[android] Use emulated thread-local storage for API 28 and earlier

Android before API 29 and a few other platforms don't support native TLS, so
fall back to LLVM's emulated TLS there, just like clang does. Also, make sure
`-Xcc -f{no-,}emulated-tls` flags passed in are also applied to control what
the Swift compiler does.

Author: finagolfin

include/swift/AST/IRGenOptions.h

@@ -551,6 +551,10 @@ class IRGenOptions {
   /// Emit a .casid file next to the object file if CAS Backend is used.
   bool EmitCASIDFile = false;
 
+  /// Tell the LLVM codegen backend to use emulated Thread-Local Storage instead
+  /// of native TLS
+  bool UseEmulatedTLS = false;
+
   /// Paths to the pass plugins registered via -load-pass-plugin.
   std::vector<std::string> LLVMPassPlugins;
 

lib/Frontend/CompilerInvocation.cpp

@@ -3143,10 +3143,14 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
     Opts.CoveragePrefixMap.addMapping(SplitMap.first, SplitMap.second);
   }
 
+  Opts.UseEmulatedTLS = Triple.hasDefaultEmulatedTLS();
   for (const Arg *A : Args.filtered(OPT_Xcc)) {
     StringRef Opt = A->getValue();
     if (Opt.starts_with("-D") || Opt.starts_with("-U"))
       Opts.ClangDefines.push_back(Opt.str());
+
+    if (Opt == "-femulated-tls" || Opt == "-fno-emulated-tls")
+      Opts.UseEmulatedTLS = Opt == "-femulated-tls";
   }
 
   for (const Arg *A : Args.filtered(OPT_l, OPT_framework)) {

lib/IRGen/IRGen.cpp

@@ -148,6 +148,8 @@ swift::getIRTargetOptions(const IRGenOptions &Opts, ASTContext &Ctx) {
   if (Clang->getTargetInfo().getTriple().isOSBinFormatWasm())
     TargetOpts.ThreadModel = llvm::ThreadModel::Single;
 
+  TargetOpts.EmulatedTLS = Opts.UseEmulatedTLS;
+
   if (Opts.EnableGlobalISel) {
     TargetOpts.EnableGlobalISel = true;
     TargetOpts.GlobalISelAbort = GlobalISelAbortMode::DisableWithDiag;

test/IRGen/Inputs/tls.h

@@ -0,0 +1,7 @@
+#include "shims/SwiftStdint.h"
+
+static inline __swift_uint32_t _swift_stdlib_gettid() {
+  static __thread __swift_uint32_t tid = 0;
+
+  return tid;
+}

test/IRGen/emulated-tls.swift

@@ -0,0 +1,7 @@
+// RUN: %target-swift-frontend -Xcc -femulated-tls %s -S -import-objc-header %S/Inputs/tls.h | %FileCheck %s --check-prefix=EMUTLS
+// RUN: %target-swift-frontend -Xcc -fno-emulated-tls %s -S -import-objc-header %S/Inputs/tls.h | %FileCheck %s --check-prefix=NOEMUTLS
+
+_swift_stdlib_gettid()
+
+// EMUTLS: __emutls_get_address
+// NOEMUTLS-NOT: __emutls_get_address