[LLD][ELF] Use start of .got.plt as the location for _GLOBAL_OFFSET_TABLE_

[peter.smith]

Summary

For most Targets the _GLOBAL_OFFSET_TABLE_ symbol is expected to be at the start of the .got.plt section so that _GLOBAL_OFFSET_TABLE_[0] = reserved value that is by convention the address of the dynamic section. Previously we had defined _GLOBAL_OFFSET_TABLE_ as either the start or end of the .got section with the intention that the .got.plt section would follow the .got. However this does not always hold with the current default section ordering so _GLOBAL_OFFSET_TABLE_[0] may not be consistent with the reserved first entry of the .got.plt.

In the patch, for X86, X86_64, Mips, Arm and AArch64 I've altered the location of the _GLOBAL_OFFSET_TABLE_ to be the start of the .got.plt section. For Power I've kept it as the ABI defined .got + 0x8000. For all except AArch64 this is consistent with Gold and BFD. They have chosen to use the start of the .got section for _GLOBAL_OFFSET_TABLE_, but I don't know of any software that depends on this, for example glibc/dl-machine.h reserves the same number of entries in the .got.plt as other targets and never directly accesses .got.plt[0] to use it for some other purpose.

Majority of the changes are to the tests as we now have to generate a .got.plt section if the _GLOBAL_OFFSET_TABLE_ symbol is defined rather than the .got.

fixes PR36555 (https://bugs.llvm.org/show_bug.cgi?id=36555)

Fixes PR36555.

[atanasyan]

test/ELF/global-offset-table-position-mips.s

8 ↗

(On Diff #137597)

On MIPS _GLOBAL_OFFSET_TABLE_ should point to the start of the .got section. I do not know any real test case uses this fact, but all GNU tools generates code which follow this rule.

[peter.smith]

Thanks for the comment, and apologies for my misreading of the binutils source. I've updated the patch to keep Mips as it was previously.

[atanasyan]

LGTM

[• rafael]

LGTM

Peter Smith via Phabricator <[e-mail]> writes:

peter.smith updated this revision to Diff 137715.
peter.smith added a comment.

Thanks for the comment, and apologies for my misreading of the binutils source. I've updated the patch to keep Mips as it was previously.

https://reviews.llvm.org/D44259

Files:

ELF/Arch/Mips.cpp
ELF/Arch/PPC.cpp
ELF/Arch/X86.cpp
ELF/Arch/X86_64.cpp
ELF/SyntheticSections.cpp
ELF/SyntheticSections.h
ELF/Target.h
ELF/Writer.cpp
test/ELF/arm-got-relative.s
test/ELF/dynamic-got.s
test/ELF/global-offset-table-position-aarch64.s
test/ELF/global-offset-table-position-arm.s
test/ELF/global-offset-table-position-i386.s
test/ELF/global-offset-table-position.s
test/ELF/global_offset_table_shared.s
test/ELF/got32x-i386.s
test/ELF/i386-gotpc.s

Index: test/ELF/i386-gotpc.s

• test/ELF/i386-gotpc.s

+++ test/ELF/i386-gotpc.s
@@ -6,15 +6,23 @@

movl $_GLOBAL_OFFSET_TABLE_, %eax

+ CHECK: Name: .got.plt
+ CHECK-NEXT: Type: SHT_PROGBITS
+ CHECK-NEXT: Flags [
+ CHECK-NEXT: SHF_ALLOC
+ CHECK-NEXT: SHF_WRITE
+ CHECK-NEXT: ]
+ CHECK-NEXT: Address: 0x2000
+
CHECK: Name: .got
CHECK-NEXT: Type: SHT_PROGBITS
CHECK-NEXT: Flags [
CHECK-NEXT: SHF_ALLOC
CHECK-NEXT: SHF_WRITE
CHECK-NEXT: ]
- CHECK-NEXT: Address: 0x2030
+// CHECK-NEXT: Address: 0x3030

DISASM: Disassembly of section .text:
DISASM-NEXT: .text:
- DISASM-NEXT: 1000: {{.*}} movl $4144, %eax
- 0x2030 - 0x1000 = 4144
+ DISASM-NEXT: 1000: {{.*}} movl $8240, %eax
+ 0x3030 - 0x1000 = 0x2030

Index: test/ELF/got32x-i386.s

• test/ELF/got32x-i386.s

+++ test/ELF/got32x-i386.s
@@ -33,13 +33,13 @@

1. 73728 == 0x12000 == ADDR(.got)
2. CHECK: _start:
3. CHECK-NEXT: 11001: 8b 05 {{.*}} movl 73728, %eax
4. CHECK-NEXT: 11007: 8b 1d {{.*}} movl 73728, %ebx

+# CHECK-NEXT: 11001: 8b 05 {{.*}} movl 77824, %eax
+# CHECK-NEXT: 11007: 8b 1d {{.*}} movl 77824, %ebx

1. CHECK-NEXT: 1100d: 8b 80 {{.*}} movl -4(%eax), %eax
2. CHECK-NEXT: 11013: 8b 83 {{.*}} movl -4(%ebx), %eax
3. CHECK: Sections:
4. CHECK: Name Size Address
5. CHECK: .got 00000004 0000000000012000

+# CHECK: .got 00000004 0000000000013000

1. RUN: not ld.lld %S/Inputs/i386-got32x-baseless.elf -o %t1 -pie 2>&1 | \
2. RUN: FileCheck %s --check-prefix=ERR

Index: test/ELF/global_offset_table_shared.s

• test/ELF/global_offset_table_shared.s

+++ test/ELF/global_offset_table_shared.s
@@ -4,11 +4,11 @@
.long _GLOBAL_OFFSET_TABLE_ - .

