diff --git a/clang/lib/Basic/Targets/ARM.cpp b/clang/lib/Basic/Targets/ARM.cpp
index 29de34bbc4fe4..1db9a863d460a 100644
--- a/clang/lib/Basic/Targets/ARM.cpp
+++ b/clang/lib/Basic/Targets/ARM.cpp
@@ -133,19 +133,24 @@ void ARMTargetInfo::setArchInfo(llvm::ARM::ArchKind Kind) {
 }
 
 void ARMTargetInfo::setAtomic() {
-  // when triple does not specify a sub arch,
-  // then we are not using inline atomics
-  bool ShouldUseInlineAtomic =
-      (ArchISA == llvm::ARM::ISAKind::ARM && ArchVersion >= 6) ||
-      (ArchISA == llvm::ARM::ISAKind::THUMB && ArchVersion >= 7);
-  // Cortex M does not support 8 byte atomics, while general Thumb2 does.
   if (ArchProfile == llvm::ARM::ProfileKind::M) {
+    // M-class only ever supports 32-bit atomics. Cortex-M0 doesn't have
+    // any atomics.
     MaxAtomicPromoteWidth = 32;
-    if (ShouldUseInlineAtomic)
+    if (ArchVersion >= 7)
       MaxAtomicInlineWidth = 32;
   } else {
+    // A-class targets have up to 64-bit atomics.
+    //
+    // On Linux, 64-bit atomics are always available through kernel helpers
+    // (which are lock-free). Otherwise, atomics are available on v6 or later.
+    //
+    // (Thumb doesn't matter; for Thumbv6, we just use a library call which
+    // switches out of Thumb mode.)
+    //
+    // This should match setMaxAtomicSizeInBitsSupported() in the backend.
     MaxAtomicPromoteWidth = 64;
-    if (ShouldUseInlineAtomic)
+    if (getTriple().getOS() == llvm::Triple::Linux || ArchVersion >= 6)
       MaxAtomicInlineWidth = 64;
   }
 }
diff --git a/clang/test/CodeGen/atomic-arm.c b/clang/test/CodeGen/atomic-arm.c
index 6952b4d0099b5..e6c2b8dca613a 100644
--- a/clang/test/CodeGen/atomic-arm.c
+++ b/clang/test/CodeGen/atomic-arm.c
@@ -2,7 +2,10 @@
 // RUN: %clang_cc1 -triple thumbv7m-apple-unknown-macho %s -emit-llvm -o - | FileCheck %s --check-prefixes=CHECK,CHECK-V7M
 // RUN: %clang_cc1 -triple thumbv7-apple-ios13.0 %s -emit-llvm -o - | FileCheck %s --check-prefixes=CHECK,CHECK-HOSTED
 // RUN: %clang_cc1 -triple thumbv7k-apple-watchos5.0 %s -emit-llvm -o - | FileCheck %s --check-prefixes=CHECK,CHECK-HOSTED
-
+// RUN: %clang_cc1 -triple arm-linux-gnueabi %s -emit-llvm -o - | FileCheck %s --check-prefixes=CHECK,CHECK-HOSTED
+// RUN: %clang_cc1 -triple armv7-none-eabi %s -emit-llvm -o - | FileCheck %s --check-prefixes=CHECK,CHECK-HOSTED
+// RUN: %clang_cc1 -triple thumbv6k-none-eabi %s -emit-llvm -o - | FileCheck %s --check-prefixes=CHECK,CHECK-HOSTED
+// RUN: %clang_cc1 -triple armv5-none-eabi %s -emit-llvm -o - | FileCheck %s --check-prefixes=CHECK,CHECK-V6M
 
 // CHECK-V6M: @always1 = global i32 0
 // CHECK-V6M: @always4 = global i32 0
@@ -22,7 +25,7 @@ int always8 = __atomic_always_lock_free(8, 0);
 
 int lock_free_1() {
   // CHECK-LABEL: @lock_free_1
-  // CHECK-V6M:   [[RES:%.*]] = call arm_aapcscc zeroext i1 @__atomic_is_lock_free(i32 noundef 1, ptr noundef null)
+  // CHECK-V6M:   [[RES:%.*]] = call{{.*}}zeroext i1 @__atomic_is_lock_free(i32 noundef 1, ptr noundef null)
   // CHECK-V6M:   [[RES32:%.*]] = zext i1 [[RES]] to i32
   // CHECK-V6M:   ret i32 [[RES32]]
 
@@ -33,7 +36,7 @@ int lock_free_1() {
 
 int lock_free_4() {
   // CHECK-LABEL: @lock_free_4
-  // CHECK-V6M:   [[RES:%.*]] = call arm_aapcscc zeroext i1 @__atomic_is_lock_free(i32 noundef 4, ptr noundef null)
+  // CHECK-V6M:   [[RES:%.*]] = call{{.*}}zeroext i1 @__atomic_is_lock_free(i32 noundef 4, ptr noundef null)
   // CHECK-V6M:   [[RES32:%.*]] = zext i1 [[RES]] to i32
   // CHECK-V6M:   ret i32 [[RES32]]
 
@@ -44,11 +47,11 @@ int lock_free_4() {
 
 int lock_free_8() {
   // CHECK-LABEL: @lock_free_8
-  // CHECK-V6M:   [[RES:%.*]] = call arm_aapcscc zeroext i1 @__atomic_is_lock_free(i32 noundef 8, ptr noundef null)
+  // CHECK-V6M:   [[RES:%.*]] = call{{.*}}zeroext i1 @__atomic_is_lock_free(i32 noundef 8, ptr noundef null)
   // CHECK-V6M:   [[RES32:%.*]] = zext i1 [[RES]] to i32
   // CHECK-V6M:   ret i32 [[RES32]]
 
-  // CHECK-V7M:   [[RES:%.*]] = call arm_aapcscc zeroext i1 @__atomic_is_lock_free(i32 noundef 8, ptr noundef null)
+  // CHECK-V7M:   [[RES:%.*]] = call{{.*}}zeroext i1 @__atomic_is_lock_free(i32 noundef 8, ptr noundef null)
   // CHECK-V7M:   [[RES32:%.*]] = zext i1 [[RES]] to i32
   // CHECK-V7M:   ret i32 [[RES32]]
 
diff --git a/clang/test/CodeGen/pr45476.cpp b/clang/test/CodeGen/pr45476.cpp
index 84e7a984a1a25..c95f7fb8cd9c3 100644
--- a/clang/test/CodeGen/pr45476.cpp
+++ b/clang/test/CodeGen/pr45476.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple arm-unknown-linux-gnueabi -emit-llvm %s -o - | FileCheck -check-prefix=LIBCALL %s
+// RUN: %clang_cc1 -triple armv6m-eabi -emit-llvm %s -o - | FileCheck -check-prefix=LIBCALL %s
 // RUN: %clang_cc1 -triple armv8-eabi -emit-llvm %s -o - | FileCheck -check-prefix=NATIVE %s
 // PR45476