[ARM][LLD] .ARM.exidx support for executables/shared libraries

[peter.smith]

Summary

This is the first part of a series of patches derived from https://reviews.llvm.org/D24728

Apologies for the delay in posting this, I wanted to wait for Phab to come back up.

In this patch support for .ARM.exidx sections has been added for Executables and Shared Libraries. The majority of the code is identical to the changes discussed in D24788 with the following differences:

• Garbage collection is fully implemented and tested, .ARM.exidx is live only when the section it depends on is live.
• The SHF_LINK_ORDER flag on the .ARM.exidx OutputSection is cleared for Executables and Shared libraries as it only has meaning in a relocatable object.
• I've moved the code to find the link order dependency to InputSection from OutputSection as MarkLive needs it, and in a future patch for relocatable links the OutputSection::finalize() will also need it.

Follow up patches to come shortly:

• Implement R_ARM_TARGET2 relocation.
• Allow R_ARM_PREL31 and R_ARM_NONE relocations to be used in shared objects.
• Maintain the full section name when doing a relocatable link, for example .text.foo should not be truncated and merged with other .text sections.
• Add support for .ARM.exidx for relocatable links.

[peter.smith]

Updated diff to fix spurious extra newline, apologies for not catching first time.

[ruiu]

Generally looking good, a few minor comments.

ELF/InputSection.cpp

125

typename ELFT::Shdr -> ELF_Shdr

ELF/MarkLive.cpp

257–272

Don't you have to repeat this process until it converges? I wonder if forEachSuccessor() makes more sections live, which in turn makes more .ARM.exidx sections live.

ELF/OutputSections.cpp

882–883

This is equivalent to this.

if (!Config->Relocatable)
  this->Header.sh_flags &= ~SHF_LINK_ORDER;

ELF/Writer.cpp

1409

Exidx -> exidx

1448

Remove the blank line.

1482

Remove {}

1483

OS -> *OS

[peter.smith]

Thanks for the comments. I've applied all suggested changes.

[ruiu]

ELF/MarkLive.cpp

259

I wonder if there's a better way to do this. How about adding

TinyPtrVector<InputSection<ELFT> *> Successors;

to InputSection<ELFT> class, adding reverse edges from .ARM.exidx sections to other sections to that vector as a preprocess, then handle that vector elements as successors in forEachSucessor? Then it is guranteed to converge by calling forEachSucessor<ELFT>(*Q.pop_back_val(), Enqueue) just once.

[peter.smith]

Split out garbage collection changes from this patch on Rafael's request which didn't get copied into the review:
"Can you split this further?

It would be nice to have a patch taking care of only which sections
are gced. That is, it just makes sure that we keep only the sections
that refer to a kept section.

Cheers,
Rafael"

The only differences to the previous version are the removal of changes to MarkLive.cpp and the removal of the corresponding test that depended on it.

I'll work on Rui's suggestion for garbage collection in a separate review. Will hopefully have something later today.

Hope I've interpreted everyone correctly.

[ruiu]

Do you think you can reduce the testcase for this patch too?

ELF/Writer.cpp

1447

Sort -> sort

[peter.smith]

Thanks for the comments. I've made the case change to the function name and removed unnecessary assembly language directives from the tests.

I'll wait for this one before committing the GC change in D25234

[ruiu]

LGTM

[peter.smith]

Committed revision 283730

Happy to make further changes if needed. The patch above applies with only line number offset differences and already has the garbage collection changes split out.

ELF/MarkLive.cpp

257–272

Sadly yes for one annoying case. The personality routines such as __gxx_personality_v0 that do the unwinding are only referenced from .ARM.extab sections and in at least the gcc libraries these routines have .ARM.exidx. I've updated the code and test, which to my shame illustrated that the .ARM.exidx entries for my personality routines were being removed.

In the general case .ARM.exidx only has relocations to one executable section (the one it has a link order dependency on). It may refer to a .ARM.extab section which contains unwinding instructions that won't fit into 4 bytes. The .ARM.extab section may refer to personality routines which are the functions that interpret the data, but no other executable sections.

ELF/InputSection.h

public:
   // Returns the size of this section (even if this is a common or BSS.)
   size_t getSize() const;
 
   static InputSectionBase<ELFT> Discarded;
 
   const Elf_Shdr *getSectionHdr() const { return Header; }
   ObjectFile<ELFT> *getFile() const { return File; }
   uintX_t getOffset(const DefinedRegular<ELFT> &Sym) const;
-
+  InputSectionBase *getLinkOrderDep() const;
   // Translate an offset in the input section to an offset in the output
   // section.
   uintX_t getOffset(uintX_t Offset) const;
 
   void uncompress();
 
   void relocate(uint8_t *Buf, uint8_t *BufEnd);
 };

ELF/InputSection.cpp

 }
 
 template <class ELFT>
 typename ELFT::uint
 InputSectionBase<ELFT>::getOffset(const DefinedRegular<ELFT> &Sym) const {
   return getOffset(Sym.Value);
 }
 
