[ELF] Refactor per-target TLS layout configuration. NFC.

[rprichard]

Summary

There are really three different kinds of TLS layouts:

• A fixed TLS-to-TP offset. On architectures like PowerPC, MIPS, and RISC-V, the thread pointer points to a fixed offset from the start of the executable's TLS segment. The offset is 0x7000 for PowerPC and MIPS, which allows a signed 16-bit offset to reach 0x1000 of per-thread implementation data and 0xf000 of the application's TLS segment. The size and layout of the TCB isn't relevant to the static linker and might not be known.
• A fixed TCB size. This is the format documented as "variant 1" in Ulrich Drepper's TLS spec. The thread pointer points to a 2-word TCB followed by the executable's TLS segment. The first word is always the DTV pointer. Used on ARM. The thread pointer must be aligned to the TLS segment's alignment, possibly creating alignment padding.
• Variant 2. This format predates variant 1 and is also documented in Drepper's TLS spec. It allocates the executable's TLS segment before the thread pointer, apparently for backwards-compatibility. It's used on x86 and SPARC.

Factor out an lld::elf::getTlsTpOffset() function for use in a
follow-up patch for Android. The TcbSize/TlsTpOffset fields are only used
in getTlsTpOffset, so replace them with a switch on Config->EMachine.

[rprichard]

This change would make it possible to model RISC-V TLS without the (Config->EMachine == EM_RISCV) special case in https://reviews.llvm.org/D39324 (InputSection.cpp). RISC-V expects a fixed offset of 0.

[jrtc27]

Hmm, I'm not sure I like making TlsTpOffset negative, the name reads as meaning the offset of the thread pointer, which is *positive*, and in general linkers, loaders and libcs like to define it as a positive constant (along with others, such as PPC64TocOffset and DynamicThreadPointerOffset in PPC64.cpp). Personally I'd leave TlsTpOffset as being positive, rename getTlsTpOffset (maybe getTlsOffsetFromTp?) and make it negate TlsTpOffset. Thoughts? Otherwise looks correct to me.

[ruiu]

ELF/InputSection.cpp

572–573

I don't think you need to define TlsLayoutKind. I'd just dispatch based on Config->EMachine in this function.

[jrtc27]

ELF/InputSection.cpp

572–573

That works, and there are certainly other parts of LLD that do things like that, but you could say the same thing about TlsTpOffset and TcbSize themselves... I personally like to see Config->EMachine used as little as possible and put the logic in ELF/Arch/$ARCH.cpp, but of course as maintainer you ultimately decide where to draw the line for when to abstract, and it's not exactly a big deal (though does then disregard the original motivation for this patch).

Also, on a separate issue, the case labels (and bodies) should be deindented one level to be flush with the switch.

[ruiu]

ELF/InputSection.cpp

572–573

TlsTpOffset and TcbSize are just member variables that doesn't have any logic, but TlsLayoutKind is a new thing with which we compute some value, and we use that member variable only in this function. So I think eliminating TlsLayoutKind and directly use Config->EMachine matches the taste of lld's code. I do understand your motivation to write target-dependent code in files under Arch/, but we are not too serious about doing that. We have target-dependent code in many other places if it makes code easier to read.

As to the indentation depth, case should be at the same nesting level as switch in the LLVM coding style.

[rprichard]

ELF/InputSection.cpp

572–573

Using Config->EMachine makes sense to me. I'm wondering if we should keep the TcbSize / TlsTpOffset fields or move them into getTlsTpOffset. I'm leaning toward removing the fields so that all the TP-to-TLS-segment logic is in one place.

[rprichard]

Replace the TargetInfo TLS layout fields with a switch on Config->EMachine.

[ruiu]

LGTM

Nice! The new code is much easier to understand IMO.

[jrtc27]

Yeah, if we're not using a field for which TLS variant the target wants, this is the clearest way to do it, putting all the logic in one place rather than some in the target and some in InputSection.

[ruiu]

ELF/InputSection.cpp

571

I think you can make this a file-scope static function.

[PkmX]

LGTM. I will update D39324 to reflect this change.

[rprichard]

Make getTlsTpOffset a file-scope static function.

[ruiu]

Still LGTM, please commit.