CHECK: Name: _GLOBAL_OFFSET_TABLE_
- CHECK-NEXT: Value: 0x2060
+ CHECK-NEXT: Value: 0x2000
CHECK-NEXT: Size: 0
CHECK-NEXT: Binding: Local
CHECK-NEXT: Type: None
CHECK-NEXT: Other [ (0x2)
CHECK-NEXT: STV_HIDDEN (0x2)
CHECK-NEXT: ]
- CHECK-NEXT: Section: .got
+// CHECK-NEXT: Section: .got.plt

Index: test/ELF/global-offset-table-position.s

• test/ELF/global-offset-table-position.s

+++ test/ELF/global-offset-table-position.s
@@ -3,7 +3,8 @@
RUN: llvm-readobj -t %t2 | FileCheck %s
REQUIRES: x86

- The X86_64 _GLOBAL_OFFSET_TABLE_ is defined at the end of the .got section.
+ The X86_64 _GLOBAL_OFFSET_TABLE_ is defined at the start of the .got.plt
+// section.
.globl a
.type a,@object
.comm a,4,4
@@ -21,11 +22,11 @@
.long _GLOBAL_OFFSET_TABLE_ - .

CHECK: Name: _GLOBAL_OFFSET_TABLE_
- CHECK-NEXT: Value: 0x30D8
+ CHECK-NEXT: Value: 0x2008
CHECK-NEXT: Size: 0
CHECK-NEXT: Binding: Local
CHECK-NEXT: Type: None (0x0)
CHECK-NEXT: Other [
CHECK-NEXT: STV_HIDDEN
CHECK-NEXT: ]
- CHECK-NEXT: Section: .got
+// CHECK-NEXT: Section: .got.plt

Index: test/ELF/global-offset-table-position-i386.s

• test/ELF/global-offset-table-position-i386.s

+++ test/ELF/global-offset-table-position-i386.s
@@ -3,7 +3,8 @@
RUN: llvm-readobj -t %t2 | FileCheck %s
REQUIRES: x86

- The X86 _GLOBAL_OFFSET_TABLE_ is defined at the end of the .got section.
+ The X86 _GLOBAL_OFFSET_TABLE_ is defined at the start of the .got.plt
+// section.
.globl a
.type a,@object
.comm a,4,4
@@ -21,11 +22,11 @@
calll f@PLT

CHECK: Name: _GLOBAL_OFFSET_TABLE_ (1)
- CHECK-NEXT: Value: 0x306C
+ CHECK-NEXT: Value: 0x2000
CHECK-NEXT: Size: 0
CHECK-NEXT: Binding: Local (0x0)
CHECK-NEXT: Type: None (0x0)
CHECK-NEXT: Other [ (0x2)
CHECK-NEXT: STV_HIDDEN (0x2)
CHECK-NEXT: ]
- CHECK-NEXT: Section: .got (0xA)
+// CHECK-NEXT: Section: .got.plt

Index: test/ELF/global-offset-table-position-arm.s

• test/ELF/global-offset-table-position-arm.s

+++ test/ELF/global-offset-table-position-arm.s
@@ -3,7 +3,8 @@
RUN: llvm-readobj -t %t2 | FileCheck %s
REQUIRES: arm

- The ARM _GLOBAL_OFFSET_TABLE_ should be defined at the start of the .got
+ The ARM _GLOBAL_OFFSET_TABLE_ should be defined at the start of the
+// .got.plt section.
.globl a
.type a,%object
.comm a,4,4
@@ -25,11 +26,11 @@
.data

CHECK: Name: _GLOBAL_OFFSET_TABLE_
- CHECK-NEXT: Value: 0x3068
+ CHECK-NEXT: Value: 0x2000
CHECK-NEXT: Size: 0
CHECK-NEXT: Binding: Local
CHECK-NEXT: Type: None
CHECK-NEXT: Other [ (0x2)
CHECK-NEXT: STV_HIDDEN (0x2)
CHECK-NEXT: ]
- CHECK-NEXT: Section: .got
+// CHECK-NEXT: Section: .got.plt

Index: test/ELF/global-offset-table-position-aarch64.s

• test/ELF/global-offset-table-position-aarch64.s

+++ test/ELF/global-offset-table-position-aarch64.s
@@ -20,11 +20,11 @@
.long _GLOBAL_OFFSET_TABLE_ - .

CHECK: Name: _GLOBAL_OFFSET_TABLE_ (11)
- CHECK-NEXT: Value: 0x30090
+ CHECK-NEXT: Value: 0x20008
CHECK-NEXT: Size: 0
CHECK-NEXT: Binding: Local (0x0)
CHECK-NEXT: Type: None (0x0)
CHECK-NEXT: Other [ (0x2)
CHECK-NEXT: STV_HIDDEN (0x2)
CHECK-NEXT: ]
- CHECK-NEXT: Section: .got
+// CHECK-NEXT: Section: .got.plt

Index: test/ELF/dynamic-got.s

• test/ELF/dynamic-got.s

+++ test/ELF/dynamic-got.s
@@ -3,6 +3,23 @@
RUN: ld.lld --hash-style=sysv %t.o -o %t.so -shared
RUN: llvm-readobj -s -l -section-data -r %t.so | FileCheck %s

