[lld][ELF][ARM] Generate PLT entries for function calls from ARM and Thumb code

[denis-protivensky]

Summary

The function call that goes through PLT table may be performed from both ARM and Thumb code. This situation requires adding a veneer to original PLT code (which is always ARM) to effect Thumb-to-ARM transition.

[ruiu]

LGTM

lib/ReaderWriter/ELF/ARM/ARMRelocationPass.cpp

125 ↗

(On Diff #22880)

We usually put protected members after public members. You may want to move this for consistency.

378 ↗

(On Diff #22880)

Maybe this is enough?

StringRef source;
#ifndef NDEBUG
source = "_ifunc_";
#endif

[denis-protivensky]

Thanks for reviewing, Rui!

lib/ReaderWriter/ELF/ARM/ARMRelocationPass.cpp

125 ↗

(On Diff #22880)

Didn't want to move the constructor to the bottom of the class, but that's not a big deal. Will do.

378 ↗

(On Diff #22880)

Right.

[shankarke]

lib/ReaderWriter/ELF/ARM/ARMRelocationPass.cpp

296–308 ↗

(On Diff #22880)

Isnt a seperate entry in the PLT for a thumb call preferred ?

555–556 ↗

(On Diff #22880)

Say for example you call the same atom from thumb/arm, the current code would reuse the veneer generated by ARM for thumb too, is this correct ?

[denis-protivensky]

lib/ReaderWriter/ELF/ARM/ARMRelocationPass.cpp

296–308 ↗

(On Diff #22880)

When the same atom is called through PLT from both ARM and Thumb, it's enough to generate one PLT with transition code on top of it. ARM code will reference ARM instructions while Thumb code will first execute instruction mode switch. Obviously, I first implemented separate PLTs for ARM & Thumb, but then saw how gcc handles it in one PLT entry and decided that it's more optimal.

555–556 ↗

(On Diff #22880)

When the same atom is called from both modes, the one mode will call the code directly as mode switch is not needed, and the other will use the veneer generated. The conditions for veneer generation are checked in handleVeneer method, while this method only creates a veneer when requested.

[• shankar.easwaran]

Thanks for the explanation. LGTM.

lld/trunk/lib/ReaderWriter/ELF/ARM/ARMRelocationPass.cpp

 /// which is not specified anywhere.
 ///
 //===----------------------------------------------------------------------===//
 
 #include "ARMRelocationPass.h"
 #include "ARMLinkingContext.h"
 #include "Atoms.h"
 #include "lld/Core/Simple.h"
-#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/Debug.h"
 
 using namespace lld;
 using namespace lld::elf;
 using namespace llvm::ELF;
 
 namespace {
static const uint8_t Veneer_THM_B_BL_StaticAtomContent[8] = {
     0xfe, 0xff, 0xff, 0xea   // b <target_symbol_address>
 };
 
 // .got values
 static const uint8_t ARMGotAtomContent[4] = {0};
 
 // .plt values (other entries)
 static const uint8_t ARMPltAtomContent[12] = {
-    0x00, 0xc0, 0x8f,
-    0xe2, // add    ip, pc, #offset[G0]
-    0x00, 0xc0, 0x8c,
-    0xe2, // add    ip, ip, #offset[G1]
+    0x00, 0xc0, 0x8f, 0xe2,  // add ip, pc, #offset[G0]
+    0x00, 0xc0, 0x8c, 0xe2,  // add ip, ip, #offset[G1]
+    0x00, 0xf0, 0xbc, 0xe5   // ldr pc, [ip, #offset[G2]]!
+};
 
-    0x00, 0xf0, 0xbc,
-    0xe5, // ldr    pc, [ip, #offset[G2]]!
+// Veneer for switching from Thumb to ARM code for PLT entries.
+static const uint8_t ARMPltVeneerAtomContent[4] = {
+    0x78, 0x47,              // bx pc
+    0x00, 0x00               // nop
 };
 
 /// \brief Atoms that hold veneer code.
 class VeneerAtom : public SimpleELFDefinedAtom {
   StringRef _section;
 
 public:
   VeneerAtom(const File &f, StringRef secName)
public:
   ArrayRef<uint8_t> rawContent() const override {
     return llvm::makeArrayRef(Veneer_THM_B_BL_StaticAtomContent);
   }
 };
 
 /// \brief Atoms that are used by ARM dynamic linking
 class ARMGOTAtom : public GOTAtom {
 public:
-  ARMGOTAtom(const File &f, StringRef secName) : GOTAtom(f, secName) {}
+  ARMGOTAtom(const File &f) : GOTAtom(f, ".got") {}
 
   ArrayRef<uint8_t> rawContent() const override {
     return llvm::makeArrayRef(ARMGotAtomContent);
   }
 
