diff --git a/clang/include/clang/Driver/ToolChain.h b/clang/include/clang/Driver/ToolChain.h
--- a/clang/include/clang/Driver/ToolChain.h
+++ b/clang/include/clang/Driver/ToolChain.h
@@ -456,6 +456,12 @@
   /// by default.
   virtual bool IsUnwindTablesDefault(const llvm::opt::ArgList &Args) const;
 
+  /// Test whether this toolchain supports outline atomics by default.
+  virtual bool
+  IsAArch64OutlineAtomicsDefault(const llvm::opt::ArgList &Args) const {
+    return false;
+  }
+
   /// Test whether this toolchain defaults to PIC.
   virtual bool isPICDefault() const = 0;
 
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -6435,6 +6435,12 @@
       CmdArgs.push_back("-target-feature");
       CmdArgs.push_back("-outline-atomics");
     }
+  } else {
+    if (Triple.isAArch64() &&
+        !getToolChain().IsAArch64OutlineAtomicsDefault(Args)) {
+      CmdArgs.push_back("-target-feature");
+      CmdArgs.push_back("-outline-atomics");
+    }
   }
 
   if (Args.hasFlag(options::OPT_faddrsig, options::OPT_fno_addrsig,
diff --git a/clang/lib/Driver/ToolChains/Linux.h b/clang/lib/Driver/ToolChains/Linux.h
--- a/clang/lib/Driver/ToolChains/Linux.h
+++ b/clang/lib/Driver/ToolChains/Linux.h
@@ -36,6 +36,8 @@
   void AddIAMCUIncludeArgs(const llvm::opt::ArgList &DriverArgs,
                            llvm::opt::ArgStringList &CC1Args) const override;
   CXXStdlibType GetDefaultCXXStdlibType() const override;
+  bool
+  IsAArch64OutlineAtomicsDefault(const llvm::opt::ArgList &Args) const override;
   bool isPIEDefault() const override;
   bool isNoExecStackDefault() const override;
   bool IsMathErrnoDefault() const override;
diff --git a/clang/lib/Driver/ToolChains/Linux.cpp b/clang/lib/Driver/ToolChains/Linux.cpp
--- a/clang/lib/Driver/ToolChains/Linux.cpp
+++ b/clang/lib/Driver/ToolChains/Linux.cpp
@@ -836,6 +836,21 @@
           getTriple().isMusl() || getSanitizerArgs().requiresPIE();
 }
 
+bool Linux::IsAArch64OutlineAtomicsDefault(const ArgList &Args) const {
+  if (!getTriple().isAArch64())
+    return false;
+  // Outline atomics for AArch64 are supported by compiler-rt
+  // and libgcc since 9.3.1
+  if (GetRuntimeLibType(Args) == ToolChain::RLT_Libgcc) {
+    const GCCVersion &Ver = GCCInstallation.getVersion();
+    if (Ver.isOlderThan(9, 3, 1))
+      return false;
+  } else if (GetRuntimeLibType(Args) != ToolChain::RLT_CompilerRT) {
+    return false;
+  }
+  return true;
+}
+
 bool Linux::isNoExecStackDefault() const {
     return getTriple().isAndroid();
 }
diff --git a/llvm/lib/Target/AArch64/AArch64.td b/llvm/lib/Target/AArch64/AArch64.td
--- a/llvm/lib/Target/AArch64/AArch64.td
+++ b/llvm/lib/Target/AArch64/AArch64.td
@@ -1084,6 +1084,7 @@
                      FeatureFPARMv8,
                      FeatureFuseAES,
                      FeatureNEON,
+                     FeatureOutlineAtomics,
                      FeaturePerfMon,
                      FeaturePostRAScheduler,
 // ETE and TRBE are future architecture extensions. We temporarily enable them
diff --git a/llvm/test/CodeGen/AArch64/arm64_32-atomics.ll b/llvm/test/CodeGen/AArch64/arm64_32-atomics.ll
--- a/llvm/test/CodeGen/AArch64/arm64_32-atomics.ll
+++ b/llvm/test/CodeGen/AArch64/arm64_32-atomics.ll
@@ -1,5 +1,5 @@
-; RUN: llc -mtriple=arm64_32-apple-ios7.0 -o - %s | FileCheck %s
-; RUN: llc -mtriple=arm64_32-apple-ios7.0 -mattr=+outline-atomics -o - %s | FileCheck %s -check-prefix=OUTLINE-ATOMICS
+; RUN: llc -mtriple=arm64_32-apple-ios7.0 -mattr=-outline-atomics -o - %s | FileCheck %s
+; RUN: llc -mtriple=arm64_32-apple-ios7.0 -o - %s | FileCheck %s -check-prefix=OUTLINE-ATOMICS
 
 define i8 @test_load_8(i8* %addr) {
 ; CHECK-LABAL: test_load_8:
diff --git a/llvm/test/CodeGen/AArch64/atomic-ops-not-barriers.ll b/llvm/test/CodeGen/AArch64/atomic-ops-not-barriers.ll
--- a/llvm/test/CodeGen/AArch64/atomic-ops-not-barriers.ll
+++ b/llvm/test/CodeGen/AArch64/atomic-ops-not-barriers.ll
@@ -1,5 +1,5 @@
-; RUN: llc -mtriple=aarch64-none-linux-gnu -verify-machineinstrs < %s | FileCheck %s
-; RUN: llc -mtriple=aarch64-none-linux-gnu -verify-machineinstrs -mattr=+outline-atomics < %s | FileCheck %s --check-prefix=OUTLINE-ATOMICS
+; RUN: llc -mtriple=aarch64-none-linux-gnu -verify-machineinstrs -mattr=-outline-atomics < %s | FileCheck %s
+; RUN: llc -mtriple=aarch64-none-linux-gnu -verify-machineinstrs < %s | FileCheck %s --check-prefix=OUTLINE-ATOMICS
 