ELF/Arch/AArch64.cpp

AArch64::AArch64() {
   DefaultMaxPageSize = 65536;
 
   // Align to the 2 MiB page size (known as a superpage or huge page).
   // FreeBSD automatically promotes 2 MiB-aligned allocations.
   DefaultImageBase = 0x200000;
 
   // It doesn't seem to be documented anywhere, but tls on aarch64 uses variant
   // 1 of the tls structures and the tcb size is 16.
+  TlsLayout = TlsLayoutKind::FixedTcbSize;
   TcbSize = 16;
   NeedsThunks = true;
 }
 
 RelExpr AArch64::getRelExpr(RelType Type, const Symbol &S,
                             const uint8_t *Loc) const {
   switch (Type) {
   case R_AARCH64_TLSDESC_ADR_PAGE21:

ELF/Arch/ARM.cpp

ARM::ARM() {
   TlsOffsetRel = R_ARM_TLS_DTPOFF32;
   GotBaseSymInGotPlt = false;
   GotEntrySize = 4;
   GotPltEntrySize = 4;
   PltEntrySize = 16;
   PltHeaderSize = 32;
   TrapInstr = 0xd4d4d4d4;
   // ARM uses Variant 1 TLS
+  TlsLayout = TlsLayoutKind::FixedTcbSize;
   TcbSize = 8;
   NeedsThunks = true;
 }
 
 uint32_t ARM::calcEFlags() const {
   // The ABIFloatType is used by loaders to detect the floating point calling
   // convention.
   uint32_t ABIFloatType = 0;

ELF/Arch/PPC64.cpp

PPC64::PPC64() {
   PltEntrySize = 4;
   GotPltEntrySize = 8;
   GotBaseSymInGotPlt = false;
   GotBaseSymOff = 0x8000;
   GotHeaderEntriesNum = 1;
   GotPltHeaderEntriesNum = 2;
   PltHeaderSize = 60;
   NeedsThunks = true;
-  TcbSize = 8;
-  TlsTpOffset = 0x7000;
+  TlsLayout = TlsLayoutKind::FixedOffset;
+  TlsTpOffset = -0x7000;
 
   TlsModuleIndexRel = R_PPC64_DTPMOD64;
   TlsOffsetRel = R_PPC64_DTPREL64;
 
   TlsGotRel = R_PPC64_TPREL64;
 
   NeedsMoreStackNonSplit = false;
 

ELF/Arch/X86.cpp

X86::X86() {
   TlsModuleIndexRel = R_386_TLS_DTPMOD32;
   TlsOffsetRel = R_386_TLS_DTPOFF32;
   GotEntrySize = 4;
   GotPltEntrySize = 4;
   PltEntrySize = 16;
   PltHeaderSize = 16;
   TlsGdRelaxSkip = 2;
   TrapInstr = 0xcccccccc; // 0xcc = INT3
+  TlsLayout = TlsLayoutKind::Variant2;
 
   // Align to the non-PAE large page size (known as a superpage or huge page).
   // FreeBSD automatically promotes large, superpage-aligned allocations.
   DefaultImageBase = 0x400000;
 }
 
 static bool hasBaseReg(uint8_t ModRM) { return (ModRM & 0xc7) != 0x5; }
 

ELF/Arch/X86_64.cpp

template <class ELFT> X86_64<ELFT>::X86_64() {
   TlsModuleIndexRel = R_X86_64_DTPMOD64;
   TlsOffsetRel = R_X86_64_DTPOFF64;
   GotEntrySize = 8;
   GotPltEntrySize = 8;
   PltEntrySize = 16;
   PltHeaderSize = 16;
   TlsGdRelaxSkip = 2;
   TrapInstr = 0xcccccccc; // 0xcc = INT3
+  TlsLayout = TlsLayoutKind::Variant2;
 
   // Align to the large page size (known as a superpage or huge page).
   // FreeBSD automatically promotes large, superpage-aligned allocations.
   DefaultImageBase = 0x200000;
 }
 
 template <class ELFT>
 RelExpr X86_64<ELFT>::getRelExpr(RelType Type, const Symbol &S,

ELF/InputSection.h

private:
 
   template <class ELFT> void copyShtGroup(uint8_t *Buf);
 };
 
 // The list of all input sections.
 extern std::vector<InputSectionBase *> InputSections;
 
 Relocation *getRISCVPCRelHi20(const Symbol *Sym, const uint64_t Addend);
+
+int64_t getTlsTpOffset();
 } // namespace elf
 
 std::string toString(const elf::InputSectionBase *);
 } // namespace lld
 
 #endif

ELF/InputSection.cpp

for (auto It = std::get<0>(Range); It != std::get<1>(Range); ++It)
     if (isRelExprOneOf<R_PC>(It->Expr))
       return &*It;
 
   error("R_RISCV_PCREL_LO12 relocation points to " + IS->getObjMsg(D->Value) +
         " without an associated R_RISCV_PCREL_HI20 relocation");
   return nullptr;
 }
 