   Alignment alignment() const override { return 4; }
+
+protected:
+  // Constructor for PLTGOT atom.
+  ARMGOTAtom(const File &f, StringRef secName) : GOTAtom(f, secName) {}
+};
+
+class ARMGOTPLTAtom : public ARMGOTAtom {
+public:
+  ARMGOTPLTAtom(const File &f) : ARMGOTAtom(f, ".got.plt") {}
 };
 
 class ARMPLTAtom : public PLTAtom {
 public:
-  ARMPLTAtom(const File &f, StringRef secName) : PLTAtom(f, secName) {}
+  ARMPLTAtom(const File &f) : PLTAtom(f, ".plt") {}
 
   ArrayRef<uint8_t> rawContent() const override {
     return llvm::makeArrayRef(ARMPltAtomContent);
   }
+
+  Alignment alignment() const override { return 4; }
+};
+
+class ARMPLTVeneerAtom : public PLTAtom {
+public:
+  ARMPLTVeneerAtom(const File &f) : PLTAtom(f, ".plt") {}
+
+  DefinedAtom::CodeModel codeModel() const override {
+    return DefinedAtom::codeARMThumb;
+  }
+
+  ArrayRef<uint8_t> rawContent() const override {
+    return llvm::makeArrayRef(ARMPltVeneerAtomContent);
+  }
+
+  Alignment alignment() const override { return 4; }
 };
 
 class ELFPassFile : public SimpleFile {
 public:
   ELFPassFile(const ELFLinkingContext &eti) : SimpleFile("ELFPassFile") {
     setOrdinal(eti.getNextOrdinalAndIncrement());
   }
 
void handleReference(const DefinedAtom &atom, const Reference &ref) {
     case R_ARM_MOVT_ABS:
     case R_ARM_THM_MOVW_ABS_NC:
     case R_ARM_THM_MOVT_ABS:
     case R_ARM_THM_CALL:
     case R_ARM_CALL:
     case R_ARM_JUMP24:
     case R_ARM_THM_JUMP24:
     case R_ARM_THM_JUMP11:
-      static_cast<Derived *>(this)->handleIFUNC(ref);
+      static_cast<Derived *>(this)->handleIFUNC(atom, ref);
       static_cast<Derived *>(this)->handleVeneer(atom, ref);
       break;
     case R_ARM_TLS_IE32:
       static_cast<Derived *>(this)->handleTLSIE32(ref);
       break;
     case R_ARM_GOT_BREL:
       static_cast<Derived *>(this)->handleGOT(ref);
       break;
     default:
       break;
     }
   }
 
 protected:
   std::error_code handleVeneer(const DefinedAtom &atom, const Reference &ref) {
+    const VeneerAtom *(Derived::*getVeneer)(const DefinedAtom *, StringRef) =
+        nullptr;
     const auto kindValue = ref.kindValue();
     switch (kindValue) {
     case R_ARM_JUMP24:
+      getVeneer = &Derived::getVeneer_ARM_B_BL;
+      break;
     case R_ARM_THM_JUMP24:
+      getVeneer = &Derived::getVeneer_THM_B_BL;
       break;
     default:
       return std::error_code();
     }
 
     // Target symbol and relocated place should have different
     // instruction sets in order a veneer to be generated in between.
     const auto *target = dyn_cast<DefinedAtom>(ref.target());
if (DefinedAtom::typeCode != target->contentType() &&
           (Twine("Reference of type ") + Twine(kindValue) + " (" + kindValStr +
            ") from " + atom.name() + "+" + Twine(ref.offsetInAtom()) + " to " +
            ref.target()->name() + "+" + Twine(ref.addend()) +
            " cannot be effected without a veneer").str();
 
       llvm_unreachable(errStr.c_str());
     }
 
-    const Atom *veneer = nullptr;
-    switch (kindValue) {
-    case R_ARM_JUMP24:
-      veneer = static_cast<Derived *>(this)
-                   ->getVeneer_ARM_B_BL(target, secName);
-      break;
-    case R_ARM_THM_JUMP24:
-      veneer = static_cast<Derived *>(this)
-                   ->getVeneer_THM_B_BL(target, secName);
-      break;
-    default:
-      llvm_unreachable("Unhandled reference type for veneer generation");
-    }
+    assert(getVeneer && "The veneer handler is missing");
+    const Atom *veneer =
+        (static_cast<Derived *>(this)->*getVeneer)(target, secName);
 
     assert(veneer && "The veneer is not set");
     const_cast<Reference &>(ref).setTarget(veneer);
     return std::error_code();
   }
 
   std::error_code handleTLSIE32(const Reference &ref) {
     if (const auto *target = dyn_cast<DefinedAtom>(ref.target())) {
       const_cast<Reference &>(ref).setTarget(
           static_cast<Derived *>(this)->getTLSTPOFF32(target));
       return std::error_code();
     }
     llvm_unreachable("R_ARM_TLS_IE32 reloc targets wrong atom type");
   }
 
   /// \brief Create a GOT entry for TLS with reloc type and addend specified.
   template <Reference::KindValue R_ARM_TLS, Reference::Addend A = 0>
   const GOTAtom *getGOTTLSEntry(const DefinedAtom *da) {
-    auto got = _gotMap.find(da);
-    if (got != _gotMap.end())
-      return got->second;
-    auto g = new (_file._alloc) ARMGOTAtom(_file, ".got");
+    if (auto got = _gotAtoms.lookup(da))
+      return got;
+    auto g = new (_file._alloc) ARMGOTAtom(_file);
     g->addReferenceELF_ARM(R_ARM_TLS, 0, da, A);
 #ifndef NDEBUG
     g->_name = "__got_tls_";
     g->_name += da->name();
 #endif
-    _gotMap[da] = g;
-    _gotVector.push_back(g);
+    _gotAtoms[da] = g;
     return g;
   }
 
-  /// \brief Create a PLT entry referencing PLTGOT entry.
+  /// \brief get a veneer for a PLT entry.
+  const PLTAtom *getPLTVeneer(const DefinedAtom *da, PLTAtom *pa,
+                              StringRef source) {
+    auto va = new (_file._alloc) ARMPLTVeneerAtom(_file);
+    // Fake reference to show connection between veneer and PLT entry.
+    va->addReferenceELF_ARM(R_ARM_NONE, 0, pa, 0);
+#ifndef NDEBUG
+    va->_name = "__plt_from_thumb";
+    va->_name += source;
+    va->_name += da->name();
+#endif
+    _pltAtoms[da] = PLTWithVeneer(pa, va);
+    return va;
+  }
+
+  typedef const GOTAtom *(Derived::*GOTFactory)(const DefinedAtom *);
+
+  /// \brief get a PLT entry referencing PLTGOT entry.
   ///
-  /// The function creates the PLT entry object and passes ownership
-  /// over it to the caller.
-  PLTAtom *createPLTforGOT(const GOTAtom *ga) {
-    auto pa = new (_file._alloc) ARMPLTAtom(_file, ".plt");
+  /// If the entry does not exist, both GOT and PLT entry are created.
+  const PLTAtom *getPLTEntry(const DefinedAtom *da, bool fromThumb,
+                             GOTFactory gotFactory, StringRef source) {
+    auto pltVeneer = _pltAtoms.lookup(da);
+    if (!pltVeneer.empty()) {
+      // Return clean PLT entry provided it is ARM code.
+      if (!fromThumb)
+        return pltVeneer._plt;
+
+      // Check if veneer is present for Thumb to ARM transition.
+      if (pltVeneer._veneer)
+        return pltVeneer._veneer;
+
+      // Create veneer for existing PLT entry.
+      return getPLTVeneer(da, pltVeneer._plt, source);
+    }
+
+    // Create specific GOT entry.
+    const auto *ga = (static_cast<Derived *>(this)->*gotFactory)(da);
+    assert(_gotAtoms.lookup(da) == ga &&
+           "GOT entry should be added to the map");
+    assert(ga->customSectionName() == ".got.plt" &&
+           "GOT entry should be in a special section");
+
+    // Create PLT entry for the GOT entry.
+    auto pa = new (_file._alloc) ARMPLTAtom(_file);
     pa->addReferenceELF_ARM(R_ARM_ALU_PC_G0_NC, 0, ga, -8);
     pa->addReferenceELF_ARM(R_ARM_ALU_PC_G1_NC, 4, ga, -4);
     pa->addReferenceELF_ARM(R_ARM_LDR_PC_G2, 8, ga, 0);
+#ifndef NDEBUG
+    pa->_name = "__plt";
+    pa->_name += source;
+    pa->_name += da->name();
+#endif
+    // Since all PLT entries are in ARM code, Thumb to ARM
+    // switching should be added if the relocated place contais Thumb code.
+    if (fromThumb)
+      return getPLTVeneer(da, pa, source);
+
+    // Otherwise just add PLT entry and return it to the caller.
+    _pltAtoms[da] = PLTWithVeneer(pa);
     return pa;
   }
 
