[Driver,CodeGen] Support -mtls-dialect= (#79256)

GCC supports -mtls-dialect= for several architectures to select TLSDESC.
This patch supports the following values

* x86: "gnu". "gnu2" (TLSDESC) is not supported yet.
* RISC-V: "trad" (general dynamic), "desc" (TLSDESC, see #66915)

AArch64 toolchains seem to support TLSDESC from the beginning, and the
general dynamic model has poor support. Nobody seems to use the option
-mtls-dialect= at all, so we don't bother with it.
There also seems very little interest in AArch32's TLSDESC support.

TLSDESC does not change IR, but affects object file generation. Without
a backend option the option is a no-op for in-process ThinLTO.

There seems no motivation to have fine-grained control mixing trad/desc
for TLS, so we just pass -mllvm, and don't bother with a modules flag
metadata or function attribute.

Co-authored-by: Paul Kirth <paulkirth@google.com>
(cherry picked from commit 36b4a9ccd9f7e04010476e6b2a311f2052a4ac20)

clang/include/clang/Basic/CodeGenOptions.def

@@ -369,6 +369,9 @@ ENUM_CODEGENOPT(VecLib, llvm::driver::VectorLibrary, 3, llvm::driver::VectorLibr
 /// The default TLS model to use.
 ENUM_CODEGENOPT(DefaultTLSModel, TLSModel, 2, GeneralDynamicTLSModel)
 
+/// Whether to enable TLSDESC. AArch64 enables TLSDESC regardless of this value.
+CODEGENOPT(EnableTLSDESC, 1, 0)
+
 /// Bit size of immediate TLS offsets (0 == use the default).
 VALUE_CODEGENOPT(TLSSize, 8, 0)
 

clang/include/clang/Driver/Options.td

@@ -4419,6 +4419,8 @@ def mtls_size_EQ : Joined<["-"], "mtls-size=">, Group<m_Group>,
   HelpText<"Specify bit size of immediate TLS offsets (AArch64 ELF only): "
            "12 (for 4KB) | 24 (for 16MB, default) | 32 (for 4GB) | 48 (for 256TB, needs -mcmodel=large)">,
   MarshallingInfoInt<CodeGenOpts<"TLSSize">>;
+def mtls_dialect_EQ : Joined<["-"], "mtls-dialect=">, Group<m_Group>,
+  Flags<[TargetSpecific]>, HelpText<"Which thread-local storage dialect to use for dynamic accesses of TLS variables">;
 def mimplicit_it_EQ : Joined<["-"], "mimplicit-it=">, Group<m_Group>;
 def mdefault_build_attributes : Joined<["-"], "mdefault-build-attributes">, Group<m_Group>;
 def mno_default_build_attributes : Joined<["-"], "mno-default-build-attributes">, Group<m_Group>;
@@ -7066,6 +7068,9 @@ def fexperimental_assignment_tracking_EQ : Joined<["-"], "fexperimental-assignme
   Values<"disabled,enabled,forced">, NormalizedValues<["Disabled","Enabled","Forced"]>,
   MarshallingInfoEnum<CodeGenOpts<"AssignmentTrackingMode">, "Enabled">;
 
+def enable_tlsdesc : Flag<["-"], "enable-tlsdesc">,
+  MarshallingInfoFlag<CodeGenOpts<"EnableTLSDESC">>;
+
 } // let Visibility = [CC1Option]
 
 //===----------------------------------------------------------------------===//

clang/lib/CodeGen/BackendUtil.cpp

@@ -401,6 +401,7 @@ static bool initTargetOptions(DiagnosticsEngine &Diags,
   Options.UniqueBasicBlockSectionNames =
       CodeGenOpts.UniqueBasicBlockSectionNames;
   Options.TLSSize = CodeGenOpts.TLSSize;
+  Options.EnableTLSDESC = CodeGenOpts.EnableTLSDESC;
   Options.EmulatedTLS = CodeGenOpts.EmulatedTLS;
   Options.DebuggerTuning = CodeGenOpts.getDebuggerTuning();
   Options.EmitStackSizeSection = CodeGenOpts.StackSizeSection;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5822,6 +5822,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     Args.AddLastArg(CmdArgs, options::OPT_mtls_size_EQ);
   }
 
+  if (isTLSDESCEnabled(TC, Args))
+    CmdArgs.push_back("-enable-tlsdesc");
+
   // Add the target cpu
   std::string CPU = getCPUName(D, Args, Triple, /*FromAs*/ false);
   if (!CPU.empty()) {

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -729,6 +729,33 @@ bool tools::isUseSeparateSections(const llvm::Triple &Triple) {
   return Triple.isPS();
 }
 
+bool tools::isTLSDESCEnabled(const ToolChain &TC,
+                             const llvm::opt::ArgList &Args) {
+  const llvm::Triple &Triple = TC.getEffectiveTriple();
+  Arg *A = Args.getLastArg(options::OPT_mtls_dialect_EQ);
+  if (!A)
+    return Triple.hasDefaultTLSDESC();
+  StringRef V = A->getValue();
+  bool SupportedArgument = false, EnableTLSDESC = false;
+  bool Unsupported = !Triple.isOSBinFormatELF();
+  if (Triple.isRISCV()) {
+    SupportedArgument = V == "desc" || V == "trad";
+    EnableTLSDESC = V == "desc";
+  } else if (Triple.isX86()) {
+    SupportedArgument = V == "gnu";
+  } else {
+    Unsupported = true;
+  }
+  if (Unsupported) {
+    TC.getDriver().Diag(diag::err_drv_unsupported_opt_for_target)
+        << A->getSpelling() << Triple.getTriple();
+  } else if (!SupportedArgument) {
+    TC.getDriver().Diag(diag::err_drv_unsupported_option_argument_for_target)
+        << A->getSpelling() << V << Triple.getTriple();
+  }
+  return EnableTLSDESC;
+}
+
 void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
                           ArgStringList &CmdArgs, const InputInfo &Output,
                           const InputInfo &Input, bool IsThinLTO) {
@@ -988,6 +1015,9 @@ void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
     CmdArgs.push_back(
         Args.MakeArgString(Twine(PluginOptPrefix) + "-emulated-tls"));
   }
+  if (isTLSDESCEnabled(ToolChain, Args))
+    CmdArgs.push_back(
+        Args.MakeArgString(Twine(PluginOptPrefix) + "-enable-tlsdesc"));
 