+ CHECK: Name: .got.plt
+ CHECK-NEXT: Type: SHT_PROGBITS
+ CHECK-NEXT: Flags [
+ CHECK-NEXT: SHF_ALLOC
+ CHECK-NEXT: SHF_WRITE
+ CHECK-NEXT: ]
+ CHECK-NEXT: Address:
+ CHECK-NEXT: Offset:
+ CHECK-NEXT: Size:
+ CHECK-NEXT: Link:
+ CHECK-NEXT: Info:
+ CHECK-NEXT: AddressAlignment:
+ CHECK-NEXT: EntrySize:
+ CHECK-NEXT: SectionData (
+ CHECK-NEXT: 0000: 00300000 00000000 00000000
+ CHECK-NEXT: )
+
CHECK: Name: .got
CHECK-NEXT: Type: SHT_PROGBITS
CHECK-NEXT: Flags [
@@ -17,19 +34,19 @@
CHECK-NEXT: AddressAlignment:
CHECK-NEXT: EntrySize:
CHECK-NEXT: SectionData (
- CHECK-NEXT: 0000: 00200000 |
+ CHECK-NEXT: 0000: 00300000
// CHECK-NEXT: )

CHECK: Relocations [
CHECK-NEXT: Section ({{.*}}) .rel.dyn {
- CHECK-NEXT: 0x2050 R_386_RELATIVE - 0x0
+ CHECK-NEXT: 0x3050 R_386_RELATIVE - 0x0
CHECK-NEXT: }
CHECK-NEXT: ]

CHECK: Type: PT_DYNAMIC
- CHECK-NEXT: Offset: 0x2000
- CHECK-NEXT: VirtualAddress: 0x2000
- CHECK-NEXT: PhysicalAddress: 0x2000
+ CHECK-NEXT: Offset: 0x3000
+ CHECK-NEXT: VirtualAddress: 0x3000
+// CHECK-NEXT: PhysicalAddress: 0x3000

calll   .L0$pb

.L0$pb:

Index: test/ELF/arm-got-relative.s

• test/ELF/arm-got-relative.s

+++ test/ELF/arm-got-relative.s
@@ -28,26 +28,26 @@

bx lr

CHECK: Dynamic Relocations {
- CHECK-NEXT: 0x2048 R_ARM_GLOB_DAT function 0x0
+// CHECK-NEXT: 0x3048 R_ARM_GLOB_DAT function 0x0

CHECK: Name: _GLOBAL_OFFSET_TABLE_
- CHECK-NEXT: Value: 0x2048
+ CHECK-NEXT: Value: 0x2000
CHECK-NEXT: Size:
CHECK-NEXT: Binding: Local
CHECK-NEXT: Type: None
CHECK-NEXT: Other [
CHECK-NEXT: STV_HIDDEN
CHECK-NEXT: ]
- CHECK-NEXT: Section: .got
+// CHECK-NEXT: Section: .got.plt

CODE: Disassembly of section .text:
CODE-NEXT: _start:
CODE-NEXT: 1000: 08 30 9f e5 ldr r3, [pc, #8]
CODE-NEXT: 1004: 08 20 9f e5 ldr r2, [pc, #8]
CODE-NEXT: 1008: 03 00 8f e0 add r0, pc, r3
CODE-NEXT: 100c: 1e ff 2f e1 bx lr
CODE:$d.1:
- (_GLOBAL_OFFSET_TABLE_ = 0x2048) - (0x1008 + 8) 0x1038
- CODE-NEXT: 1010: 38 10 00 00
+ (_GLOBAL_OFFSET_TABLE_ = 0x2000) - (0x1008 + 8) = 0xff0
+ CODE-NEXT: 1010: f0 0f 00 00
(Got(function) - GotBase = 0x0
// CODE-NEXT: 1014: 00 00 00 00

Index: ELF/Writer.cpp

• ELF/Writer.cpp

+++ ELF/Writer.cpp
@@ -868,11 +868,12 @@
// defining these symbols explicitly in the linker script.
template <class ELFT> void Writer<ELFT>::setReservedSymbolSections() {

if (ElfSym::GlobalOffsetTable) {

• // The _GLOBAL_OFFSET_TABLE_ symbol is defined by target convention to
• // be at some offset from the base of the .got section, usually 0 or the end
• // of the .got
• InputSection *GotSection = InX::MipsGot ? cast<InputSection>(InX::MipsGot)
• : cast<InputSection>(InX::Got);

+ The _GLOBAL_OFFSET_TABLE_ symbol is defined by target convention usually
+ to the start of the .got or .got.plt section.
+ InputSection *GotSection = InX::GotPlt;
+ if (!Target->GotBaseSymInGotPlt)
+ GotSection = InX::MipsGot ? cast<InputSection>(InX::MipsGot)
+ : cast<InputSection>(InX::Got);

  ElfSym::GlobalOffsetTable->Section = GotSection;
}

Index: ELF/Target.h

• ELF/Target.h

+++ ELF/Target.h
@@ -71,9 +71,10 @@

uint64_t getImageBase();

• // Offset of _GLOBAL_OFFSET_TABLE_ from base of .got section. Use -1 for
• // end of .got

+ // Offset of _GLOBAL_OFFSET_TABLE_ from base of .got or .got.plt section.

uint64_t GotBaseSymOff = 0;

+ // True if _GLOBAL_OFFSET_TABLE_ is relative to .got.plt, false if .got.
+ bool GotBaseSymInGotPlt = true;

// On systems with range extensions we place collections of Thunks at
// regular spacings that enable the majority of branches reach the Thunks.

