[ELF] Implement Local Dynamic style TLSDESC for x86-64

For the Local Dynamic case of TLSDESC, _TLS_MODULE_BASE_ is defined as a
special TLS symbol that makes:

1) Without relaxation: it produces a dynamic TLSDESC relocation that
computes 0. Adding @dtpoff to access a TLS symbol.
2) With LD->LE relaxation: _TLS_MODULE_BASE_@tpoff = 0 (lowest address in
the TLS block). Adding @tpoff to access a TLS symbol.

For 1), this saves dynamic relocations and GOT slots as otherwise
(General Dynamic) we would create an R_X86_64_TLSDESC and reserve two
GOT slots for each symbol.

Add ElfSym::TlsModuleBase and change the signature of getTlsTpOffset()
to special case _TLS_MODULE_BASE_.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D62577

llvm-svn: 362078

1 files changed, 10 insertions, 6 deletions

diff --git a/lld/ELF/InputSection.cpp b/lld/ELF/InputSection.cpp
index 2ff6b4800b7..45bafd321de 100644
--- a/lld/ELF/InputSection.cpp
+++ b/lld/ELF/InputSection.cpp
@@ -584,24 +584,28 @@ static Relocation *getRISCVPCRelHi20(const Symbol *Sym, uint64_t Addend) {
 
 // A TLS symbol's virtual address is relative to the TLS segment. Add a
 // target-specific adjustment to produce a thread-pointer-relative offset.
-static int64_t getTlsTpOffset() {
+static int64_t getTlsTpOffset(const Symbol &S) {
+  // On targets that support TLSDESC, _TLS_MODULE_BASE_@tpoff = 0.
+  if (&S == ElfSym::TlsModuleBase)
+    return 0;
+
   switch (Config->EMachine) {
   case EM_ARM:
   case EM_AARCH64:
     // Variant 1. The thread pointer points to a TCB with a fixed 2-word size,
     // followed by a variable amount of alignment padding, followed by the TLS
     // segment.
-    return alignTo(Config->Wordsize * 2, Out::TlsPhdr->p_align);
+    return S.getVA(0) + alignTo(Config->Wordsize * 2, Out::TlsPhdr->p_align);
   case EM_386:
   case EM_X86_64:
     // Variant 2. The TLS segment is located just before the thread pointer.
-    return -alignTo(Out::TlsPhdr->p_memsz, Out::TlsPhdr->p_align);
+    return S.getVA(0) - alignTo(Out::TlsPhdr->p_memsz, Out::TlsPhdr->p_align);
   case EM_PPC64:
     // The thread pointer points to a fixed offset from the start of the
     // executable's TLS segment. An offset of 0x7000 allows a signed 16-bit
     // offset to reach 0x1000 of TCB/thread-library data and 0xf000 of the
     // program's TLS segment.
-    return -0x7000;
+    return S.getVA(0) - 0x7000;
   default:
     llvm_unreachable("unhandled Config->EMachine");
   }
@@ -745,12 +749,12 @@ static uint64_t getRelocTargetVA(const InputFile *File, RelType Type, int64_t A,
     // loaders.
     if (Sym.isUndefined())
       return A;
-    return Sym.getVA(A) + getTlsTpOffset();
+    return getTlsTpOffset(Sym) + A;
   case R_RELAX_TLS_GD_TO_LE_NEG:
   case R_NEG_TLS:
     if (Sym.isUndefined())
       return A;
-    return -Sym.getVA(0) - getTlsTpOffset() + A;
+    return -getTlsTpOffset(Sym) + A;
   case R_SIZE:
     return Sym.getSize() + A;
   case R_TLSDESC: