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
@@ -6481,6 +6481,10 @@
       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
@@ -845,6 +845,19 @@
           getTriple().isMusl() || getSanitizerArgs().requiresPIE();
 }
 
+bool Linux::IsAArch64OutlineAtomicsDefault(const ArgList &Args) const {
+  // Outline atomics for AArch64 are supported by compiler-rt
+  // and libgcc since 9.3.1
+  assert(getTriple().isAArch64() && "expected AArch64 target!");
+  ToolChain::RuntimeLibType RtLib = GetRuntimeLibType(Args);
+  if (RtLib == ToolChain::RLT_CompilerRT)
+    return true;
+  assert(RtLib == ToolChain::RLT_Libgcc && "unexpected runtime library type!");
+  if (GCCInstallation.getVersion().isOlderThan(9, 3, 1))
+    return false;
+  return true;
+}
+
 bool Linux::isNoExecStackDefault() const {
     return getTriple().isAndroid();
 }
diff --git a/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-10/lib/gcc/aarch64-unknown-linux-gnu/10/crtbegin.o b/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-10/lib/gcc/aarch64-unknown-linux-gnu/10/crtbegin.o
new file mode 100644
diff --git a/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-10/lib/gcc/aarch64-unknown-linux-gnu/10/libgcc.a b/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-10/lib/gcc/aarch64-unknown-linux-gnu/10/libgcc.a
new file mode 100644
diff --git a/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-7.5.0/lib/gcc/aarch64-unknown-linux-gnu/7.5.0/crtbegin.o b/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-7.5.0/lib/gcc/aarch64-unknown-linux-gnu/7.5.0/crtbegin.o
new file mode 100644
diff --git a/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-7.5.0/lib/gcc/aarch64-unknown-linux-gnu/7.5.0/libgcc.a b/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-7.5.0/lib/gcc/aarch64-unknown-linux-gnu/7.5.0/libgcc.a
new file mode 100644
diff --git a/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-9.3.0/lib/gcc/aarch64-unknown-linux-gnu/9.3.0/crtbegin.o b/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-9.3.0/lib/gcc/aarch64-unknown-linux-gnu/9.3.0/crtbegin.o
new file mode 100644
diff --git a/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-9.3.0/lib/gcc/aarch64-unknown-linux-gnu/9.3.0/libgcc.a b/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-9.3.0/lib/gcc/aarch64-unknown-linux-gnu/9.3.0/libgcc.a
new file mode 100644
diff --git a/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-9.3.1/lib/gcc/aarch64-unknown-linux-gnu/9.3.1/crtbegin.o b/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-9.3.1/lib/gcc/aarch64-unknown-linux-gnu/9.3.1/crtbegin.o
new file mode 100644
diff --git a/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-9.3.1/lib/gcc/aarch64-unknown-linux-gnu/9.3.1/libgcc.a b/clang/test/Driver/Inputs/aarch64-linux-gnu-tree/gcc-9.3.1/lib/gcc/aarch64-unknown-linux-gnu/9.3.1/libgcc.a
new file mode 100644
diff --git a/clang/test/Driver/aarch64-features.c b/clang/test/Driver/aarch64-features.c
--- a/clang/test/Driver/aarch64-features.c
+++ b/clang/test/Driver/aarch64-features.c
@@ -6,3 +6,60 @@
 // The AArch64 PCS states that chars should be unsigned.
 // CHECK: fno-signed-char
 
+// Check for AArch64 out-of-line atomics default settings.
+// RUN: %clang -target aarch64-linux-android -rtlib=compiler-rt \
+// RUN: -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-OUTLINE-ATOMICS-ON %s
+
+// RUN: %clang -target aarch64-linux-gnu -rtlib=compiler-rt \
+// RUN: -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-OUTLINE-ATOMICS-ON %s
+
+// RUN: %clang -target arm64-unknown-linux -rtlib=compiler-rt \
+// RUN: -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-OUTLINE-ATOMICS-ON %s
+
+// RUN: %clang -target aarch64--none-eabi -rtlib=compiler-rt \
+// RUN: -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-OUTLINE-ATOMICS-OFF %s
+
+// RUN: %clang -target aarch64-apple-darwin -rtlib=compiler-rt \
+// RUN: -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-OUTLINE-ATOMICS-OFF %s
+
+// RUN: %clang -target aarch64-windows-gnu -rtlib=compiler-rt \
+// RUN: -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-OUTLINE-ATOMICS-OFF %s
+
+// RUN: %clang -target aarch64-unknown-openbsd -rtlib=compiler-rt \
+// RUN: -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-OUTLINE-ATOMICS-OFF %s
+
+// RUN: %clang -target aarch64-linux-gnu -rtlib=libgcc \
+// RUN: --gcc-toolchain=%S/Inputs/aarch64-linux-gnu-tree/gcc-10 \
+// RUN: -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-OUTLINE-ATOMICS-ON %s
+
+// RUN: %clang -target aarch64-linux-gnu -rtlib=libgcc \
+// RUN: --gcc-toolchain=%S/Inputs/aarch64-linux-gnu-tree/gcc-7.5.0 \
+// RUN: -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-OUTLINE-ATOMICS-OFF %s
+
+// RUN: %clang -target aarch64-linux-gnu -rtlib=libgcc \
+// RUN: --gcc-toolchain=%S/Inputs/aarch64-linux-gnu-tree/gcc-9.3.1 \
+// RUN: -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-OUTLINE-ATOMICS-ON %s
+
+// RUN: %clang -target aarch64-linux-gnu -rtlib=libgcc \
+// RUN: --gcc-toolchain=%S/Inputs/aarch64-linux-gnu-tree/gcc-9.3.0 \
+// RUN: -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-OUTLINE-ATOMICS-OFF %s
+
+// RUN: %clang -target arm64-linux -rtlib=compiler-rt -mno-outline-atomics \
+// RUN: -### -c %s 2>&1 | FileCheck \
+// RUN: -check-prefixes=CHECK-OUTLINE-ATOMICS-OFF,CHECK-NO-OUTLINE-ATOMICS %s
+
+// RUN: %clang -target aarch64-linux-gnu -rtlib=libgcc -mno-outline-atomics \
+// RUN: --gcc-toolchain=%S/Inputs/aarch64-linux-gnu-tree/gcc-10 \
+// RUN: -### -c %s 2>&1 | FileCheck \
+// RUN: -check-prefixes=CHECK-OUTLINE-ATOMICS-OFF,CHECK-NO-OUTLINE-ATOMICS %s
+
+// RUN: %clang -target aarch64-apple-darwin -rtlib=compiler-rt -moutline-atomics \
+// RUN: -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-OUTLINE-ATOMICS-ON %s
+
+// RUN: %clang -target aarch64-windows-gnu -rtlib=libgcc -moutline-atomics \
+// RUN: --gcc-toolchain=%S/Inputs/aarch64-linux-gnu-tree/gcc-7.5.0 \
+// RUN: -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-OUTLINE-ATOMICS-ON %s
+
+// CHECK-OUTLINE-ATOMICS-ON: "-target-feature" "+outline-atomics"
+// CHECK-OUTLINE-ATOMICS-OFF-NOT: "-target-feature" "+outline-atomics"
+// CHECK-NO-OUTLINE-ATOMICS: "-target-feature" "-outline-atomics"