Index: ELF/SyntheticSections.h

• ELF/SyntheticSections.h

+++ ELF/SyntheticSections.h
@@ -269,7 +269,7 @@

void addEntry(Symbol &Sym);
size_t getSize() const override;
void writeTo(uint8_t *Buf) override;

• bool empty() const override { return Entries.empty(); }

+ bool empty() const override;

private:

std::vector<const Symbol *> Entries;

Index: ELF/SyntheticSections.cpp

• ELF/SyntheticSections.cpp

+++ ELF/SyntheticSections.cpp
@@ -628,8 +628,9 @@
bool GotSection::empty() const {

// We need to emit a GOT even if it's empty if there's a relocation that is
// relative to GOT(such as GOTOFFREL) or there's a symbol that points to a GOT

• // (i.e. _GLOBAL_OFFSET_TABLE_).
• return NumEntries == 0 && !HasGotOffRel && !ElfSym::GlobalOffsetTable;

+ // (i.e. _GLOBAL_OFFSET_TABLE_) that the target defines relative to the .got.
+ return NumEntries == 0 && !HasGotOffRel &&
+ !(ElfSym::GlobalOffsetTable && !Target->GotBaseSymInGotPlt);
}

void GotSection::writeTo(uint8_t *Buf) {
@@ -900,6 +901,14 @@

}

}

+bool GotPltSection::empty() const {
+ We need to emit a GOT.PLT even if it's empty if there's a symbol that
+ references the _GLOBAL_OFFSET_TABLE_ and the Target defines the symbol
+ relative to the .got.plt section.
+ return Entries.empty() &&
+ !(ElfSym::GlobalOffsetTable && Target->GotBaseSymInGotPlt);
+}
+
On ARM the IgotPltSection is part of the GotSection, on other Targets it is
// part of the .got.plt
IgotPltSection::IgotPltSection()

Index: ELF/Arch/X86_64.cpp

• ELF/Arch/X86_64.cpp

+++ ELF/Arch/X86_64.cpp
@@ -51,7 +51,6 @@
} // namespace

template <class ELFT> X86_64<ELFT>::X86_64() {

• GotBaseSymOff = -1; CopyRel = R_X86_64_COPY; GotRel = R_X86_64_GLOB_DAT; PltRel = R_X86_64_JUMP_SLOT;

Index: ELF/Arch/X86.cpp

• ELF/Arch/X86.cpp

+++ ELF/Arch/X86.cpp
@@ -46,7 +46,6 @@
} // namespace

X86::X86() {

• GotBaseSymOff = -1; CopyRel = R_386_COPY; GotRel = R_386_GLOB_DAT; PltRel = R_386_JUMP_SLOT;

Index: ELF/Arch/PPC.cpp

• ELF/Arch/PPC.cpp

+++ ELF/Arch/PPC.cpp
@@ -21,13 +21,18 @@
namespace {
class PPC final : public TargetInfo {
public:

• PPC() { GotBaseSymOff = 0x8000; }

+ PPC();

void relocateOne(uint8_t *Loc, RelType Type, uint64_t Val) const override;
RelExpr getRelExpr(RelType Type, const Symbol &S,
                   const uint8_t *Loc) const override;

};
} // namespace

+PPC::PPC() {
+ GotBaseSymOff = 0x8000;
+ GotBaseSymInGotPlt = false;
+}
+
RelExpr PPC::getRelExpr(RelType Type, const Symbol &S,

                      const uint8_t *Loc) const {
switch (Type) {

Index: ELF/Arch/Mips.cpp

• ELF/Arch/Mips.cpp

+++ ELF/Arch/Mips.cpp
@@ -50,6 +50,7 @@

DefaultMaxPageSize = 65536;
GotEntrySize = sizeof(typename ELFT::uint);
GotPltEntrySize = sizeof(typename ELFT::uint);

+ GotBaseSymInGotPlt = false;

PltEntrySize = 16;
PltHeaderSize = 32;
CopyRel = R_MIPS_COPY;

lld/trunk/ELF/Arch/Mips.cpp

 };
 } // namespace
 
 template <class ELFT> MIPS<ELFT>::MIPS() {
   GotPltHeaderEntriesNum = 2;
   DefaultMaxPageSize = 65536;
   GotEntrySize = sizeof(typename ELFT::uint);
   GotPltEntrySize = sizeof(typename ELFT::uint);
+  GotBaseSymInGotPlt = false;
   PltEntrySize = 16;
   PltHeaderSize = 32;
   CopyRel = R_MIPS_COPY;
   PltRel = R_MIPS_JUMP_SLOT;
   NeedsThunks = true;
   TrapInstr = 0xefefefef;
 
   if (ELFT::Is64Bits) {

lld/trunk/ELF/Arch/PPC.cpp

 using namespace llvm::support::endian;
 using namespace llvm::ELF;
 using namespace lld;
 using namespace lld::elf;
 
 namespace {
 class PPC final : public TargetInfo {
 public:
-  PPC() { GotBaseSymOff = 0x8000; }
+  PPC();
   void relocateOne(uint8_t *Loc, RelType Type, uint64_t Val) const override;
   RelExpr getRelExpr(RelType Type, const Symbol &S,
                      const uint8_t *Loc) const override;
 };
 } // namespace
 
