[AArch64][Clang][Linux] Enable out-of-line atomics by default.

Generate outline atomics if compiling for armv8-a non-LSE AArch64 Linux
(including Android) targets to use LSE instructions, if they are available,
at runtime. Library support is checked by clang driver which doesn't enable
outline atomics if no proper libraries (libgcc >= 9.3.1 or compiler-rt) found.

Differential Revision: https://reviews.llvm.org/D93585

Author: ilinpv

clang/include/clang/Driver/ToolChain.h

@@ -456,6 +456,12 @@ class ToolChain {
   /// 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;
 

clang/lib/Driver/ToolChains/Clang.cpp

@@ -6500,6 +6500,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
       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,

clang/lib/Driver/ToolChains/Linux.cpp

@@ -836,6 +836,19 @@ bool Linux::isPIEDefault() const {
           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();
 }

clang/lib/Driver/ToolChains/Linux.h

@@ -36,6 +36,8 @@ class LLVM_LIBRARY_VISIBILITY Linux : public Generic_ELF {
   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;