+  /// \brief Create the GOT entry for a given IFUNC Atom.
+  const GOTAtom *createIFUNCGOTEntry(const DefinedAtom *da) {
+    assert(!_gotAtoms.lookup(da) && "IFUNC GOT entry already exists");
+    auto g = new (_file._alloc) ARMGOTPLTAtom(_file);
+    g->addReferenceELF_ARM(R_ARM_ABS32, 0, da, 0);
+    g->addReferenceELF_ARM(R_ARM_IRELATIVE, 0, da, 0);
+#ifndef NDEBUG
+    g->_name = "__got_ifunc_";
+    g->_name += da->name();
+#endif
+    _gotAtoms[da] = g;
+    return g;
+  }
+
   /// \brief get the PLT entry for a given IFUNC Atom.
-  ///
-  /// If the entry does not exist. Both the GOT and PLT entry is created.
-  const PLTAtom *getIFUNCPLTEntry(const DefinedAtom *da) {
-    auto plt = _pltMap.find(da);
-    if (plt != _pltMap.end())
-      return plt->second;
-    auto ga = new (_file._alloc) ARMGOTAtom(_file, ".got.plt");
-    ga->addReferenceELF_ARM(R_ARM_ABS32, 0, da, 0);
-    ga->addReferenceELF_ARM(R_ARM_IRELATIVE, 0, da, 0);
-    auto pa = createPLTforGOT(ga);
+  const PLTAtom *getIFUNCPLTEntry(const DefinedAtom *da, bool fromThumb) {
+    StringRef source;
 #ifndef NDEBUG
-    ga->_name = "__got_ifunc_";
-    ga->_name += da->name();
-    pa->_name = "__plt_ifunc_";
-    pa->_name += da->name();
+    source = "_ifunc_";
 #endif
-    _gotMap[da] = ga;
-    _pltMap[da] = pa;
-    _gotVector.push_back(ga);
-    _pltVector.push_back(pa);
-    return pa;
+    return getPLTEntry(da, fromThumb, &Derived::createIFUNCGOTEntry, source);
   }
 
   /// \brief Redirect the call to the PLT stub for the target IFUNC.
   ///
   /// This create a PLT and GOT entry for the IFUNC if one does not exist. The
   /// GOT entry and a IRELATIVE relocation to the original target resolver.
-  std::error_code handleIFUNC(const Reference &ref) {
+  std::error_code handleIFUNC(const DefinedAtom &atom, const Reference &ref) {
     auto target = dyn_cast<const DefinedAtom>(ref.target());
     if (target && target->contentType() == DefinedAtom::typeResolver) {
-      const_cast<Reference &>(ref).setTarget(getIFUNCPLTEntry(target));
+      const_cast<Reference &>(ref).setTarget(
+          getIFUNCPLTEntry(target, isThumbCode(atom.codeModel())));
     }
     return std::error_code();
   }
 
   /// \brief Create a GOT entry containing 0.
   const GOTAtom *getNullGOT() {
     if (!_null) {
-      _null = new (_file._alloc) ARMGOTAtom(_file, ".got.plt");
+      _null = new (_file._alloc) ARMGOTPLTAtom(_file);
 #ifndef NDEBUG
       _null->_name = "__got_null";
 #endif
     }
     return _null;
   }
 
   const GOTAtom *getGOT(const DefinedAtom *da) {
-    auto got = _gotMap.find(da);
-    if (got != _gotMap.end())
-      return got->second;
-    auto g = new (_file._alloc) ARMGOTAtom(_file, ".got");
+    if (auto got = _gotAtoms.lookup(da))
+      return got;
+    auto g = new (_file._alloc) ARMGOTAtom(_file);
     g->addReferenceELF_ARM(R_ARM_ABS32, 0, da, 0);
 #ifndef NDEBUG
     g->_name = "__got_";
     g->_name += da->name();
 #endif
-    _gotMap[da] = g;
-    _gotVector.push_back(g);
+    _gotAtoms[da] = g;
     return g;
   }
 
 public:
   ARMRelocationPass(const ELFLinkingContext &ctx) : _file(ctx), _ctx(ctx) {}
 
   /// \brief Do the pass.
   ///
void perform(std::unique_ptr<MutableFile> &mf) override {
     for (const auto &atom : mf->defined()) {
       for (const auto &ref : *atom) {
         handleReference(*atom, *ref);
       }
     }
 
     // Add all created atoms to the link.
     uint64_t ordinal = 0;
-    for (auto &plt : _pltVector) {
-      plt->setOrdinal(ordinal++);
-      mf->addAtom(*plt);
+    for (auto &pltKV : _pltAtoms) {
+      auto &plt = pltKV.second;
+      if (auto *v = plt._veneer) {
+        v->setOrdinal(ordinal++);
+        mf->addAtom(*v);
+      }
+      auto *p = plt._plt;
+      p->setOrdinal(ordinal++);
+      mf->addAtom(*p);
     }
     if (_null) {
       _null->setOrdinal(ordinal++);
       mf->addAtom(*_null);
     }
-    for (auto &got : _gotVector) {
+    for (auto &gotKV : _gotAtoms) {
+      auto &got = gotKV.second;
       got->setOrdinal(ordinal++);
       mf->addAtom(*got);
     }
-    for (auto &veneer : _veneerVector) {
+    for (auto &veneerKV : _veneerAtoms) {
+      auto &veneer = veneerKV.second;
       veneer->setOrdinal(ordinal++);
       mf->addAtom(*veneer);
     }
   }
 
 protected:
   /// \brief Owner of all the Atoms created by this pass.
   ELFPassFile _file;
   const ELFLinkingContext &_ctx;
 