-template <class ELFT>
+template<class ELFT>
+InputSectionBase<ELFT>*
+InputSectionBase<ELFT>::getLinkOrderDep() const {
+  const typename ELFT::Shdr *Hdr = getSectionHdr();
+  if ((Hdr->sh_flags & SHF_LINK_ORDER) && Hdr->sh_link != 0)
+    return getFile()->getSections()[Hdr->sh_link];
+  return nullptr;
+}
+
+template <class ELFT>
 InputSection<ELFT>::InputSection(elf::ObjectFile<ELFT> *F,
                                  const Elf_Shdr *Header, StringRef Name)
     : InputSectionBase<ELFT>(F, Header, Name, Base::Regular) {}
 
 template <class ELFT>
 bool InputSection<ELFT>::classof(const InputSectionBase<ELFT> *S) {
   return S->kind() == Base::Regular;
 }

ELF/MarkLive.cpp

for (InputSectionBase<ELFT> *Sec : F->getSections()) {
           Enqueue({Sec, 0});
       }
     }
   }
 
   // Mark all reachable sections.
   while (!Q.empty())
     forEachSuccessor<ELFT>(*Q.pop_back_val(), Enqueue);
+
+  // Mark all .ARM.exidx sections that reference live sections
+  if (Config->EMachine == EM_ARM) {
+    for (ObjectFile<ELFT> *F : Symtab<ELFT>::X->getObjectFiles()) {
+      for (InputSectionBase<ELFT> *Sec : F->getSections()) {
+        if (Sec && Sec != &InputSection<ELFT>::Discarded &&
+            Sec->Name.startswith(".ARM.exidx")) {
+          auto* D = Sec->getLinkOrderDep();
+          if (D && D->Live)
+            Enqueue({Sec, 0});
+        }
+      }
+    }
+    // Mark all reachable sections from live .ARM.exidx
+    while (!Q.empty())
+      forEachSuccessor<ELFT>(*Q.pop_back_val(), Enqueue);
+  }
 }
 
 template void elf::markLive<ELF32LE>();
 template void elf::markLive<ELF32BE>();
 template void elf::markLive<ELF64LE>();
 template void elf::markLive<ELF64BE>();

ELF/OutputSections.cpp

OutputSection<ELFT>::OutputSection(StringRef Name, uint32_t Type, uintX_t Flags)
   if (Type == SHT_RELA)
     this->Header.sh_entsize = sizeof(Elf_Rela);
   else if (Type == SHT_REL)
     this->Header.sh_entsize = sizeof(Elf_Rel);
 }
 
 template <class ELFT> void OutputSection<ELFT>::finalize() {
   uint32_t Type = this->Header.sh_type;
+  // SHF_LINK_ORDER only has meaning in relocatable objects
+  if (!Config->Relocatable && (this->Header.sh_flags & SHF_LINK_ORDER))
+    this->Header.sh_flags &= ~SHF_LINK_ORDER;
   if (Type != SHT_RELA && Type != SHT_REL)
     return;
   this->Header.sh_link = Out<ELFT>::SymTab->SectionIndex;
   // sh_info for SHT_REL[A] sections should contain the section header index of
   // the section to which the relocation applies.
   InputSectionBase<ELFT> *S = Sections[0]->getRelocatedSection();
   this->Header.sh_info = S->OutSec->SectionIndex;
 }

ELF/Writer.cpp

 #include "llvm/Support/FileOutputBuffer.h"
 #include "llvm/Support/StringSaver.h"
 #include "llvm/Support/raw_ostream.h"
 #include <climits>
 
 using namespace llvm;
 using namespace llvm::ELF;
 using namespace llvm::object;
+using namespace llvm::support;
+using namespace llvm::support::endian;
 
 using namespace lld;
 using namespace lld::elf;
 
 namespace {
 // The writer writes a SymbolTable result to a file.
 template <class ELFT> class Writer {
 public:
std::vector<PhdrEntry<ELFT>> Writer<ELFT>::createPhdrs() {
   if (!ScriptConfig->HasSections) {
     Load->add(Out<ELFT>::ElfHeader);
     Load->add(Out<ELFT>::ProgramHeaders);
   }
 
   Phdr TlsHdr(PT_TLS, PF_R);
   Phdr RelRo(PT_GNU_RELRO, PF_R);
   Phdr Note(PT_NOTE, PF_R);
+  Phdr ARMExidx(PT_ARM_EXIDX, PF_R);
   for (OutputSectionBase<ELFT> *Sec : OutputSections) {
     if (!(Sec->getFlags() & SHF_ALLOC))
       break;
 
     // If we meet TLS section then we create TLS header
     // and put all TLS sections inside for further use when
     // assign addresses.
     if (Sec->getFlags() & SHF_TLS)
for (OutputSectionBase<ELFT> *Sec : OutputSections) {
     }
 