+int64_t lld::elf::getTlsTpOffset() {
+  // The TLS symbol's virtual address is relative to the TLS segment. Add a
+  // target-specific adjustment to produce a thread-pointer-relative offset.
+  switch (Target->TlsLayout) {
+    case TlsLayoutKind::FixedOffset:
+      return Target->TlsTpOffset;
+    case TlsLayoutKind::FixedTcbSize:
+      return alignTo(Target->TcbSize, Out::TlsPhdr->p_align);
+    case TlsLayoutKind::Variant2:
+      return -Out::TlsPhdr->p_memsz;
+  }
+}
+
 static uint64_t getRelocTargetVA(const InputFile *File, RelType Type, int64_t A,
                                  uint64_t P, const Symbol &Sym, RelExpr Expr) {
   switch (Expr) {
   case R_INVALID:
     return 0;
   case R_ABS:
   case R_RELAX_TLS_LD_TO_LE_ABS:
   case R_RELAX_GOT_PC_NOPIC:
static uint64_t getRelocTargetVA(const InputFile *File, RelType Type, int64_t A,
   case R_TLS:
     // A weak undefined TLS symbol resolves to the base of the TLS
     // block, i.e. gets a value of zero. If we pass --gc-sections to
     // lld and .tbss is not referenced, it gets reclaimed and we don't
     // create a TLS program header. Therefore, we resolve this
     // statically to zero.
     if (Sym.isTls() && Sym.isUndefWeak())
       return 0;
-
-    // For TLS variant 1 the TCB is a fixed size, whereas for TLS variant 2 the
-    // TCB is on unspecified size and content. Targets that implement variant 1
-    // should set TcbSize.
-    if (Target->TcbSize) {
-      // PPC64 V2 ABI has the thread pointer offset into the middle of the TLS
-      // storage area by TlsTpOffset for efficient addressing TCB and up to
-      // 4KB – 8 B of other thread library information (placed before the TCB).
-      // Subtracting this offset will get the address of the first TLS block.
-      if (Target->TlsTpOffset)
-        return Sym.getVA(A) - Target->TlsTpOffset;
-
-      // If thread pointer is not offset into the middle, the first thing in the
-      // TLS storage area is the TCB. Add the TcbSize to get the address of the
-      // first TLS block.
-      return Sym.getVA(A) + alignTo(Target->TcbSize, Out::TlsPhdr->p_align);
-    }
-    return Sym.getVA(A) - Out::TlsPhdr->p_memsz;
+    return Sym.getVA(A) + getTlsTpOffset();
   case R_RELAX_TLS_GD_TO_LE_NEG:
   case R_NEG_TLS:
     return Out::TlsPhdr->p_memsz - Sym.getVA(A);
   case R_SIZE:
     return Sym.getSize() + A;
   case R_TLSDESC:
     return In.Got->getGlobalDynAddr(Sym) + A;
   case R_TLSDESC_PAGE:

ELF/Target.h

 namespace lld {
 std::string toString(elf::RelType Type);
 
 namespace elf {
 class Defined;
 class InputFile;
 class Symbol;
 
+// To calculate LE-model TLS relocations, the linker must know where the
+// executable's TLS segment is located relative to the thread pointer.
+// Each target has its own method of determining this offset.
+enum class TlsLayoutKind {
+  // The offset is fixed (e.g. -0x7000 for MIPS/PPC).
+  FixedOffset,
+  // The thread pointer points to a TCB with a fixed size, followed by a
+  // variable amount of alignment padding, followed by the TLS segment.
+  FixedTcbSize,
+  // The TLS segment is located just before the thread pointer.
+  Variant2,
+};
+
 class TargetInfo {
 public:
   virtual uint32_t calcEFlags() const { return 0; }
   virtual RelType getDynRel(RelType Type) const { return Type; }
   virtual void writeGotPltHeader(uint8_t *Buf) const {}
   virtual void writeGotHeader(uint8_t *Buf) const {}
   virtual void writeGotPlt(uint8_t *Buf, const Symbol &S) const {};
   virtual void writeIgotPlt(uint8_t *Buf, const Symbol &S) const;
public:
 
   // At least on x86_64 positions 1 and 2 are used by the first plt entry
   // to support lazy loading.
   unsigned GotPltHeaderEntriesNum = 3;
 
   // On PPC ELF V2 abi, the first entry in the .got is the .TOC.
   unsigned GotHeaderEntriesNum = 0;
 
-  // For TLS variant 1, the TCB is a fixed size specified by the Target.
-  // For variant 2, the TCB is an unspecified size.
-  // Set to 0 for variant 2.
-  unsigned TcbSize = 0;
+  TlsLayoutKind TlsLayout = TlsLayoutKind::FixedOffset;
 
-  // Set to the offset (in bytes) that the thread pointer is initialized to
-  // point to, relative to the start of the thread local storage.
-  unsigned TlsTpOffset = 0;
+  // For TlsLayoutKind::FixedOffset, the offset (in bytes) from the thread
+  // pointer to the start of the thread local storage.
+  int TlsTpOffset = 0;
+
+  // For TlsLayoutKind::FixedTcbSize, the size of the TCB in bytes.
+  unsigned TcbSize = 0;
 
   bool NeedsThunks = false;
 
   // A 4-byte field corresponding to one or more trap instructions, used to pad
   // executable OutputSections.
   uint32_t TrapInstr = 0;
 
   // If a target needs to rewrite calls to __morestack to instead call