[LDC] Custom TLS emulation for Android targets

The Bionic C library ignores thread-local data stored in the normal .tbss/.tdata
ELF sections, which are marked with the SHF_TLS/STT_TLS flags. LDC rolls its own
emulated TLS scheme for Android instead, by keeping TLS data in the .tdata/.tbss
sections but removing the SHF_TLS/STT_TLS flags, and replacing direct access to
these globals by a call to __tls_get_addr() (implemented in druntime's
rt.sections_android). The function is expected to translate an address in the
TLS static data to the corresponding address in the actual TLS dynamic
per-thread data.

Author: kinke

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -4525,6 +4525,9 @@ class TargetLowering : public TargetLoweringBase {
   virtual SDValue LowerToTLSEmulatedModel(const GlobalAddressSDNode *GA,
                                           SelectionDAG &DAG) const;
 
+  SDValue LowerToAndroidEmulatedTLSAddress(SDValue Op, SDValue Result,
+                                           SelectionDAG &DAG, bool is64bit) const; // LDC
+
   /// Expands target specific indirect branch for the case of JumpTable
   /// expanasion.
   virtual SDValue expandIndirectJTBranch(const SDLoc& dl, SDValue Value, SDValue Addr,

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -7572,6 +7572,33 @@ SDValue TargetLowering::getVectorElementPointer(SelectionDAG &DAG,
   return DAG.getMemBasePlusOffset(VecPtr, Index, dl);
 }
 
+SDValue
+TargetLowering::LowerToAndroidEmulatedTLSAddress(SDValue Op, SDValue Result,
+                SelectionDAG &DAG, bool is64bit) const { // LDC
+  SDLoc DL(Op);
+  SDValue Chain = DAG.getEntryNode();
+  ArgListTy Args;
+  ArgListEntry Entry;
+  Type *Ty;
+  if (is64bit)
+    Ty = (Type *)Type::getInt64Ty(*DAG.getContext());
+  else
+    Ty = (Type *)Type::getInt32Ty(*DAG.getContext());
+  Entry.Node = Result;
+  Entry.Ty = Ty;
+  Args.push_back(Entry);
+
+  // copied, modified from ARMTargetLowering::LowerToTLSGeneralDynamicModel
+  TargetLowering::CallLoweringInfo CLI(DAG);
+  CLI.setDebugLoc(DL).setChain(Chain).setLibCallee(
+      CallingConv::C, Ty,
+      DAG.getExternalSymbol("__tls_get_addr",
+                            getPointerTy(DAG.getDataLayout())),
+      std::move(Args));
+  std::pair<SDValue, SDValue> CallResult = LowerCallTo(CLI);
+  return CallResult.first;
+}
+
 //===----------------------------------------------------------------------===//
 // Implementation of Emulated TLS Model
 //===----------------------------------------------------------------------===//

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -490,7 +490,7 @@ static unsigned getELFSectionType(StringRef Name, SectionKind K) {
   return ELF::SHT_PROGBITS;
 }
 
-static unsigned getELFSectionFlags(SectionKind K) {
+static unsigned getELFSectionFlags(SectionKind K, const Triple &TargetTriple) {
   unsigned Flags = 0;
 
   if (!K.isMetadata())
@@ -505,7 +505,7 @@ static unsigned getELFSectionFlags(SectionKind K) {
   if (K.isWriteable())
     Flags |= ELF::SHF_WRITE;
 
-  if (K.isThreadLocal())
+  if (K.isThreadLocal() && !TargetTriple.isAndroid()) // LDC
     Flags |= ELF::SHF_TLS;
 
   if (K.isMergeableCString() || K.isMergeableConst())
@@ -669,7 +669,7 @@ MCSection *TargetLoweringObjectFileELF::getExplicitSectionGlobal(
   Kind = getELFKindForNamedSection(SectionName, Kind);
 
   StringRef Group = "";
-  unsigned Flags = getELFSectionFlags(Kind);
+  unsigned Flags = getELFSectionFlags(Kind, getTargetTriple());
   if (const Comdat *C = getELFComdat(GO)) {
     Group = C->getName();
     Flags |= ELF::SHF_GROUP;
@@ -793,7 +793,7 @@ static MCSectionELF *selectELFSectionForGlobal(
 
 MCSection *TargetLoweringObjectFileELF::SelectSectionForGlobal(
     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
-  unsigned Flags = getELFSectionFlags(Kind);
+  unsigned Flags = getELFSectionFlags(Kind, getTargetTriple());
 
   // If we have -ffunction-section or -fdata-section then we should emit the
   // global value to a uniqued section specifically for it.