   if (Args.hasFlag(options::OPT_fstack_size_section,
                    options::OPT_fno_stack_size_section, false))

clang/lib/Driver/ToolChains/CommonArgs.h

@@ -144,6 +144,9 @@ llvm::StringRef getLTOParallelism(const llvm::opt::ArgList &Args,
 bool areOptimizationsEnabled(const llvm::opt::ArgList &Args);
 
 bool isUseSeparateSections(const llvm::Triple &Triple);
+// Parse -mtls-dialect=. Return true if the target supports both general-dynamic
+// and TLSDESC, and TLSDESC is requested.
+bool isTLSDESCEnabled(const ToolChain &TC, const llvm::opt::ArgList &Args);
 
 /// \p EnvVar is split by system delimiter for environment variables.
 /// If \p ArgName is "-I", "-L", or an empty string, each entry from \p EnvVar

clang/test/CodeGen/RISCV/tls-dialect.c

@@ -0,0 +1,14 @@
+// REQUIRES: riscv-registered-target
+/// cc1 -enable-tlsdesc (due to -mtls-dialect=desc) enables TLSDESC.
+// RUN: %clang_cc1 -triple riscv64 -S -mrelocation-model pic -pic-level 1 -enable-tlsdesc %s -o - | FileCheck %s --check-prefix=DESC
+// RUN: %clang_cc1 -triple riscv64 -S -mrelocation-model pic -pic-level 1 %s -o - | FileCheck %s --check-prefix=NODESC
+
+__thread int x;
+
+// DESC:       %tlsdesc_hi
+// DESC-NOT:   %tls_gd_pcrel_hi
+// NODESC:     %tls_gd_pcrel_hi
+// NODESC-NOT: %tlsdesc_hi
+int use() {
+  return x;
+}

clang/test/Driver/tls-dialect.c

@@ -0,0 +1,25 @@
+// RUN: %clang -### --target=riscv64-freebsd -mtls-dialect=desc %s 2>&1 | FileCheck --check-prefix=DESC %s
+// RUN: %clang -### --target=riscv64-linux -mtls-dialect=trad %s 2>&1 | FileCheck --check-prefix=NODESC %s
+// RUN: %clang -### --target=riscv64-linux %s 2>&1 | FileCheck --check-prefix=NODESC %s
+// RUN: %clang -### --target=x86_64-linux -mtls-dialect=gnu %s 2>&1 | FileCheck --check-prefix=NODESC %s
+
+/// LTO
+// RUN: %clang -### --target=riscv64-linux -flto -mtls-dialect=desc %s 2>&1 | FileCheck --check-prefix=LTO-DESC %s
+// RUN: %clang -### --target=riscv64-linux -flto %s 2>&1 | FileCheck --check-prefix=LTO-NODESC %s
+
+/// Unsupported target
+/// GCC supports -mtls-dialect= for AArch64, but we just unsupport it for AArch64 as it is very rarely used.
+// RUN: not %clang --target=aarch64-linux -mtls-dialect=desc %s 2>&1 | FileCheck --check-prefix=UNSUPPORTED-TARGET %s
+// RUN: not %clang --target=x86_64-apple-macos -mtls-dialect=desc -flto %s 2>&1 | FileCheck -check-prefix=UNSUPPORTED-TARGET %s
+
+/// Unsupported argument
+// RUN: not %clang -### --target=riscv64-linux -mtls-dialect=gnu2 %s 2>&1 | FileCheck --check-prefix=UNSUPPORTED-ARG %s
+// RUN: not %clang -### --target=x86_64-linux -mtls-dialect=gnu2 %s 2>&1 | FileCheck --check-prefix=UNSUPPORTED-ARG %s
+
+// DESC:       "-cc1" {{.*}}"-enable-tlsdesc"
+// NODESC-NOT: "-enable-tlsdesc"
+// LTO-DESC:       "-plugin-opt=-enable-tlsdesc"
+// LTO-NODESC-NOT: "-plugin-opt=-enable-tlsdesc"
+
+// UNSUPPORTED-TARGET: error: unsupported option '-mtls-dialect=' for target
+// UNSUPPORTED-ARG: error: unsupported argument 'gnu2' to option '-mtls-dialect=' for target

llvm/include/llvm/TargetParser/Triple.h

@@ -1033,11 +1033,11 @@ public:
            isWindowsCygwinEnvironment() || isOHOSFamily();
   }
 
-  /// Tests whether the target uses TLS Descriptor by default.
+  /// True if the target supports both general-dynamic and TLSDESC, and TLSDESC
+  /// is enabled by default.
   bool hasDefaultTLSDESC() const {
     // TODO: Improve check for other platforms, like Android, and RISC-V
-    // Note: This is currently only used on RISC-V.
-    return isOSBinFormatELF() && isAArch64();
+    return false;
   }
 
   /// Tests whether the target uses -data-sections as default.