   /// \brief Map Atoms to their GOT entries.
-  llvm::DenseMap<const Atom *, GOTAtom *> _gotMap;
+  llvm::MapVector<const Atom *, GOTAtom *> _gotAtoms;
 
-  /// \brief Map Atoms to their PLT entries.
-  llvm::DenseMap<const Atom *, PLTAtom *> _pltMap;
+  /// \brief Map Atoms to their PLT entries depending on the code model.
+  struct PLTWithVeneer {
+    PLTWithVeneer(PLTAtom *p = nullptr, PLTAtom *v = nullptr)
+        : _plt(p), _veneer(v) {}
 
-  /// \brief Map Atoms to their veneers.
-  llvm::DenseMap<const Atom *, VeneerAtom *> _veneerMap;
+    bool empty() const {
+      assert((_plt || !_veneer) && "Veneer appears without PLT entry");
+      return !_plt && !_veneer;
+    }
 
-  /// \brief the list of GOT/PLT atoms
-  std::vector<GOTAtom *> _gotVector;
-  std::vector<PLTAtom *> _pltVector;
+    PLTAtom *_plt;
+    PLTAtom *_veneer;
+  };
+  llvm::MapVector<const Atom *, PLTWithVeneer> _pltAtoms;
 
-  /// \brief the list of veneer atoms.
-  std::vector<VeneerAtom *> _veneerVector;
+  /// \brief Map Atoms to their veneers.
+  llvm::MapVector<const Atom *, VeneerAtom *> _veneerAtoms;
 
   /// \brief GOT entry that is always 0. Used for undefined weaks.
   GOTAtom *_null = nullptr;
 };
 
 /// This implements the static relocation model. Meaning GOT and PLT entries are
 /// not created for references that can be directly resolved. These are
 /// converted to a direct relocation. For entries that do require a GOT or PLT
 /// entry, that entry is statically bound.
 ///
 /// TLS always assumes module 1 and attempts to remove indirection.
 class ARMStaticRelocationPass final
     : public ARMRelocationPass<ARMStaticRelocationPass> {
 public:
   ARMStaticRelocationPass(const elf::ARMLinkingContext &ctx)
       : ARMRelocationPass(ctx) {}
 
   /// \brief Get the veneer for ARM B/BL instructions.
   const VeneerAtom *getVeneer_ARM_B_BL(const DefinedAtom *da,
                                        StringRef secName) {
-    auto veneer = _veneerMap.find(da);
-    if (_veneerMap.end() != veneer)
-      return veneer->second;
-
+    if (auto veneer = _veneerAtoms.lookup(da))
+      return veneer;
     auto v = new (_file._alloc) Veneer_ARM_B_BL_StaticAtom(_file, secName);
     v->addReferenceELF_ARM(R_ARM_ABS32, 4, da, 0);
 
     v->_name = "__";
     v->_name += da->name();
     v->_name += "_from_arm";
 
-    _veneerMap[da] = v;
-    _veneerVector.push_back(v);
+    _veneerAtoms[da] = v;
     return v;
   }
 
   /// \brief Get the veneer for Thumb B/BL instructions.
   const VeneerAtom *getVeneer_THM_B_BL(const DefinedAtom *da,
                                        StringRef secName) {
-    auto veneer = _veneerMap.find(da);
-    if (_veneerMap.end() != veneer)
-      return veneer->second;
-
+    if (auto veneer = _veneerAtoms.lookup(da))
+      return veneer;
     auto v = new (_file._alloc) Veneer_THM_B_BL_StaticAtom(_file, secName);
     v->addReferenceELF_ARM(R_ARM_JUMP24, 4, da, 0);
 
     v->_name = "__";
     v->_name += da->name();
     v->_name += "_from_thumb";
 
-    _veneerMap[da] = v;
-    _veneerVector.push_back(v);
+    _veneerAtoms[da] = v;
     return v;
   }
 