llvm/lib/MC/MCELFStreamer.cpp

@@ -464,7 +464,12 @@ void MCELFStreamer::fixSymbolsInTLSFixups(const MCExpr *expr) {
       break;
     }
     getAssembler().registerSymbol(symRef.getSymbol());
-    cast<MCSymbolELF>(symRef.getSymbol()).setType(ELF::STT_TLS);
+    // LDC
+    {
+      auto ofi = getContext().getObjectFileInfo();
+      if (!(ofi && ofi->getTargetTriple().isAndroid()))
+        cast<MCSymbolELF>(symRef.getSymbol()).setType(ELF::STT_TLS);
+    }
     break;
   }
 

llvm/lib/MC/MCObjectFileInfo.cpp

@@ -367,12 +367,14 @@ void MCObjectFileInfo::initELFMCObjectFileInfo(const Triple &T, bool Large) {
   ReadOnlySection =
       Ctx->getELFSection(".rodata", ELF::SHT_PROGBITS, ELF::SHF_ALLOC);
 
-  TLSDataSection =
-      Ctx->getELFSection(".tdata", ELF::SHT_PROGBITS,
-                         ELF::SHF_ALLOC | ELF::SHF_TLS | ELF::SHF_WRITE);
+  // LDC
+  const auto tlsFlag = (!getTargetTriple().isAndroid() ? ELF::SHF_TLS : 0);
 
-  TLSBSSSection = Ctx->getELFSection(
-      ".tbss", ELF::SHT_NOBITS, ELF::SHF_ALLOC | ELF::SHF_TLS | ELF::SHF_WRITE);
+  TLSDataSection = Ctx->getELFSection(
+      ".tdata", ELF::SHT_PROGBITS, ELF::SHF_ALLOC | tlsFlag | ELF::SHF_WRITE);
+
+  TLSBSSSection = Ctx->getELFSection(".tbss", ELF::SHT_NOBITS,
+                                     ELF::SHF_ALLOC | tlsFlag | ELF::SHF_WRITE);
 
   DataRelROSection = Ctx->getELFSection(".data.rel.ro", ELF::SHT_PROGBITS,
                                         ELF::SHF_ALLOC | ELF::SHF_WRITE);

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -6270,8 +6270,12 @@ SDValue AArch64TargetLowering::LowerGlobalTLSAddress(SDValue Op,
 
   if (Subtarget->isTargetDarwin())
     return LowerDarwinGlobalTLSAddress(Op, DAG);
-  if (Subtarget->isTargetELF())
-    return LowerELFGlobalTLSAddress(Op, DAG);
+  if (Subtarget->isTargetELF()) {
+    if (Subtarget->isTargetAndroid())
+      return LowerToAndroidEmulatedTLSAddress(Op, LowerGlobalAddress(Op, DAG), DAG, true); // LDC
+    else
+      return LowerELFGlobalTLSAddress(Op, DAG);
+  }
   if (Subtarget->isTargetWindows())
     return LowerWindowsGlobalTLSAddress(Op, DAG);
 

llvm/lib/Target/AArch64/MCTargetDesc/AArch64MCExpr.cpp

@@ -13,7 +13,9 @@
 
 #include "AArch64MCExpr.h"
 #include "llvm/BinaryFormat/ELF.h"
+#include "llvm/MC/MCAssembler.h" // LDC
 #include "llvm/MC/MCContext.h"
+#include "llvm/MC/MCObjectFileInfo.h" // LDC
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSymbolELF.h"
 #include "llvm/MC/MCValue.h"
@@ -130,7 +132,12 @@ static void fixELFSymbolsInTLSFixupsImpl(const MCExpr *Expr, MCAssembler &Asm) {
     // We're known to be under a TLS fixup, so any symbol should be
     // modified. There should be only one.
     const MCSymbolRefExpr &SymRef = *cast<MCSymbolRefExpr>(Expr);
-    cast<MCSymbolELF>(SymRef.getSymbol()).setType(ELF::STT_TLS);
+    // LDC
+    {
+      auto ofi = Asm.getContext().getObjectFileInfo();
+      if (!(ofi && ofi->getTargetTriple().isAndroid()))
+        cast<MCSymbolELF>(SymRef.getSymbol()).setType(ELF::STT_TLS);
+    }
     break;
   }
 

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -3534,6 +3534,8 @@ ARMTargetLowering::LowerGlobalTLSAddress(SDValue Op, SelectionDAG &DAG) const {
 
   // TODO: implement the "local dynamic" model
   assert(Subtarget->isTargetELF() && "Only ELF implemented here");
+  if (Subtarget->isTargetAndroid())
+    return LowerToAndroidEmulatedTLSAddress(Op, LowerGlobalAddress(Op, DAG), DAG, false); // LDC
   TLSModel::Model model = getTargetMachine().getTLSModel(GA->getGlobal());
 
   switch (model) {

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -19366,6 +19366,9 @@ X86TargetLowering::LowerGlobalTLSAddress(SDValue Op, SelectionDAG &DAG) const {
   bool PositionIndependent = isPositionIndependent();
 
   if (Subtarget.isTargetELF()) {
+    if (Subtarget.isTargetAndroid())
+      return LowerToAndroidEmulatedTLSAddress(Op, LowerGlobalAddress(Op, DAG), DAG, Subtarget.is64Bit()); // LDC
+
     TLSModel::Model model = DAG.getTarget().getTLSModel(GV);
     switch (model) {
       case TLSModel::GeneralDynamic: