[Driver,CodeGen] Support -mtls-dialect=

[MaskRay]

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 [RISCV] Support Global Dynamic TLSDESC in the RISC-V backend #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 [email protected]

[llvmbot]

@llvm/pr-subscribers-clang-driver
@llvm/pr-subscribers-clang-codegen

@llvm/pr-subscribers-clang

Author: Fangrui Song (MaskRay)

Changes

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.

Co-authored-by: Paul Kirth <[email protected]>

---

Full diff: https://github.com/llvm/llvm-project/pull/79256.diff

6 Files Affected:

• (modified) clang/include/clang/Basic/CodeGenOptions.def (+3)
• (modified) clang/include/clang/Driver/Options.td (+5)
• (modified) clang/lib/CodeGen/BackendUtil.cpp (+1)
• (modified) clang/lib/Driver/ToolChains/Clang.cpp (+23)
• (added) clang/test/CodeGen/RISCV/tls-dialect.c (+13)
• (added) clang/test/Driver/tls-dialect.c (+19)

diff --git a/clang/include/clang/Basic/CodeGenOptions.def b/clang/include/clang/Basic/CodeGenOptions.def
index 2f2e45d5cf63dfa..7c0bfe328496147 100644
--- a/clang/include/clang/Basic/CodeGenOptions.def
+++ b/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)
 
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
index 7f4fa33748facaf..773bc1dcda01d5c 100644
--- a/clang/include/clang/Driver/Options.td
+++ b/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]
 
 //===----------------------------------------------------------------------===//
diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index ec203f6f28bc173..7877e20d77f7724 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/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;
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 5dc614e11aab599..93fd579eb92ba50 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -5822,6 +5822,29 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
     Args.AddLastArg(CmdArgs, options::OPT_mtls_size_EQ);
   }
 
+  if (Arg *A = Args.getLastArg(options::OPT_mtls_dialect_EQ)) {
+    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) {
+      D.Diag(diag::err_drv_unsupported_opt_for_target)
+          << A->getSpelling() << TripleStr;
+    } else if (!SupportedArgument) {
+      D.Diag(diag::err_drv_unsupported_option_argument_for_target)
+          << A->getSpelling() << V << TripleStr;
+    } else if (EnableTLSDESC) {
+      CmdArgs.push_back("-enable-tlsdesc");
+    }
+  }
+
   // Add the target cpu
   std::string CPU = getCPUName(D, Args, Triple, /*FromAs*/ false);
   if (!CPU.empty()) {
diff --git a/clang/test/CodeGen/RISCV/tls-dialect.c b/clang/test/CodeGen/RISCV/tls-dialect.c
new file mode 100644
index 000000000000000..5a1bfa539321269
--- /dev/null
+++ b/clang/test/CodeGen/RISCV/tls-dialect.c
@@ -0,0 +1,13 @@
+/// 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;
+}
diff --git a/clang/test/Driver/tls-dialect.c b/clang/test/Driver/tls-dialect.c
new file mode 100644
index 000000000000000..d90200998e3b06b
--- /dev/null
+++ b/clang/test/Driver/tls-dialect.c
@@ -0,0 +1,19 @@
+// 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
+
+/// 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 %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"
+
+// UNSUPPORTED-TARGET: error: unsupported option '-mtls-dialect=' for target
+// UNSUPPORTED-ARG: error: unsupported argument 'gnu2' to option '-mtls-dialect=' for target

[efriedma-quic]

What's the expected interaction here with LTO? Modifying TargetOptions has no effect if we're generating a bitcode file.

[MaskRay]

What's the expected interaction here with LTO? Modifying TargetOptions has no effect if we're generating a bitcode file.

Thanks for reminding me this. There seems no motivation to mix trad/desc (so modules flag metadata or function attribute would be overkill), so I think we can do it the -ffunction-sections/-femulated-tls way by passing an option to the linker...

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[ilovepi]

LGTM