   /// \brief Create a GOT entry for R_ARM_TLS_TPOFF32 reloc.
   const GOTAtom *getTLSTPOFF32(const DefinedAtom *da) {
     return getGOTTLSEntry<R_ARM_TLS_LE32>(da);
   }
 

lld/trunk/test/elf/ARM/plt-interwork.test

+# Check that PLT entries are properly generated for cases when
+# referenced from both ARM and Thumb code.
+#
+# 1. ARM code generates PLT without transition veneer.
+# RUN: yaml2obj -format=elf -docnum 1 %s > %t-f1-arm.o
+# RUN: lld -flavor gnu -target arm-linux-gnu \
+# RUN: -Bstatic --noinhibit-exec %t-f1-arm.o -o %t
+# RUN: llvm-objdump -s %t | FileCheck -check-prefix=ARM-ONE %s
+
+# ARM-ONE: Contents of section .plt:
+# ARM-ONE-NEXT: 40007c 00c68fe2 00ca8ce2 7cffbce5
+#                          ^ regular veneer ^
+# Next line shows that .plt section ends.
+# ARM-ONE-NEXT: Contents of section .text:
+
+# 2. Thumb code generates PLT with transition veneer.
+# RUN: yaml2obj -format=elf -docnum 2 %s > %t-f1-thm.o
+# RUN: lld -flavor gnu -target arm-linux-gnu \
+# RUN: -Bstatic --noinhibit-exec %t-f1-thm.o -o %t
+# RUN: llvm-objdump -s %t | FileCheck -check-prefix=THM-ONE %s
+
+# THM-ONE: Contents of section .plt:
+# THM-ONE-NEXT: 40007c 78470000 00c68fe2 00ca8ce2 78ffbce5
+#                   bx pc ^^        ^ regular veneer ^
+# Next line shows that .plt section ends.
+# THM-ONE-NEXT: Contents of section .text:
+
+# 3. ARM + Thumb code generate same single PLT with transition veneer.
+# RUN: yaml2obj -format=elf -docnum 1 %s > %t-f1-arm.o
+# RUN: yaml2obj -format=elf -docnum 3 %s > %t-f1-thm-for-arm.o
+# RUN: lld -flavor gnu -target arm-linux-gnu \
+# RUN: -Bstatic --noinhibit-exec %t-f1-arm.o %t-f1-thm-for-arm.o -o %t
+# RUN: llvm-objdump -s %t | FileCheck -check-prefix=THM-ONE %s
+
+# 4. Thumb + ARM code generate same single PLT with transition veneer.
+# RUN: yaml2obj -format=elf -docnum 3 %s > %t-f1-thm-for-arm.o
+# RUN: yaml2obj -format=elf -docnum 1 %s > %t-f1-arm.o
+# RUN: lld -flavor gnu -target arm-linux-gnu \
+# RUN: -Bstatic --noinhibit-exec %t-f1-thm-for-arm.o %t-f1-arm.o -o %t
+# RUN: llvm-objdump -s %t | FileCheck -check-prefix=THM-ONE %s
+
+# 5. ARM + ARM code generate two PLTs without transition veneers.
+# RUN: yaml2obj -format=elf -docnum 1 %s > %t-f1-arm.o
+# RUN: yaml2obj -format=elf -docnum 4 %s > %t-f2-arm.o
+# RUN: lld -flavor gnu -target arm-linux-gnu \
+# RUN: -Bstatic --noinhibit-exec %t-f1-arm.o %t-f2-arm.o -o %t
+# RUN: llvm-objdump -s %t | FileCheck -check-prefix=ARM-TWO %s
+
+# ARM-TWO: Contents of section .plt:
+# ARM-TWO-NEXT: 400084 00c68fe2 00ca8ce2 74ffbce5 00c68fe2
+#                         ^ regular veneer 1 ^        ^ --
+# ARM-TWO-NEXT: 400094 00ca8ce2 6cffbce5
+#             --- regular veneer 2 ^
+# Next line shows that .plt section ends.
+# ARM-TWO-NEXT: Contents of section .text:
+
+# 6. ARM + Thumb code generate two PLTs without and with transition veneer.
+# RUN: yaml2obj -format=elf -docnum 1 %s > %t-f1-arm.o
+# RUN: yaml2obj -format=elf -docnum 5 %s > %t-f2-thm.o
+# RUN: lld -flavor gnu -target arm-linux-gnu \
+# RUN: -Bstatic --noinhibit-exec %t-f1-arm.o %t-f2-thm.o -o %t
+# RUN: llvm-objdump -s %t | FileCheck -check-prefix=ARM-THM-TWO %s
+
+# ARM-THM-TWO: Contents of section .plt:
+# ARM-THM-TWO-NEXT: 400084 00c68fe2 00ca8ce2 74ffbce5 78470000
+#                             ^ regular veneer 1 ^       ^^ bx pc 2
+# ARM-THM-TWO-NEXT: 400094 00c68fe2 00ca8ce2 68ffbce5
+#                             ^ regular veneer 2 ^
+# Next line shows that .plt section ends.
+# ARM-THM-TWO-NEXT: Contents of section .text:
+
+# 7. Thumb + ARM code generate two PLTs with and without transition veneer.
+# RUN: yaml2obj -format=elf -docnum 2 %s > %t-f1-thm.o
+# RUN: yaml2obj -format=elf -docnum 4 %s > %t-f2-arm.o
+# RUN: lld -flavor gnu -target arm-linux-gnu \
+# RUN: -Bstatic --noinhibit-exec %t-f1-thm.o %t-f2-arm.o -o %t
+# RUN: llvm-objdump -s %t | FileCheck -check-prefix=THM-ARM-TWO %s
+
+# THM-ARM-TWO: Contents of section .plt:
+# THM-ARM-TWO-NEXT: 400084 78470000 00c68fe2 00ca8ce2 70ffbce
+#                     bx pc 1 ^^       ^ regular veneer 1 ^
+# THM-ARM-TWO-NEXT: 400094 00c68fe2 00ca8ce2 68ffbce5
+#                             ^ regular veneer 2 ^
+# Next line shows that .plt section ends.
+# THM-ARM-TWO-NEXT: Contents of section .text:
+
+# 8. Thumb + Thumb code generate two PLTs with transition veneers.
+# RUN: yaml2obj -format=elf -docnum 2 %s > %t-f1-thm.o
+# RUN: yaml2obj -format=elf -docnum 5 %s > %t-f2-thm.o
+# RUN: lld -flavor gnu -target arm-linux-gnu \
+# RUN: -Bstatic --noinhibit-exec %t-f1-thm.o %t-f2-thm.o -o %t
+# RUN: llvm-objdump -s %t | FileCheck -check-prefix=THM-TWO %s
+
+# THM-TWO: Contents of section .plt:
+# THM-TWO-NEXT: 400084 78470000 00c68fe2 00ca8ce2 70ffbce5
+#                 bx pc 1 ^^       ^ regular veneer 1 ^
+# THM-TWO-NEXT: 400094 78470000 00c68fe2 00ca8ce2 64ffbce5
+#                 bx pc 2 ^^       ^ regular veneer 2 ^
+# Next line shows that .plt section ends.
+# THM-TWO-NEXT: Contents of section .text:
+
+# f1-arm.o
+---
+FileHeader:
+  Class:           ELFCLASS32
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_ARM
+  Flags:           [ EF_ARM_EABI_VER5 ]
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    AddressAlign:    0x0000000000000004
+    Content:         04B02DE500B08DE2003000E3003040E30300A0E100D04BE204B09DE41EFF2FE100482DE904B08DE2FEFFFFEB0030A0E10300A0E10088BDE8
+  - Name:            .rel.text
+    Type:            SHT_REL
+    Link:            .symtab
+    AddressAlign:    0x0000000000000004
+    Info:            .text
+    Relocations:
+      - Offset:          0x0000000000000008
+        Symbol:          main
+        Type:            R_ARM_MOVW_ABS_NC
+      - Offset:          0x000000000000000C
+        Symbol:          main
+        Type:            R_ARM_MOVT_ABS
+      - Offset:          0x0000000000000028
+        Symbol:          f1
+        Type:            R_ARM_CALL
+  - Name:            .data
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+  - Name:            .bss
+    Type:            SHT_NOBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+Symbols:
+  Global:
+    - Name:            f1
+      Type:            STT_GNU_IFUNC
+      Section:         .text
+    - Name:            main
+      Type:            STT_FUNC
+      Section:         .text
+      Value:           0x0000000000000020
+
+# f1-thm.o
+---
+FileHeader:
+  Class:           ELFCLASS32
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_ARM
+  Flags:           [ EF_ARM_EABI_VER5 ]
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    AddressAlign:    0x0000000000000004
+    Content:         80B400AF40F20003C0F200031846BD465DF8047B704700BF80B500AFFFF7FEFF0346184680BD00BF
+  - Name:            .rel.text
+    Type:            SHT_REL
+    Link:            .symtab
+    AddressAlign:    0x0000000000000004
+    Info:            .text
+    Relocations:
+      - Offset:          0x0000000000000004
+        Symbol:          main
+        Type:            R_ARM_THM_MOVW_ABS_NC
+      - Offset:          0x0000000000000008
+        Symbol:          main
+        Type:            R_ARM_THM_MOVT_ABS
+      - Offset:          0x000000000000001C
+        Symbol:          f1
+        Type:            R_ARM_THM_CALL
+  - Name:            .data
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+  - Name:            .bss
+    Type:            SHT_NOBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+Symbols:
+ Global:
+    - Name:            f1
+      Type:            STT_GNU_IFUNC
+      Section:         .text
+      Value:           0x0000000000000001
+    - Name:            main
+      Type:            STT_FUNC
+      Section:         .text
+      Value:           0x0000000000000019
+
+# f1-thm-for-arm.o
+---
+FileHeader:
+  Class:           ELFCLASS32
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_ARM
+  Flags:           [ EF_ARM_EABI_VER5 ]
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    AddressAlign:    0x0000000000000004
+    Content:         80B500AFFFF7FEFF0346184680BD00BF
+  - Name:            .rel.text
+    Type:            SHT_REL
+    Link:            .symtab
+    AddressAlign:    0x0000000000000004
+    Info:            .text
+    Relocations:
+      - Offset:          0x0000000000000004
+        Symbol:          f1
+        Type:            R_ARM_THM_CALL
+  - Name:            .data
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+  - Name:            .bss
+    Type:            SHT_NOBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+Symbols:
+  Global:
+    - Name:            f
+      Type:            STT_FUNC
+      Section:         .text
+      Value:           0x0000000000000001
+    - Name:            f1
+
+# f2-arm.o
+---
+FileHeader:
+  Class:           ELFCLASS32
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_ARM
+  Flags:           [ EF_ARM_EABI_VER5 ]
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    AddressAlign:    0x0000000000000004
+    Content:         04B02DE500B08DE2003000E3003040E30300A0E100D04BE204B09DE41EFF2FE100482DE904B08DE2FEFFFFEB0030A0E10300A0E10088BDE8
+  - Name:            .rel.text
+    Type:            SHT_REL
+    Link:            .symtab
+    AddressAlign:    0x0000000000000004
+    Info:            .text
+    Relocations:
+      - Offset:          0x0000000000000008
+        Symbol:          f
+        Type:            R_ARM_MOVW_ABS_NC
+      - Offset:          0x000000000000000C
+        Symbol:          f
+        Type:            R_ARM_MOVT_ABS
+      - Offset:          0x0000000000000028
+        Symbol:          f2
+        Type:            R_ARM_CALL
+  - Name:            .data
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+  - Name:            .bss
+    Type:            SHT_NOBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+Symbols:
+  Global:
+    - Name:            f2
+      Type:            STT_GNU_IFUNC
+      Section:         .text
+    - Name:            f
+      Type:            STT_FUNC
+      Section:         .text
+      Value:           0x0000000000000020
+
+# f2-thm.o
+---
+FileHeader:
+  Class:           ELFCLASS32
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_ARM
+  Flags:           [ EF_ARM_EABI_VER5 ]
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    AddressAlign:    0x0000000000000004
+    Content:         80B400AF40F20003C0F200031846BD465DF8047B704700BF80B500AFFFF7FEFF0346184680BD00BF
+  - Name:            .rel.text
+    Type:            SHT_REL
+    Link:            .symtab
+    AddressAlign:    0x0000000000000004
+    Info:            .text
+    Relocations:
+      - Offset:          0x0000000000000004
+        Symbol:          f
+        Type:            R_ARM_THM_MOVW_ABS_NC
+      - Offset:          0x0000000000000008
+        Symbol:          f
+        Type:            R_ARM_THM_MOVT_ABS
+      - Offset:          0x000000000000001C
+        Symbol:          f2
+        Type:            R_ARM_THM_CALL
+  - Name:            .data
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+  - Name:            .bss
+    Type:            SHT_NOBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+Symbols:
+ Global:
+    - Name:            f2
+      Type:            STT_GNU_IFUNC
+      Section:         .text
+      Value:           0x0000000000000001
+    - Name:            f
+      Type:            STT_FUNC
+      Section:         .text
+      Value:           0x0000000000000019
+...