     Load->add(Sec);
 
     if (isRelroSection(Sec))
       RelRo.add(Sec);
     if (Sec->getType() == SHT_NOTE)
       Note.add(Sec);
+    if (Config->EMachine == EM_ARM && Sec->getType() == SHT_ARM_EXIDX)
+      ARMExidx.add(Sec);
   }
 
   // Add the TLS segment unless it's empty.
   if (TlsHdr.First)
     Ret.push_back(std::move(TlsHdr));
 
   // Add an entry for .dynamic.
   if (Out<ELFT>::DynSymTab) {
     Phdr &H = *AddHdr(PT_DYNAMIC, Out<ELFT>::Dynamic->getPhdrFlags());
     H.add(Out<ELFT>::Dynamic);
   }
 
   // PT_GNU_RELRO includes all sections that should be marked as
   // read-only by dynamic linker after proccessing relocations.
   if (RelRo.First)
     Ret.push_back(std::move(RelRo));
 
   // PT_GNU_EH_FRAME is a special section pointing on .eh_frame_hdr.
   if (!Out<ELFT>::EhFrame->empty() && Out<ELFT>::EhFrameHdr) {
     Phdr &Hdr = *AddHdr(PT_GNU_EH_FRAME, Out<ELFT>::EhFrameHdr->getPhdrFlags());
     Hdr.add(Out<ELFT>::EhFrameHdr);
   }
 
+  // PT_ARM_EXIDX is the ARM EHABI equivalent of PT_GNU_EH_FRAME
+  if (ARMExidx.First)
+    Ret.push_back(std::move(ARMExidx));
+
   // PT_GNU_STACK is a special section to tell the loader to make the
   // pages for the stack non-executable.
   if (!Config->ZExecStack) {
     Phdr &Hdr = *AddHdr(PT_GNU_STACK, PF_R | PF_W);
     if (Config->ZStackSize != uint64_t(-1))
       Hdr.H.p_memsz = Config->ZStackSize;
   }
 

 
 template <class ELFT> void Writer<ELFT>::writeSectionsBinary() {
   uint8_t *Buf = Buffer->getBufferStart();
   for (OutputSectionBase<ELFT> *Sec : OutputSections)
     if (Sec->getFlags() & SHF_ALLOC)
       Sec->writeTo(Buf + Sec->getFileOff());
 }
 
+// Convert the .ARM.Exidx table entries that use relative PREL31 offsets to
+// Absolute addresses. This form is internal to LLD and is only used to
+// make reordering the table simpler.
+static void ARMExidxEntryPrelToAbs(uint8_t *Loc, uint64_t EntryVA) {
+  uint64_t Addr = Target->getImplicitAddend(Loc, R_ARM_PREL31) + EntryVA;
+  bool InlineEntry =
+      (read32le(Loc + 4) == 1 || (read32le(Loc + 4) & 0x80000000));
+  if (InlineEntry)
+    // Set flag in unused bit of code address so that when we convert back we
+    // know which table entries to leave alone.
+    Addr |= 0x1;
+  else
+    write32le(Loc + 4,
+              Target->getImplicitAddend(Loc + 4, R_ARM_PREL31) + EntryVA + 4);
+  write32le(Loc, Addr);
+}
+
+// Convert the .ARM.exidx table entries from the internal to LLD form using
+// absolute addresses back to relative PREL31 offsets.
+static void ARMExidxEntryAbsToPrel(uint8_t *Loc, uint64_t EntryVA) {
+  uint64_t Off = read32le(Loc) - EntryVA;
+  // ARMExidxEntryPreltoAbs sets bit 0 if the table entry has inline data
+  // that is not an address
+  bool InlineEntry = Off & 0x1;
+  Target->relocateOne(Loc, R_ARM_PREL31, Off & ~0x1);
+  if (!InlineEntry)
+    Target->relocateOne(Loc + 4, R_ARM_PREL31,
+                        read32le(Loc + 4) - (EntryVA + 4));
+}
+
+// The table formed by the .ARM.exidx OutputSection has entries with two
+// 4-byte fields:
+// | PREL31 offset to function | Action to take for function |
+// The table must be ordered in ascending virtual address of the functions
+// identified by the first field of the table. Instead of using the
+// SHF_LINK_ORDER dependency to reorder the sections prior to relocation we
+// sort the table post-relocation.
+// Ref: Exception handling ABI for the ARM architecture
+static void SortARMExidx(uint8_t *Buf, uint64_t OutSecVA, uint64_t Size) {
+
+  struct ARMExidxEntry {
+    ulittle32_t Target;
+    ulittle32_t Action;
+  };
+  ARMExidxEntry *Start = (ARMExidxEntry *)Buf;
+  size_t NumEnt = Size / sizeof(ARMExidxEntry);
+  for (uint64_t Off = 0; Off < Size; Off += 8)
+    ARMExidxEntryPrelToAbs(Buf + Off, OutSecVA + Off);
+  std::stable_sort(Start, Start + NumEnt,
+                   [](const ARMExidxEntry &A, const ARMExidxEntry &B) {
+                     return A.Target < B.Target;
+                   });
+  for (uint64_t Off = 0; Off < Size; Off += 8)
+    ARMExidxEntryAbsToPrel(Buf + Off, OutSecVA + Off);
+}
+
 // Write section contents to a mmap'ed file.
 template <class ELFT> void Writer<ELFT>::writeSections() {
   uint8_t *Buf = Buffer->getBufferStart();
 