+PPC::PPC() {
+  GotBaseSymOff = 0x8000;
+  GotBaseSymInGotPlt = false;
+}
+
 RelExpr PPC::getRelExpr(RelType Type, const Symbol &S,
                         const uint8_t *Loc) const {
   switch (Type) {
   case R_PPC_REL24:
   case R_PPC_REL32:
     return R_PC;
   case R_PPC_PLTREL24:
     return R_PLT_PC;

lld/trunk/ELF/Arch/X86.cpp

public:
   void relaxTlsGdToIe(uint8_t *Loc, RelType Type, uint64_t Val) const override;
   void relaxTlsGdToLe(uint8_t *Loc, RelType Type, uint64_t Val) const override;
   void relaxTlsIeToLe(uint8_t *Loc, RelType Type, uint64_t Val) const override;
   void relaxTlsLdToLe(uint8_t *Loc, RelType Type, uint64_t Val) const override;
 };
 } // namespace
 
 X86::X86() {
-  GotBaseSymOff = -1;
   CopyRel = R_386_COPY;
   GotRel = R_386_GLOB_DAT;
   PltRel = R_386_JUMP_SLOT;
   IRelativeRel = R_386_IRELATIVE;
   RelativeRel = R_386_RELATIVE;
   TlsGotRel = R_386_TLS_TPOFF;
   TlsModuleIndexRel = R_386_TLS_DTPMOD32;
   TlsOffsetRel = R_386_TLS_DTPOFF32;

lld/trunk/ELF/Arch/X86_64.cpp

 
 private:
   void relaxGotNoPic(uint8_t *Loc, uint64_t Val, uint8_t Op,
                      uint8_t ModRm) const;
 };
 } // namespace
 
 template <class ELFT> X86_64<ELFT>::X86_64() {
-  GotBaseSymOff = -1;
   CopyRel = R_X86_64_COPY;
   GotRel = R_X86_64_GLOB_DAT;
   PltRel = R_X86_64_JUMP_SLOT;
   RelativeRel = R_X86_64_RELATIVE;
   IRelativeRel = R_X86_64_IRELATIVE;
   TlsGotRel = R_X86_64_TPOFF64;
   TlsModuleIndexRel = R_X86_64_DTPMOD64;
   TlsOffsetRel = R_X86_64_DTPOFF64;

lld/trunk/ELF/SyntheticSections.h

 };
 
 class GotPltSection final : public SyntheticSection {
 public:
   GotPltSection();
   void addEntry(Symbol &Sym);
   size_t getSize() const override;
   void writeTo(uint8_t *Buf) override;
-  bool empty() const override { return Entries.empty(); }
+  bool empty() const override;
 
 private:
   std::vector<const Symbol *> Entries;
 };
 
 // The IgotPltSection is a Got associated with the PltSection for GNU Ifunc
 // Symbols that will be relocated by Target->IRelativeRel.
 // On most Targets the IgotPltSection will immediately follow the GotPltSection

lld/trunk/ELF/SyntheticSections.cpp

uint64_t GotSection::getGlobalDynOffset(const Symbol &B) const {
   return B.GlobalDynIndex * Config->Wordsize;
 }
 
 void GotSection::finalizeContents() { Size = NumEntries * Config->Wordsize; }
 
 bool GotSection::empty() const {
   // We need to emit a GOT even if it's empty if there's a relocation that is
   // relative to GOT(such as GOTOFFREL) or there's a symbol that points to a GOT
-  // (i.e. _GLOBAL_OFFSET_TABLE_).
-  return NumEntries == 0 && !HasGotOffRel && !ElfSym::GlobalOffsetTable;
+  // (i.e. _GLOBAL_OFFSET_TABLE_) that the target defines relative to the .got.
+  return NumEntries == 0 && !HasGotOffRel &&
+         !(ElfSym::GlobalOffsetTable && !Target->GotBaseSymInGotPlt);
 }
 
 void GotSection::writeTo(uint8_t *Buf) {
   // Buf points to the start of this section's buffer,
   // whereas InputSectionBase::relocateAlloc() expects its argument
   // to point to the start of the output section.
   relocateAlloc(Buf - OutSecOff, Buf - OutSecOff + Size);
 }
void GotPltSection::writeTo(uint8_t *Buf) {
   Target->writeGotPltHeader(Buf);
   Buf += Target->GotPltHeaderEntriesNum * Target->GotPltEntrySize;
   for (const Symbol *B : Entries) {
     Target->writeGotPlt(Buf, *B);
     Buf += Config->Wordsize;
   }
 }
 
+bool GotPltSection::empty() const {
+  // We need to emit a GOT.PLT even if it's empty if there's a symbol that
+  // references the _GLOBAL_OFFSET_TABLE_ and the Target defines the symbol
+  // relative to the .got.plt section.
+  return Entries.empty() &&
+         !(ElfSym::GlobalOffsetTable && Target->GotBaseSymInGotPlt);
+}
+
 // On ARM the IgotPltSection is part of the GotSection, on other Targets it is
 // part of the .got.plt
 IgotPltSection::IgotPltSection()
     : SyntheticSection(SHF_ALLOC | SHF_WRITE, SHT_PROGBITS,
                        Target->GotPltEntrySize,
                        Config->EMachine == EM_ARM ? ".got" : ".got.plt") {}
 
 void IgotPltSection::addEntry(Symbol &Sym) {

lld/trunk/ELF/Target.h

public:
   virtual ~TargetInfo();
 
   unsigned TlsGdRelaxSkip = 1;
   unsigned PageSize = 4096;
   unsigned DefaultMaxPageSize = 4096;
 
   uint64_t getImageBase();
 