lld/trunk/test/elf/ARM/rel-ifunc.test

 # Check handling of IFUNC (gnu_indirect_function).
 # RUN: yaml2obj -format=elf %s > %t-o.o
 # RUN: lld -flavor gnu -target arm -m armelf_linux_eabi -Bstatic \
 # RUN: %t-o.o -o %t
 # RUN: llvm-objdump -s -t %t | FileCheck %s
 
 # CHECK: Contents of section .rel.plt:
 # CHECK: 400074 00104000
 # CHECK: Contents of section .plt:
-# CHECK: 400080 00c68fe2 00ca8ce2 78ffbce5
+# CHECK: 40007c 00c68fe2 00ca8ce2 7cffbce5
 # CHECK: Contents of section .text:
-# CHECK: 4001ac 00482de9 04b08de2 d4ffffeb b0ffffeb
+# CHECK: 4001a8 00482de9 04b08de2 d4ffffeb b0ffffeb
 #                                             ^ bl #-320 (to address 0x400080=__plt_ifunc_myfunc)
 # CHECK: Contents of section .got.plt:
-# CHECK: 401000 c4004000
+# CHECK: 401000 c0004000
 # CHECK: SYMBOL TABLE:
-# CHECK: 004000c4 g       .text 00000020 myfunc
+# CHECK: 004000c0 g       .text 00000020 myfunc
 # CHECK: 00400074 g       *ABS* 00000000 __rel_iplt_start
 # CHECK: 0040007c g       *ABS* 00000000 __rel_iplt_end
 
 ---
 FileHeader:
   Class:           ELFCLASS32
   Data:            ELFDATA2LSB
   Type:            ET_REL