; BASELINE64: bl __sync_val_compare_and_swap_8
 entry:
   %0 = atomicrmw max i64* @atomic_i64, i64 1 monotonic
   ret i64 %0
@@ -441,10 +441,10 @@
 define i64 @test_min_i64() {
 ; COMMON-LABEL: test_min_i64:
 ; EXPAND64: ldrexd
-; EXPAND64: str
+; EXPAND64-NOT: str
 ; EXPAND64: strexd
 ; THUMB1: bl __sync_fetch_and_min_8
-; BASELINE64: bl __sync_fetch_and_min_8
+; BASELINE64: bl __sync_val_compare_and_swap_8
 entry:
   %0 = atomicrmw min i64* @atomic_i64, i64 1 monotonic
   ret i64 %0
@@ -452,10 +452,10 @@
 define i64 @test_umax_i64() {
 ; COMMON-LABEL: test_umax_i64:
 ; EXPAND64: ldrexd
-; EXPAND64: str
+; EXPAND64-NOT: str
 ; EXPAND64: strexd
 ; THUMB1: bl __sync_fetch_and_umax_8
-; BASELINE64: bl __sync_fetch_and_umax_8
+; BASELINE64: bl __sync_val_compare_and_swap_8
 entry:
   %0 = atomicrmw umax i64* @atomic_i64, i64 1 monotonic
   ret i64 %0
@@ -463,10 +463,10 @@
 define i64 @test_umin_i64() {
 ; COMMON-LABEL: test_umin_i64:
 ; EXPAND64: ldrexd
-; EXPAND64: str
+; EXPAND64-NOT: str
 ; EXPAND64: strexd
 ; THUMB1: bl __sync_fetch_and_umin_8
-; BASELINE64: bl __sync_fetch_and_umin_8
+; BASELINE64: bl __sync_val_compare_and_swap_8
 entry:
   %0 = atomicrmw umin i64* @atomic_i64, i64 1 monotonic
   ret i64 %0