-  // Offset of _GLOBAL_OFFSET_TABLE_ from base of .got section. Use -1 for
-  // end of .got
+  // Offset of _GLOBAL_OFFSET_TABLE_ from base of .got or .got.plt section.
   uint64_t GotBaseSymOff = 0;
+  // True if _GLOBAL_OFFSET_TABLE_ is relative to .got.plt, false if .got.
+  bool GotBaseSymInGotPlt = true;
 
   // On systems with range extensions we place collections of Thunks at
   // regular spacings that enable the majority of branches reach the Thunks.
   uint32_t ThunkSectionSpacing = 0;
 
   RelType CopyRel;
   RelType GotRel;
   RelType PltRel;

lld/trunk/ELF/Writer.cpp

 
 // This function generates assignments for predefined symbols (e.g. _end or
 // _etext) and inserts them into the commands sequence to be processed at the
 // appropriate time. This ensures that the value is going to be correct by the
 // time any references to these symbols are processed and is equivalent to
 // defining these symbols explicitly in the linker script.
 template <class ELFT> void Writer<ELFT>::setReservedSymbolSections() {
   if (ElfSym::GlobalOffsetTable) {
-    // The _GLOBAL_OFFSET_TABLE_ symbol is defined by target convention to
-    // be at some offset from the base of the .got section, usually 0 or the end
-    // of the .got
-    InputSection *GotSection = InX::MipsGot ? cast<InputSection>(InX::MipsGot)
-                                            : cast<InputSection>(InX::Got);
+    // The _GLOBAL_OFFSET_TABLE_ symbol is defined by target convention usually
+    // to the start of the .got or .got.plt section.
+    InputSection *GotSection = InX::GotPlt;
+    if (!Target->GotBaseSymInGotPlt)
+      GotSection = InX::MipsGot ? cast<InputSection>(InX::MipsGot)
     ElfSym::GlobalOffsetTable->Section = GotSection;
   }
 
   PhdrEntry *Last = nullptr;
   PhdrEntry *LastRO = nullptr;
 
   for (PhdrEntry *P : Phdrs) {
     if (P->p_type != PT_LOAD)

lld/trunk/test/ELF/arm-got-relative.s

 
  .globl function
  .align 2
 function:
  .type function, %function
  bx lr
 
 // CHECK: Dynamic Relocations {
-// CHECK-NEXT:  0x2048 R_ARM_GLOB_DAT function 0x0
+// CHECK-NEXT:  0x3048 R_ARM_GLOB_DAT function 0x0
 
 // CHECK: Name: _GLOBAL_OFFSET_TABLE_
-// CHECK-NEXT:    Value: 0x2048
+// CHECK-NEXT:    Value: 0x2000
 // CHECK-NEXT:    Size:
 // CHECK-NEXT:    Binding: Local
 // CHECK-NEXT:    Type: None
 // CHECK-NEXT:    Other [
 // CHECK-NEXT:      STV_HIDDEN
 // CHECK-NEXT:    ]
-// CHECK-NEXT:    Section: .got
+// CHECK-NEXT:    Section: .got.plt
 
 // CODE: Disassembly of section .text:
 // CODE-NEXT: _start:
 // CODE-NEXT:    1000:        08 30 9f e5    ldr     r3, [pc, #8]
 // CODE-NEXT:    1004:        08 20 9f e5    ldr     r2, [pc, #8]
 // CODE-NEXT:    1008:        03 00 8f e0    add     r0, pc, r3
 // CODE-NEXT:    100c:        1e ff 2f e1    bx      lr
 // CODE:$d.1:
-// (_GLOBAL_OFFSET_TABLE_ = 0x2048) - (0x1008 + 8) 0x1038
-// CODE-NEXT:    1010:        38 10 00 00
+// (_GLOBAL_OFFSET_TABLE_ = 0x2000) - (0x1008 + 8) = 0xff0
+// CODE-NEXT:    1010:        f0 0f 00 00
 // (Got(function) - GotBase = 0x0
 // CODE-NEXT:    1014:        00 00 00 00

lld/trunk/test/ELF/dynamic-got.s

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=i386-pc-linux %s -o %t.o
 // RUN: ld.lld --hash-style=sysv %t.o -o %t.so -shared
 // RUN: llvm-readobj -s -l -section-data -r %t.so | FileCheck %s
 
+// CHECK:     Name: .got.plt
+// CHECK-NEXT:     Type: SHT_PROGBITS
+// CHECK-NEXT:     Flags [
+// CHECK-NEXT:       SHF_ALLOC
+// CHECK-NEXT:       SHF_WRITE
+// CHECK-NEXT:     ]
+// CHECK-NEXT:     Address:
+// CHECK-NEXT:     Offset:
+// CHECK-NEXT:     Size:
+// CHECK-NEXT:     Link:
+// CHECK-NEXT:     Info:
+// CHECK-NEXT:     AddressAlignment:
+// CHECK-NEXT:     EntrySize:
+// CHECK-NEXT:     SectionData (
+// CHECK-NEXT:       0000: 00300000 00000000 00000000
+// CHECK-NEXT:     )
+
 // CHECK:      Name: .got
 // CHECK-NEXT: Type: SHT_PROGBITS
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC
 // CHECK-NEXT:   SHF_WRITE
 // CHECK-NEXT: ]
 // CHECK-NEXT: Address:
 // CHECK-NEXT: Offset:
 // CHECK-NEXT: Size:
 // CHECK-NEXT: Link:
 // CHECK-NEXT: Info:
 // CHECK-NEXT: AddressAlignment:
 // CHECK-NEXT: EntrySize:
 // CHECK-NEXT: SectionData (
-// CHECK-NEXT:   0000: 00200000                |
+// CHECK-NEXT:   0000: 00300000
 // CHECK-NEXT: )
 