   // PPC64 needs to process relocations in the .opd section
   // before processing relocations in code-containing sections.
   Out<ELFT>::Opd = findSection(".opd");
   if (Out<ELFT>::Opd) {
     Out<ELFT>::OpdBuf = Buf + Out<ELFT>::Opd->getFileOff();
     Out<ELFT>::Opd->writeTo(Buf + Out<ELFT>::Opd->getFileOff());
   }
 
   for (OutputSectionBase<ELFT> *Sec : OutputSections)
     if (Sec != Out<ELFT>::Opd && Sec != Out<ELFT>::EhFrameHdr)
       Sec->writeTo(Buf + Sec->getFileOff());
 
+  OutputSectionBase<ELFT> *ARMExidx = findSection(".ARM.exidx");
+  if (!Config->Relocatable) {
+    if (auto OS = dyn_cast_or_null<OutputSection<ELFT>>(ARMExidx))
+      SortARMExidx(Buf + OS->getFileOff(), OS->getVA(), OS->getSize());
+  }
+
   // The .eh_frame_hdr depends on .eh_frame section contents, therefore
   // it should be written after .eh_frame is written.
   if (!Out<ELFT>::EhFrame->empty() && Out<ELFT>::EhFrameHdr)
     Out<ELFT>::EhFrameHdr->writeTo(Buf + Out<ELFT>::EhFrameHdr->getFileOff());
 }
 
 template <class ELFT> void Writer<ELFT>::writeBuildId() {
   if (!Out<ELFT>::BuildId)

test/ELF/Inputs/arm-exidx-cantunwind.s

+// Functions that will generate a .ARM.exidx section with SHF_LINK_ORDER
+// dependency on the progbits section containing the .cantunwind directive
+ .syntax unified
+ .section .func1, "ax",%progbits
+ .globl func1
+ .align 2
+ .type func1,%function
+func1:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+ .section .func2, "ax", %progbits
+ .globl func2
+ .align 2
+ .type func2,%function
+func2:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+ .section .func3, "ax",%progbits
+ .globl func3
+ .align 2
+ .type func3,%function
+func3:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+ .section .text, "ax",%progbits
+ .globl func4
+ .align 2
+ .type func4,%function
+func4:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+ .globl func5
+ .align 2
+ .type func5,%function
+func5:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend

test/ELF/arm-exidx-canunwind.s

+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t
+// RUN: ld.lld %t -o %t2 2>&1
+// RUN: llvm-objdump -d -triple=armv7a-none-linux-gnueabi %t2 | FileCheck %s
+// RUN: llvm-objdump -s -triple=armv7a-none-linux-gnueabi %t2 | FileCheck -check-prefix=CHECK-EXIDX %s
+// RUN: llvm-readobj --program-headers --sections %t2 | FileCheck -check-prefix=CHECK-PT %s
+// REQUIRES: arm
+
+// Test that inline unwinding table entries and references to .ARM.extab
+// entries survive the re-ordering of the .ARM.exidx section
+
+ .syntax unified
+ // Will produce an ARM.exidx entry with inline unwinding instructions
+ .section .text.func1, "ax",%progbits
+ .align 2
+ .global func1
+ .type func1, %function
+func1:
+ .fnstart
+ bx lr
+ .save {r7, lr}
+ .setfp r7, sp, #0
+ .fnend
+
+ // Unwinding instructions for .text2 too large for an inline entry ARM.exidx
+ // entry. A separate .ARM.extab section is created to hold the unwind entries
+ // The .ARM.exidx table entry has a reference to the .ARM.extab section.
+ .section .text.func2, "ax",%progbits
+ .align 2
+ .global func2
+ .type func2, %function
+func2:
+ .fnstart
+ .save {r7, lr}
+ .setfp r7, sp
+ bx lr
+ .personality __gxx_personality_v0
+ .handlerdata
+ .align 2
+ .byte 0xff
+ .byte 0x90
+ .uleb128 0x16
+ .byte 0x1
+ .uleb128 0x32
+ .uleb128 0
+ .byte 0x1
+ .byte 0
+ .section .text.func2
+ .fnend
+
+ // Dummy implementation of personality routines to satisfy reference from
+ // exception tables
+ .section .text.__gcc_personality_v0, "ax", %progbits
+ .global __gxx_personality_v0
+ .type __gxx_personality_v0, %function
+__gxx_personality_v0:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+ .section .text.__aeabi_unwind_cpp_pr0, "ax", %progbits
+ .global __aeabi_unwind_cpp_pr0
+ .type __aeabi_unwind_cpp_pr0, %function
+__aeabi_unwind_cpp_pr0:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+ .text
+ .global _start
+ .type _start, %function
+_start:
+ .fnstart
+ .cantunwind
+ bl func1
+ bl func2
+ bx lr
+ .fnend
+// CHECK: Disassembly of section .text:
+// CHECK-NEXT: _start:
+// CHECK-NEXT:    11000:       01 00 00 eb     bl      #4 <func1>
+// CHECK-NEXT:    11004:       01 00 00 eb     bl      #4 <func2>
+// CHECK-NEXT:    11008:       1e ff 2f e1     bx      lr
+// CHECK:      func1:
+// CHECK-NEXT:    1100c:       1e ff 2f e1     bx      lr
+// CHECK:      func2:
+// CHECK-NEXT:    11010:       1e ff 2f e1     bx      lr
+// CHECK:      __gxx_personality_v0:
+// CHECK-NEXT:    11014:       1e ff 2f e1     bx      lr
+// CHECK:      __aeabi_unwind_cpp_pr0:
+// CHECK-NEXT:    11018:       1e ff 2f e1     bx      lr
+
+// CHECK-EXIDX: Contents of section .ARM.exidx:
+// 100d4 + f2c = 11000 = _start (cantunwind)
+// 100dc + f30 = 1100c = func1 (Inline unwind 80978408)
+// CHECK-EXIDX-NEXT:  100d4 2c0f0000 01000000 300f0000 08849780
+// 100e4 + f2c = 11010 = func2 (EHT 100e8 + 14 = 100fc = extab entry for func2)
+// 100ec + f28 = 11014 = __gxx_personality_v0 (cantunwind)
+// CHECK-EXIDX-NEXT:  100e4 2c0f0000 14000000 280f0000 01000000
+// 100f4 + f24 = 11018 = __aeabi_unwind_cpp_pr0 (cantunwind)
+// CHECK-EXIDX-NEXT:  100f4 240f0000 01000000
+// CHECK-EXIDX-NEXT: Contents of section .ARM.extab.text.func2:
+// CHECK-EXIDX-NEXT:  100fc 180f0000 08849700 ff901601 32000100
+
+// CHECK-PT:          Name: .ARM.exidx
+// CHECK-PT-NEXT:     Type: SHT_ARM_EXIDX (0x70000001)
+// CHECK-PT-NEXT:     Flags [ (0x2)
+// CHECK-PT-NEXT:       SHF_ALLOC (0x2)
+// CHECK-PT-NEXT:     ]
+// CHECK-PT-NEXT:     Address: 0x100D4
+// CHECK-PT-NEXT:     Offset: 0xD4
+// CHECK-PT-NEXT:     Size: 40
+
+// CHECK-PT:          Type: PT_ARM_EXIDX (0x70000001)
+// CHECK-PT-NEXT:     Offset: 0xD4
+// CHECK-PT-NEXT:     VirtualAddress: 0x100D4
+// CHECK-PT-NEXT:     PhysicalAddress: 0x100D4
+// CHECK-PT-NEXT:     FileSize: 40
+// CHECK-PT-NEXT:     MemSize: 40
+// CHECK-PT-NEXT:     Flags [ (0x4)
+// CHECK-PT-NEXT:       PF_R (0x4)
+// CHECK-PT-NEXT:     ]
+// CHECK-PT-NEXT:     Alignment: 4
+// CHECK-PT-NEXT:   }

