Add emulated TLS support

Currently LLVM uses emutls by default
for some targets (such as android, openbsd),
but rust does not use it, because `has_thread_local` is false.

This commit has some changes to allow users to enable emutls:

1. add `-Zhas-thread-local` flag to specify
    that std uses `#[thread_local]` instead of pthread key.
2. when using emutls, decorate symbol names
    to find thread local symbol correctly.
3. change `-Zforce-emulated-tls` to `-Ztls-model=emulated`
    to explicitly specify whether to generate emutls.

compiler/rustc_codegen_llvm/src/back/owned_target_machine.rs

@@ -39,7 +39,7 @@ impl OwnedTargetMachine {
         split_dwarf_file: &CStr,
         output_obj_file: &CStr,
         debug_info_compression: &CStr,
-        force_emulated_tls: bool,
+        use_emulated_tls: bool,
         args_cstr_buff: &[u8],
     ) -> Result<Self, LlvmError<'static>> {
         assert!(args_cstr_buff.len() > 0);
@@ -71,7 +71,7 @@ impl OwnedTargetMachine {
                 split_dwarf_file.as_ptr(),
                 output_obj_file.as_ptr(),
                 debug_info_compression.as_ptr(),
-                force_emulated_tls,
+                use_emulated_tls,
                 args_cstr_buff.as_ptr() as *const c_char,
                 args_cstr_buff.len(),
             )

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -33,7 +33,7 @@ use rustc_session::config::{self, Lto, OutputType, Passes, SplitDwarfKind, Switc
 use rustc_session::Session;
 use rustc_span::symbol::sym;
 use rustc_span::InnerSpan;
-use rustc_target::spec::{CodeModel, RelocModel, SanitizerSet, SplitDebuginfo};
+use rustc_target::spec::{CodeModel, RelocModel, SanitizerSet, SplitDebuginfo, TlsModel};
 
 use crate::llvm::diagnostic::OptimizationDiagnosticKind;
 use libc::{c_char, c_int, c_uint, c_void, size_t};
@@ -223,7 +223,7 @@ pub fn target_machine_factory(
 
     let path_mapping = sess.source_map().path_mapping().clone();
 
-    let force_emulated_tls = sess.target.force_emulated_tls;
+    let use_emulated_tls = matches!(sess.tls_model(), TlsModel::Emulated);
 
     // copy the exe path, followed by path all into one buffer
     // null terminating them so we can use them as null terminated strings
@@ -297,7 +297,7 @@ pub fn target_machine_factory(
             &split_dwarf_file,
             &output_obj_file,
             &debuginfo_compression,
-            force_emulated_tls,
+            use_emulated_tls,
             &args_cstr_buff,
         )
     })

compiler/rustc_codegen_llvm/src/context.rs

@@ -120,6 +120,7 @@ fn to_llvm_tls_model(tls_model: TlsModel) -> llvm::ThreadLocalMode {
         TlsModel::LocalDynamic => llvm::ThreadLocalMode::LocalDynamic,
         TlsModel::InitialExec => llvm::ThreadLocalMode::InitialExec,
         TlsModel::LocalExec => llvm::ThreadLocalMode::LocalExec,
+        TlsModel::Emulated => llvm::ThreadLocalMode::GeneralDynamic,
     }
 }
 

compiler/rustc_codegen_llvm/src/lib.rs

@@ -306,7 +306,9 @@ impl CodegenBackend for LlvmCodegenBackend {
             }
             PrintKind::TlsModels => {
                 writeln!(out, "Available TLS models:");
-                for name in &["global-dynamic", "local-dynamic", "initial-exec", "local-exec"] {
+                for name in
+                    &["global-dynamic", "local-dynamic", "initial-exec", "local-exec", "emulated"]
+                {
                     writeln!(out, "    {name}");
                 }
                 writeln!(out);

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2159,7 +2159,7 @@ extern "C" {
         SplitDwarfFile: *const c_char,
         OutputObjFile: *const c_char,
         DebugInfoCompression: *const c_char,
-        ForceEmulatedTls: bool,
+        UseEmulatedTls: bool,
         ArgsCstrBuff: *const c_char,
         ArgsCstrBuffLen: usize,
     ) -> *mut TargetMachine;