 // CHECK:      Relocations [
 // CHECK-NEXT:   Section ({{.*}}) .rel.dyn {
-// CHECK-NEXT:     0x2050 R_386_RELATIVE - 0x0
+// CHECK-NEXT:     0x3050 R_386_RELATIVE - 0x0
 // CHECK-NEXT:   }
 // CHECK-NEXT: ]
 
 // CHECK:      Type: PT_DYNAMIC
-// CHECK-NEXT: Offset: 0x2000
-// CHECK-NEXT: VirtualAddress: 0x2000
-// CHECK-NEXT: PhysicalAddress: 0x2000
+// CHECK-NEXT: Offset: 0x3000
+// CHECK-NEXT: VirtualAddress: 0x3000
+// CHECK-NEXT: PhysicalAddress: 0x3000
 
         calll   .L0$pb
 .L0$pb:
         popl    %eax
 .Ltmp0:
         addl    $_GLOBAL_OFFSET_TABLE_+(.Ltmp0-.L0$pb), %eax
         movl    _DYNAMIC@GOT(%eax), %eax

lld/trunk/test/ELF/global-offset-table-position-aarch64.s

 .global _start
 .type _start,@function
 _start:
  bl f
 .data
 .long _GLOBAL_OFFSET_TABLE_ - .
 
 // CHECK: Name: _GLOBAL_OFFSET_TABLE_ (11)
-// CHECK-NEXT:     Value: 0x30090
+// CHECK-NEXT:     Value: 0x20008
 // CHECK-NEXT:     Size: 0
 // CHECK-NEXT:     Binding: Local (0x0)
 // CHECK-NEXT:     Type: None (0x0)
 // CHECK-NEXT:     Other [ (0x2)
 // CHECK-NEXT:       STV_HIDDEN (0x2)
 // CHECK-NEXT:     ]
-// CHECK-NEXT:     Section: .got
+// CHECK-NEXT:     Section: .got.plt

lld/trunk/test/ELF/global-offset-table-position-arm.s

 // RUN: llvm-mc -filetype=obj -triple=armv7a-linux-gnueabihf %s -o %t
 // RUN: ld.lld --hash-style=sysv -shared %t -o %t2
 // RUN: llvm-readobj -t %t2 | FileCheck %s
 // REQUIRES: arm
 
-// The ARM _GLOBAL_OFFSET_TABLE_ should be defined at the start of the .got
+// The ARM _GLOBAL_OFFSET_TABLE_ should be defined at the start of the
+// .got.plt section.
 .globl  a
 .type   a,%object
 .comm   a,4,4
 
 .globl  f
 .type   f,%function
 f:
  ldr r2, .L1
 .L0:
  add r2, pc
 .L1:
 .word _GLOBAL_OFFSET_TABLE_ - (.L0+4)
 .word a(GOT)
 
 .global _start
 .type _start,%function
 _start:
  bl f
 .data
 
 // CHECK:     Name: _GLOBAL_OFFSET_TABLE_
-// CHECK-NEXT:     Value: 0x3068
+// CHECK-NEXT:     Value: 0x2000
 // CHECK-NEXT:     Size: 0
 // CHECK-NEXT:     Binding: Local
 // CHECK-NEXT:     Type: None
 // CHECK-NEXT:     Other [ (0x2)
 // CHECK-NEXT:       STV_HIDDEN (0x2)
 // CHECK-NEXT:     ]
-// CHECK-NEXT:     Section: .got
+// CHECK-NEXT:     Section: .got.plt

lld/trunk/test/ELF/global-offset-table-position-i386.s

 // RUN: llvm-mc -filetype=obj -triple=i386-pc-linux %s -o %t
 // RUN: ld.lld --hash-style=sysv -shared %t -o %t2
 // RUN: llvm-readobj -t %t2 | FileCheck %s
 // REQUIRES: x86
 
-// The X86 _GLOBAL_OFFSET_TABLE_ is defined at the end of the .got section.
+// The X86 _GLOBAL_OFFSET_TABLE_ is defined at the start of the .got.plt
+// section.
 .globl  a
 .type   a,@object
 .comm   a,4,4
 
 .globl  f
 .type   f,@function
 f:
 addl    $_GLOBAL_OFFSET_TABLE_, %eax
 movl    a@GOT(%eax), %eax
 
 .global _start
 .type _start,@function
 _start:
 addl    $_GLOBAL_OFFSET_TABLE_, %eax
 calll   f@PLT
 
 // CHECK:     Name: _GLOBAL_OFFSET_TABLE_ (1)
-// CHECK-NEXT:     Value: 0x306C
+// CHECK-NEXT:     Value: 0x2000
 // CHECK-NEXT:     Size: 0
 // CHECK-NEXT:     Binding: Local (0x0)
 // CHECK-NEXT:     Type: None (0x0)
 // CHECK-NEXT:     Other [ (0x2)
 // CHECK-NEXT:       STV_HIDDEN (0x2)
 // CHECK-NEXT:     ]
-// CHECK-NEXT:     Section: .got (0xA)
+// CHECK-NEXT:     Section: .got.plt

lld/trunk/test/ELF/global-offset-table-position.s

 // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t
 // RUN: ld.lld --hash-style=sysv -shared %t -o %t2
 // RUN: llvm-readobj -t %t2 | FileCheck %s
 // REQUIRES: x86
 