test/ELF/arm-exidx-gc.s

+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t
+// RUN: ld.lld %t -o %t2 --gc-sections 2>&1
+// RUN: llvm-objdump -d -triple=armv7a-none-linux-gnueabi %t2 | FileCheck %s
+// RUN: llvm-objdump -s -triple=armv7a-none-linux-gnueabi %t2 | FileCheck -check-prefix=CHECK-EXIDX %s
+// REQUIRES: arm
+
+// Test the behavior of .ARM.exidx sections under garbage collection
+// A .ARM.exidx section is live if it has a relocation to a live executable
+// section.
+// A .ARM.exidx section may have a relocation to a .ARM.extab section, if the
+// .ARM.exidx is live then the .ARM.extab section is live
+
+ .syntax unified
+ .section .text.func1, "ax",%progbits
+ .align 2
+ .global func1
+ .type func1, %function
+func1:
+ .fnstart
+ bx lr
+ .save {r7, lr}
+ .setfp r7, sp, #0
+ .fnend
+
+ .section .text.unusedfunc1, "ax",%progbits
+ .align 2
+ .global unusedfunc1
+ .type unusedfunc1, %function
+unusedfunc1:
+ .fnstart
+ bx lr
+ .save {r7, lr}
+ .setfp r7, sp, #0
+ .fnend
+
+ // Unwinding instructions for .text2 too large for an inline entry ARM.exidx
+ // entry. A separate .ARM.extab section is created to hold the unwind entries
+ // The .ARM.exidx table entry has a reference to the .ARM.extab section.
+ .section .text.func2, "ax",%progbits
+ .align 2
+ .global func2
+ .type func2, %function
+func2:
+ .fnstart
+ .save {r7, lr}
+ .setfp r7, sp
+ bx lr
+ .personality __gxx_personality_v0
+ .handlerdata
+ .align 2
+ .byte 0xff
+ .byte 0x90
+ .uleb128 0x16
+ .byte 0x1
+ .uleb128 0x32
+ .uleb128 0
+ .byte 0x1
+ .byte 0
+ .section .text.func2
+ .fnend
+
+ // An unused function with a reference to a .ARM.extab section. Both should
+ // be removed by gc.
+ .section .text.unusedfunc2, "ax",%progbits
+ .align 2
+ .global unusedfunc2
+ .type unusedfunc2, %function
+unusedfunc2:
+ .fnstart
+ .save {r7, lr}
+ .setfp r7, sp
+ bx lr
+ .personality __gxx_personality_v0
+ .handlerdata
+ .align 2
+ .byte 0xff
+ .byte 0x90
+ .uleb128 0x16
+ .byte 0x1
+ .uleb128 0x32
+ .uleb128 0
+ .byte 0x1
+ .byte 0
+ .section .text.unusedfunc2
+ .fnend
+
+ // Dummy implementation of personality routines to satisfy reference from
+ // exception tables
+ .section .text.__gcc_personality_v0, "ax", %progbits
+ .global __gxx_personality_v0
+ .type __gxx_personality_v0, %function
+__gxx_personality_v0:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+ .section .text.__aeabi_unwind_cpp_pr0, "ax", %progbits
+ .global __aeabi_unwind_cpp_pr0
+ .type __aeabi_unwind_cpp_pr0, %function
+__aeabi_unwind_cpp_pr0:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+ .text
+ .global _start
+ .type _start, %function
+_start:
+ .fnstart
+ .cantunwind
+ bl func1
+ bl func2
+ bx lr
+ .fnend
+
+// GC should have only removed unusedfunc1 and unusedfunc2 the personality
+// routines are kept alive by references from live .ARM.exidx and .ARM.extab
+// sections
+// CHECK: Disassembly of section .text:
+// CHECK-NEXT: _start:
+// CHECK-NEXT:   11000:       01 00 00 eb     bl      #4 <func1>
+// CHECK-NEXT:   11004:       01 00 00 eb     bl      #4 <func2>
+// CHECK-NEXT:   11008:       1e ff 2f e1     bx      lr
+// CHECK: func1:
+// CHECK-NEXT:   1100c:       1e ff 2f e1     bx      lr
+// CHECK: func2:
+// CHECK-NEXT:   11010:       1e ff 2f e1     bx      lr
+// CHECK: __gxx_personality_v0:
+// CHECK-NEXT:   11014:       1e ff 2f e1     bx      lr
+// CHECK: __aeabi_unwind_cpp_pr0:
+// CHECK-NEXT:   11018:       1e ff 2f e1     bx      lr
+
+// CHECK-NOT: unusedfunc1
+// CHECK-NOT: unusedfunc2
+
+// GC should have removed table entries for unusedfunc1 and unusedfunc2
+// CHECK-EXIDX: Contents of section .ARM.exidx:
+// 100d4 + f2c = 11000 = _start (cantunwind)
+// 100dc + f30 = 1100c = func1 (Inline unwind 80978408)
+// CHECK-EXIDX-NEXT: 100d4 2c0f0000 01000000 300f0000 08849780
+// CHECK-EXIDX-NEXT: 100e4 2c0f0000 04000000
+// 100e4 + f2c = 11010 = func2 (EHT 100e8 + 14 = 100fc = extab entry for func2)
+// 100ec + f28 = 11014 = __gxx_personality_v0 (cantunwind)
+// CHECK-EXIDX: Contents of section .ARM.extab.text.func2:
+// CHECK-EXIDX-NEXT: 100ec 280f0000 08849700 ff901601 32000100
+// 100f4 + f24 = 11018 = __aeabi_unwind_cpp_pr0 (cantunwind)
+// CHECK-EXIDX-NOT: Contents of section .ARM.extab.text.unusedfunc2:

