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
@@ -18758,9 +18758,11 @@
   // 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 we need a late-expanded pseudo-inst instead.
+  unsigned Size = AI->getOperand(1)->getType()->getPrimitiveSizeInBits();
   bool HasAtomicCmpXchg =
       !Subtarget->isThumb() || Subtarget->hasV8MBaselineOps();
-  if (getTargetMachine().getOptLevel() != 0 && HasAtomicCmpXchg)
+  if (getTargetMachine().getOptLevel() != 0 && HasAtomicCmpXchg &&
+      Size <= (Subtarget->isMClass() ? 32U : 64U))
     return AtomicExpansionKind::LLSC;
   return AtomicExpansionKind::None;
 }
diff --git a/llvm/test/CodeGen/ARM/atomic-64bit.ll b/llvm/test/CodeGen/ARM/atomic-64bit.ll
--- a/llvm/test/CodeGen/ARM/atomic-64bit.ll
+++ b/llvm/test/CodeGen/ARM/atomic-64bit.ll
@@ -2,6 +2,7 @@
 ; RUN: llc < %s -mtriple=thumbv7-none-linux-gnueabihf | FileCheck %s --check-prefix=CHECK-THUMB --check-prefix=CHECK-THUMB-LE
 ; RUN: llc < %s -mtriple=armebv7 -target-abi apcs | FileCheck %s --check-prefix=CHECK --check-prefix=CHECK-BE
 ; RUN: llc < %s -mtriple=thumbebv7-none-linux-gnueabihf | FileCheck %s --check-prefix=CHECK-THUMB --check-prefix=CHECK-THUMB-BE
+; RUN: llc < %s -mtriple=armv7m--none-eabi | FileCheck %s --check-prefix=CHECK-ARMV7M
 
 define i64 @test1(i64* %ptr, i64 %val) {
 ; CHECK-LABEL: test1:
@@ -28,6 +29,8 @@
 ; CHECK-THUMB: bne
 ; CHECK-THUMB: dmb {{ish$}}
 
+; CHECK-ARMV7M: __sync_fetch_and_add_8
+
   %r = atomicrmw add i64* %ptr, i64 %val seq_cst
   ret i64 %r
 }
@@ -57,6 +60,8 @@
 ; CHECK-THUMB: bne
 ; CHECK-THUMB: dmb {{ish$}}
 
+; CHECK-ARMV7M: __sync_fetch_and_sub_8
+
   %r = atomicrmw sub i64* %ptr, i64 %val seq_cst
   ret i64 %r
 }
@@ -86,6 +91,8 @@
 ; CHECK-THUMB: bne
 ; CHECK-THUMB: dmb {{ish$}}
 
+; CHECK-ARMV7M: __sync_fetch_and_and_8
+
   %r = atomicrmw and i64* %ptr, i64 %val seq_cst
   ret i64 %r
 }
@@ -115,6 +122,8 @@
 ; CHECK-THUMB: bne
 ; CHECK-THUMB: dmb {{ish$}}
 
+; CHECK-ARMV7M: __sync_fetch_and_or_8
+
   %r = atomicrmw or i64* %ptr, i64 %val seq_cst
   ret i64 %r
 }
@@ -144,6 +153,8 @@
 ; CHECK-THUMB: bne
 ; CHECK-THUMB: dmb {{ish$}}
 
+; CHECK-ARMV7M: __sync_fetch_and_xor_8
+
   %r = atomicrmw xor i64* %ptr, i64 %val seq_cst
   ret i64 %r
 }
@@ -165,6 +176,8 @@
 ; CHECK-THUMB: bne
 ; CHECK-THUMB: dmb {{ish$}}
 
+; CHECK-ARMV7M: __sync_lock_test_and_set_8
+
   %r = atomicrmw xchg i64* %ptr, i64 %val seq_cst
   ret i64 %r
 }
@@ -199,12 +212,15 @@
 ; CHECK-THUMB: beq
 ; CHECK-THUMB: dmb {{ish$}}
 
+; CHECK-ARMV7M: __sync_val_compare_and_swap_8
+
   %pair = cmpxchg i64* %ptr, i64 %val1, i64 %val2 seq_cst seq_cst
   %r = extractvalue { i64, i1 } %pair, 0
   ret i64 %r
 }
 
-; Compiles down to a single ldrexd
+; Compiles down to a single ldrexd, except on M class devices where ldrexd
+; isn't supported.
 define i64 @test8(i64* %ptr) {
 ; CHECK-LABEL: test8:
 ; CHECK: ldrexd [[REG1:(r[0-9]?[02468])]], [[REG2:(r[0-9]?[13579])]]
@@ -220,12 +236,15 @@
 ; CHECK-THUMB-NOT: strexd
 ; CHECK-THUMB: dmb {{ish$}}
 
+; CHECK-ARMV7M: __sync_val_compare_and_swap_8
+
   %r = load atomic i64, i64* %ptr seq_cst, align 8
   ret i64 %r
 }
 
 ; Compiles down to atomicrmw xchg; there really isn't any more efficient
-; way to write it.
+; way to write it. Except on M class devices, where ldrexd/strexd aren't
+; supported.
 define void @test9(i64* %ptr, i64 %val) {
 ; CHECK-LABEL: test9:
 ; CHECK: dmb {{ish$}}
@@ -243,6 +262,8 @@
 ; CHECK-THUMB: bne
 ; CHECK-THUMB: dmb {{ish$}}
 
+; CHECK-ARMV7M: __sync_lock_test_and_set_8
+
   store atomic i64 %val, i64* %ptr seq_cst, align 8
   ret void
 }
@@ -286,6 +307,8 @@
 ; CHECK-THUMB: bne
 ; CHECK-THUMB: dmb {{ish$}}
 
+; CHECK-ARMV7M: __sync_fetch_and_min_8
+
   %r = atomicrmw min i64* %ptr, i64 %val seq_cst
   ret i64 %r
 }
@@ -329,6 +352,8 @@
 ; CHECK-THUMB: bne
 ; CHECK-THUMB: dmb {{ish$}}
 
+; CHECK-ARMV7M: __sync_fetch_and_umin_8
+
   %r = atomicrmw umin i64* %ptr, i64 %val seq_cst
   ret i64 %r
 }
@@ -372,6 +397,8 @@
 ; CHECK-THUMB: bne
 ; CHECK-THUMB: dmb {{ish$}}
 
+; CHECK-ARMV7M: __sync_fetch_and_max_8
+
   %r = atomicrmw max i64* %ptr, i64 %val seq_cst
   ret i64 %r
 }
@@ -414,6 +441,9 @@
 ; CHECK-THUMB: cmp
 ; CHECK-THUMB: bne
 ; CHECK-THUMB: dmb {{ish$}}
+
+; CHECK-ARMV7M: __sync_fetch_and_umax_8
+
   %r = atomicrmw umax i64* %ptr, i64 %val seq_cst
   ret i64 %r
 }