-// The X86_64 _GLOBAL_OFFSET_TABLE_ is defined at the end of the .got section.
+// The X86_64 _GLOBAL_OFFSET_TABLE_ is defined at the start of the .got.plt
+// section.
 .globl  a
 .type   a,@object
 .comm   a,4,4
 
 .globl  f
 .type   f,@function
 f:
 movq  a@GOTPCREL(%rip), %rax
 
 .global _start
 .type _start,@function
 _start:
 callq f@PLT
 .data
 .long _GLOBAL_OFFSET_TABLE_ - .
 
 // CHECK:     Name: _GLOBAL_OFFSET_TABLE_
-// CHECK-NEXT:     Value: 0x30D8
+// CHECK-NEXT:     Value: 0x2008
 // CHECK-NEXT:     Size: 0
 // CHECK-NEXT:     Binding: Local
 // CHECK-NEXT:     Type: None (0x0)
 // CHECK-NEXT:     Other [
 // CHECK-NEXT:       STV_HIDDEN
 // CHECK-NEXT:     ]
-// CHECK-NEXT:     Section: .got
+// CHECK-NEXT:     Section: .got.plt

lld/trunk/test/ELF/global_offset_table_shared.s

 // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t
 // RUN: ld.lld --hash-style=sysv -shared %t -o %t2
 // RUN: llvm-readobj -t %t2 | FileCheck %s
 .long _GLOBAL_OFFSET_TABLE_ - .
 
 // CHECK:      Name: _GLOBAL_OFFSET_TABLE_
-// CHECK-NEXT: Value: 0x2060
+// CHECK-NEXT: Value: 0x2000
 // CHECK-NEXT: Size: 0
 // CHECK-NEXT: Binding: Local
 // CHECK-NEXT: Type: None
 // CHECK-NEXT: Other [ (0x2)
 // CHECK-NEXT: STV_HIDDEN (0x2)
 // CHECK-NEXT:    ]
-// CHECK-NEXT: Section: .got
+// CHECK-NEXT: Section: .got.plt

lld/trunk/test/ELF/got32x-i386.s

 ## register, it should be calculated as G + A. Using without base register
 ## is only allowed for non-PIC code.
 ##
 # RUN: ld.lld %S/Inputs/i386-got32x-baseless.elf -o %t1
 # RUN: llvm-objdump -section-headers -d %t1 | FileCheck %s
 
 ## 73728 == 0x12000 == ADDR(.got)
 # CHECK:       _start:
-# CHECK-NEXT:   11001: 8b 05 {{.*}} movl 73728, %eax
-# CHECK-NEXT:   11007: 8b 1d {{.*}} movl 73728, %ebx
+# CHECK-NEXT:   11001: 8b 05 {{.*}} movl 77824, %eax
+# CHECK-NEXT:   11007: 8b 1d {{.*}} movl 77824, %ebx
 # CHECK-NEXT:   1100d: 8b 80 {{.*}} movl -4(%eax), %eax
 # CHECK-NEXT:   11013: 8b 83 {{.*}} movl -4(%ebx), %eax
 # CHECK: Sections:
 # CHECK:  Name Size     Address
-# CHECK:  .got 00000004 0000000000012000
+# CHECK:  .got 00000004 0000000000013000
 
 # RUN: not ld.lld %S/Inputs/i386-got32x-baseless.elf -o %t1 -pie 2>&1 | \
 # RUN:   FileCheck %s --check-prefix=ERR
 # ERR: error: can't create dynamic relocation R_386_GOT32X against symbol: foo in readonly segment; recompile object files with -fPIC
 # ERR: error: can't create dynamic relocation R_386_GOT32X against symbol: foo in readonly segment; recompile object files with -fPIC

lld/trunk/test/ELF/i386-gotpc.s

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=i686-pc-linux %s -o %t.o
 // RUN: ld.lld --hash-style=sysv %t.o -o %t.so -shared
 // RUN: llvm-readobj -s %t.so | FileCheck %s
 // RUN: llvm-objdump -d %t.so | FileCheck --check-prefix=DISASM %s
 
 movl $_GLOBAL_OFFSET_TABLE_, %eax
 
+// CHECK:     Name: .got.plt
+// CHECK-NEXT: Type: SHT_PROGBITS
+// CHECK-NEXT: Flags [
+// CHECK-NEXT:   SHF_ALLOC
+// CHECK-NEXT:   SHF_WRITE
+// CHECK-NEXT: ]
+// CHECK-NEXT: Address: 0x2000
+
 // CHECK:      Name: .got
 // CHECK-NEXT: Type: SHT_PROGBITS
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC
 // CHECK-NEXT:   SHF_WRITE
 // CHECK-NEXT: ]
-// CHECK-NEXT: Address: 0x2030
+// CHECK-NEXT: Address: 0x3030
 
 // DISASM:      Disassembly of section .text:
 // DISASM-NEXT: .text:
-// DISASM-NEXT:    1000: {{.*}}         movl    $4144, %eax
-//                                              0x2030 - 0x1000 = 4144
+// DISASM-NEXT:    1000: {{.*}}         movl    $8240, %eax
+//                                              0x3030 - 0x1000 = 0x2030