test/ELF/arm-exidx-order.s

+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %S/Inputs/arm-exidx-cantunwind.s -o %tcantunwind
+// RUN: ld.lld %t %tcantunwind -o %t2 2>&1
+// RUN: llvm-objdump -d -triple=armv7a-none-linux-gnueabi %t2 | FileCheck %s
+// RUN: llvm-objdump -s -triple=armv7a-none-linux-gnueabi %t2 | FileCheck -check-prefix=CHECK-EXIDX %s
+// RUN: llvm-readobj --program-headers --sections %t2 | FileCheck -check-prefix=CHECK-PT %s
+// Use Linker script to place .ARM.exidx in between .text and orphan sections
+// RUN: echo "SECTIONS { \
+// RUN:          .text 0x11000 : { *(.text*) } \
+// RUN:          .ARM.exidx : { *(.ARM.exidx) } } " > %t.script
+// RUN: ld.lld --script %t.script %tcantunwind %t -o %t3 2>&1
+// RUN: llvm-objdump -d -triple=armv7a-none-linux-gnueabi %t3 | FileCheck -check-prefix=CHECK-SCRIPT %s
+// RUN: llvm-objdump -s -triple=armv7a-none-linux-gnueabi %t3 | FileCheck -check-prefix=CHECK-SCRIPT-EXIDX %s
+// REQUIRES: arm
+
+// Each assembler created .ARM.exidx section has the SHF_LINK_ORDER flag set
+// with the sh_link containing the section index of the executable section
+// containing the function it describes. The linker must combine the .ARM.exidx
+// InputSections in the same order that it has combined the executable section,
+// such that the combined .ARM.exidx OutputSection can be used as a binary
+// search table.
+
+ .syntax unified
+ .section .text, "ax",%progbits
+ .globl _start
+ .align 2
+ .type _start,%function
+_start:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+ .section .text.f1, "ax", %progbits
+ .globl f1
+ .align 2
+ .type f1,%function
+f1:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+ .section .text.f2, "ax", %progbits
+ .globl f2
+ .align 2
+ .type f2,%function
+f2:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+ .globl f3
+ .align 2
+ .type f3,%function
+f3:
+ .fnstart
+ bx lr
+ .cantunwind
+ .fnend
+
+// Check default no linker script order.
+
+// CHECK: Disassembly of section .text:
+// CHECK: _start:
+// CHECK-NEXT:    11000:       1e ff 2f e1     bx      lr
+// CHECK: f1:
+// CHECK-NEXT:    11004:       1e ff 2f e1     bx      lr
+// CHECK: f2:
+// CHECK-NEXT:    11008:       1e ff 2f e1     bx      lr
+// CHECK: f3:
+// CHECK-NEXT:    1100c:       1e ff 2f e1     bx      lr
+// CHECK: func4:
+// CHECK-NEXT:    11010:       1e ff 2f e1     bx      lr
+// CHECK: func5:
+// CHECK-NEXT:    11014:       1e ff 2f e1     bx      lr
+// CHECK: Disassembly of section .func1:
+// CHECK-NEXT: func1:
+// CHECK-NEXT:    11018:       1e ff 2f e1     bx      lr
+// CHECK: Disassembly of section .func2:
+// CHECK-NEXT: func2:
+// CHECK-NEXT:    1101c:       1e ff 2f e1     bx      lr
+// CHECK: Disassembly of section .func3:
+// CHECK-NEXT: func3:
+// CHECK-NEXT:    11020:       1e ff 2f e1     bx      lr
+
+// Each .ARM.exidx section has two 4 byte fields
+// Field 1 is the 31-bit offset to the function. The top bit is used to
+// indicate whether Field 2 is a pointer or an inline table entry.
+// Field 2 is either a pointer to a .ARM.extab section or an inline table
+// In this example all Field 2 entries are inline can't unwind (0x1)
+// We expect to see the entries in the same order as the functions
+
+// CHECK-EXIDX: Contents of section .ARM.exidx:
+// 100d4 + f2c = 11000 = _start
+// 100dc + f28 = 11004 = f1
+// CHECK-EXIDX-NEXT:       100d4 2c0f0000 01000000 280f0000 01000000
+// 100e4 + f24 = 11008 = f2
+// 100ec + f20 = 1100c = f3
+// CHECK-EXIDX-NEXT:  100e4 240f0000 01000000 200f0000 01000000
+// 100f4 + f1c = 11010 = func4