 define i32 @foo(i32* %var, i1 %cond) {
 ; OUTLINE-ATOMICS: bl __aarch64_ldadd4_relax
diff --git a/llvm/test/CodeGen/AArch64/atomic-ops.ll b/llvm/test/CodeGen/AArch64/atomic-ops.ll
--- a/llvm/test/CodeGen/AArch64/atomic-ops.ll
+++ b/llvm/test/CodeGen/AArch64/atomic-ops.ll
@@ -1,6 +1,6 @@
-; RUN: llc -mtriple=aarch64-none-linux-gnu -disable-post-ra -verify-machineinstrs < %s | FileCheck %s
-; RUN: llc -mtriple=aarch64-none-linux-gnu -disable-post-ra -verify-machineinstrs < %s | FileCheck %s --check-prefix=CHECK-REG
-; RUN: llc -mtriple=aarch64-none-linux-gnu -disable-post-ra -verify-machineinstrs -mattr=+outline-atomics < %s | FileCheck %s --check-prefix=OUTLINE_ATOMICS
+; RUN: llc -mtriple=aarch64-none-linux-gnu -disable-post-ra -verify-machineinstrs -mattr=-outline-atomics < %s | FileCheck %s
+; RUN: llc -mtriple=aarch64-none-linux-gnu -disable-post-ra -verify-machineinstrs -mattr=-outline-atomics < %s | FileCheck %s --check-prefix=CHECK-REG
+; RUN: llc -mtriple=aarch64-none-linux-gnu -disable-post-ra -verify-machineinstrs < %s | FileCheck %s --check-prefix=OUTLINE_ATOMICS
 
 
 ; Point of CHECK-REG is to make sure UNPREDICTABLE instructions aren't created
diff --git a/llvm/test/CodeGen/AArch64/cmpxchg-O0.ll b/llvm/test/CodeGen/AArch64/cmpxchg-O0.ll
--- a/llvm/test/CodeGen/AArch64/cmpxchg-O0.ll
+++ b/llvm/test/CodeGen/AArch64/cmpxchg-O0.ll
@@ -1,5 +1,5 @@
-; RUN: llc -verify-machineinstrs -mtriple=aarch64-linux-gnu -O0 -fast-isel=0 -global-isel=false %s -o - | FileCheck -enable-var-scope %s
-; RUN: llc -verify-machineinstrs -mtriple=aarch64-linux-gnu -O0 -fast-isel=0 -global-isel=false -mattr=+outline-atomics %s -o - | FileCheck -enable-var-scope %s --check-prefix=OUTLINE-ATOMICS
+; RUN: llc -verify-machineinstrs -mtriple=aarch64-linux-gnu -O0 -fast-isel=0 -global-isel=false -mattr=-outline-atomics %s -o - | FileCheck -enable-var-scope %s
+; RUN: llc -verify-machineinstrs -mtriple=aarch64-linux-gnu -O0 -fast-isel=0 -global-isel=false %s -o - | FileCheck -enable-var-scope %s --check-prefix=OUTLINE-ATOMICS
 
 define { i8, i1 } @test_cmpxchg_8(i8* %addr, i8 %desired, i8 %new) nounwind {
 ; OUTLINE-ATOMICS: bl __aarch64_cas1_acq_rel
diff --git a/llvm/test/CodeGen/AArch64/cmpxchg-idioms.ll b/llvm/test/CodeGen/AArch64/cmpxchg-idioms.ll
--- a/llvm/test/CodeGen/AArch64/cmpxchg-idioms.ll
+++ b/llvm/test/CodeGen/AArch64/cmpxchg-idioms.ll
@@ -1,5 +1,5 @@
-; RUN: llc -mtriple=aarch64-apple-ios7.0 -o - %s | FileCheck %s
-; RUN: llc -mtriple=aarch64-apple-ios7.0 -mattr=+outline-atomics -o - %s | FileCheck %s --check-prefix=OUTLINE-ATOMICS
+; RUN: llc -mtriple=aarch64-apple-ios7.0 -mattr=-outline-atomics -o - %s | FileCheck %s
+; RUN: llc -mtriple=aarch64-apple-ios7.0 -o - %s | FileCheck %s --check-prefix=OUTLINE-ATOMICS
 
 define i32 @test_return(i32* %p, i32 %oldval, i32 %newval) {
 ; OUTLINE-ATOMICS: bl ___aarch64_cas4_acq_rel
diff --git a/llvm/test/Transforms/AtomicExpand/AArch64/expand-atomicrmw-xchg-fp.ll b/llvm/test/Transforms/AtomicExpand/AArch64/expand-atomicrmw-xchg-fp.ll
--- a/llvm/test/Transforms/AtomicExpand/AArch64/expand-atomicrmw-xchg-fp.ll
+++ b/llvm/test/Transforms/AtomicExpand/AArch64/expand-atomicrmw-xchg-fp.ll
@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -S -mtriple=aarch64-- -atomic-expand %s | FileCheck %s
-; RUN: opt -S -mtriple=aarch64-- -mattr=+outline-atomics -atomic-expand %s | FileCheck %s --check-prefix=OUTLINE-ATOMICS
+; RUN: opt -S -mtriple=aarch64-- -mattr=-outline-atomics -atomic-expand %s | FileCheck %s
+; RUN: opt -S -mtriple=aarch64-- -atomic-expand %s | FileCheck %s --check-prefix=OUTLINE-ATOMICS
 
 define void @atomic_swap_f16(half* %ptr, half %val) nounwind {
 ; CHECK-LABEL: @atomic_swap_f16(