+// 100fc + f18 = 11014 = func5
+// CHECK-EXIDX-NEXT:  100f4 1c0f0000 01000000 180f0000 01000000
+// 10104 + f14 = 11018 = func1
+// 1010c + f10 = 1101c = func2
+// CHECK-EXIDX-NEXT:  10104 140f0000 01000000 100f0000 01000000
+// 10114 + f0c = 11020 = func3
+// CHECK-EXIDX-NEXT:  10114 0c0f0000 01000000
+
+// Check that PT_ARM_EXIDX program header has been generated that describes
+// the .ARM.exidx output section
+// CHECK-PT:          Name: .ARM.exidx
+// CHECK-PT-NEXT:     Type: SHT_ARM_EXIDX (0x70000001)
+// CHECK-PT-NEXT:     Flags [ (0x2)
+// CHECK-PT-NEXT:       SHF_ALLOC (0x2)
+// CHECK-PT-NEXT:     ]
+// CHECK-PT-NEXT:     Address: 0x100D4
+// CHECK-PT-NEXT:     Offset: 0xD4
+// CHECK-PT-NEXT:     Size: 72
+
+// CHECK-PT:          Type: PT_ARM_EXIDX (0x70000001)
+// CHECK-PT-NEXT:     Offset: 0xD4
+// CHECK-PT-NEXT:     VirtualAddress: 0x100D4
+// CHECK-PT-NEXT:     PhysicalAddress: 0x100D4
+// CHECK-PT-NEXT:     FileSize: 72
+// CHECK-PT-NEXT:     MemSize: 72
+// CHECK-PT-NEXT:     Flags [ (0x4)
+// CHECK-PT-NEXT:       PF_R (0x4)
+// CHECK-PT-NEXT:     ]
+// CHECK-PT-NEXT:     Alignment: 4
+// CHECK-PT-NEXT:   }
+
+
+// Check linker script order. The .ARM.exidx section will be inserted after
+// the .text section but before the orphan sections
+
+// CHECK-SCRIPT: Disassembly of section .text:
+// CHECK-SCRIPT-NEXT: func4:
+// CHECK-SCRIPT-NEXT:    11000:       1e ff 2f e1     bx      lr
+// CHECK-SCRIPT:      func5:
+// CHECK-SCRIPT-NEXT:    11004:       1e ff 2f e1     bx      lr
+// CHECK-SCRIPT:      _start:
+// CHECK-SCRIPT-NEXT:    11008:       1e ff 2f e1     bx      lr
+// CHECK-SCRIPT:      f1:
+// CHECK-SCRIPT-NEXT:    1100c:       1e ff 2f e1     bx      lr
+// CHECK-SCRIPT:      f2:
+// CHECK-SCRIPT-NEXT:    11010:       1e ff 2f e1     bx      lr
+// CHECK-SCRIPT:      f3:
+// CHECK-SCRIPT-NEXT:    11014:       1e ff 2f e1     bx      lr
+// CHECK-SCRIPT-NEXT: Disassembly of section .func1:
+// CHECK-SCRIPT-NEXT: func1:
+// CHECK-SCRIPT-NEXT:    11060:       1e ff 2f e1     bx      lr
+// CHECK-SCRIPT-NEXT: Disassembly of section .func2:
+// CHECK-SCRIPT-NEXT: func2:
+// CHECK-SCRIPT-NEXT:    11064:       1e ff 2f e1     bx      lr
+// CHECK-SCRIPT-NEXT: Disassembly of section .func3:
+// CHECK-SCRIPT-NEXT: func3:
+// CHECK-SCRIPT-NEXT:    11068:       1e ff 2f e1     bx      lr
+
+// Check that the .ARM.exidx section is sorted in order as the functions
+// The offset in field 1, is 32-bit so in the binary the most significant bit
+// CHECK-SCRIPT-EXIDX: Contents of section .ARM.exidx:
+// 11018 - 18 = 11000 func4
+// 11020 - 1c = 11004 func5
+// CHECK-SCRIPT-EXIDX-NEXT:  11018 e8ffff7f 01000000 e4ffff7f 01000000
+// 11028 - 20 = 11008 _start
+// 11030 - 24 = 1100c f1
+// CHECK-SCRIPT-EXIDX-NEXT:  11028 e0ffff7f 01000000 dcffff7f 01000000
+// 11038 - 28 = 11010 f2
+// 11040 - 2c = 11014 f3
+// CHECK-SCRIPT-EXIDX-NEXT:  11038 d8ffff7f 01000000 d4ffff7f 01000000
+// 11048 + 18 = 11060 = func1
+// 11050 + 14 = 11064 = func2
+// CHECK-SCRIPT-EXIDX-NEXT:  11048 18000000 01000000 14000000 01000000
+// 11058 + 10 = 11068 = func3
+// CHECK-SCRIPT-EXIDX-NEXT:  11058 10000000 01000000

test/ELF/arm-exidx-output.s

  .fnstart
  bx lr
  .cantunwind
  .fnend
 
 // CHECK:         Section {
 // CHECK:         Name: .ARM.exidx
 // CHECK-NEXT:    Type: SHT_ARM_EXIDX (0x70000001)
-// CHECK-NEXT:    Flags [ (0x82)
+// CHECK-NEXT:    Flags [ (0x2)
 // CHECK-NEXT:      SHF_ALLOC (0x2)
-// CHECK-NEXT:      SHF_LINK_ORDER (0x80)
 // CHECK-NEXT:    ]
 
 // CHECK-NOT:     Name: .ARM.exidx.text.f1
 // CHECK-NOT:     Name: .ARM.exidx.text.f2