ELF: Add support for emitting dynamic relocations in the Android relocation packing format.

The Android relocation packing format is a more compact
format for dynamic relocations in executables and DSOs
that is based on delta encoding and SLEBs. An overview
of the format can be found in the Android source code:
https://android.googlesource.com/platform/bionic/+/refs/heads/master/tools/relocation_packer/src/delta_encoder.h

This patch implements relocation packing using that format.

This implementation uses a more intelligent algorithm for compressing
relative relocations than Android's own relocation packer. As a
result it can generally create smaller relocation sections than
that packer. If I link Chromium for Android targeting ARM32 I get a
.rel.dyn of size 174693 bytes, as compared to 371832 bytes with gold
and the Android packer.

Differential Revision: https://reviews.llvm.org/D39152

llvm-svn: 316775

Author: pcc

lld/ELF/Config.h

@@ -105,6 +105,7 @@ struct Configuration {
   std::vector<SymbolVersion> VersionScriptLocals;
   std::vector<uint8_t> BuildIdVector;
   bool AllowMultipleDefinition;
+  bool AndroidPackDynRelocs = false;
   bool AsNeeded = false;
   bool Bsymbolic;
   bool BsymbolicFunctions;

lld/ELF/Driver.cpp

@@ -775,6 +775,14 @@ void LinkerDriver::readConfigs(opt::InputArgList &Args) {
 
   std::tie(Config->BuildId, Config->BuildIdVector) = getBuildId(Args);
 
+  if (auto *Arg = Args.getLastArg(OPT_pack_dyn_relocs_eq)) {
+    StringRef S = Arg->getValue();
+    if (S == "android")
+      Config->AndroidPackDynRelocs = true;
+    else if (S != "none")
+      error("unknown -pack-dyn-relocs format: " + S);
+  }
+
   if (auto *Arg = Args.getLastArg(OPT_symbol_ordering_file))
     if (Optional<MemoryBufferRef> Buffer = readFile(Arg->getValue()))
       Config->SymbolOrderingFile = getLines(*Buffer);

lld/ELF/Options.td

@@ -222,6 +222,9 @@ def omagic: Flag<["--"], "omagic">, MetaVarName<"<magic>">,
 defm orphan_handling: Eq<"orphan-handling">, 
   HelpText<"Control how orphan sections are handled when linker script used">;
 
+def pack_dyn_relocs_eq: J<"pack-dyn-relocs=">, MetaVarName<"<format>">,
+  HelpText<"Pack dynamic relocations in the given format (none or android)">;
+
 def pie: F<"pie">, HelpText<"Create a position independent executable">;
 
 def print_gc_sections: F<"print-gc-sections">,

lld/ELF/SyntheticSections.cpp

@@ -33,6 +33,7 @@
 #include "llvm/Object/Decompressor.h"
 #include "llvm/Object/ELFObjectFile.h"
 #include "llvm/Support/Endian.h"
+#include "llvm/Support/LEB128.h"
 #include "llvm/Support/MD5.h"
 #include "llvm/Support/RandomNumberGenerator.h"
 #include "llvm/Support/SHA1.h"
@@ -817,7 +818,7 @@ unsigned MipsGotSection::getLocalEntriesNum() const {
 
 void MipsGotSection::finalizeContents() { updateAllocSize(); }
 
-void MipsGotSection::updateAllocSize() {
+bool MipsGotSection::updateAllocSize() {
   PageEntriesNum = 0;
   for (std::pair<const OutputSection *, size_t> &P : PageIndexMap) {
     // For each output section referenced by GOT page relocations calculate
@@ -831,6 +832,7 @@ void MipsGotSection::updateAllocSize() {
   }
   Size = (getLocalEntriesNum() + GlobalEntries.size() + TlsEntries.size()) *
          Config->Wordsize;
+  return false;
 }
 
 bool MipsGotSection::empty() const {
@@ -1063,10 +1065,11 @@ template <class ELFT> void DynamicSection<ELFT>::finalizeContents() {
 
   this->Link = InX::DynStrTab->getParent()->SectionIndex;
   if (In<ELFT>::RelaDyn->getParent() && !In<ELFT>::RelaDyn->empty()) {
-    bool IsRela = Config->IsRela;
-    add({IsRela ? DT_RELA : DT_REL, In<ELFT>::RelaDyn});
-    add({IsRela ? DT_RELASZ : DT_RELSZ, In<ELFT>::RelaDyn->getParent(),
+    add({In<ELFT>::RelaDyn->DynamicTag, In<ELFT>::RelaDyn});
+    add({In<ELFT>::RelaDyn->SizeDynamicTag, In<ELFT>::RelaDyn->getParent(),
          Entry::SecSize});
+
+    bool IsRela = Config->IsRela;
     add({IsRela ? DT_RELAENT : DT_RELENT,
          uint64_t(IsRela ? sizeof(Elf_Rela) : sizeof(Elf_Rel))});
 
@@ -1202,21 +1205,57 @@ uint32_t DynamicReloc::getSymIndex() const {
   return 0;
 }
 
-template <class ELFT>
-RelocationSection<ELFT>::RelocationSection(StringRef Name, bool Sort)
-    : SyntheticSection(SHF_ALLOC, Config->IsRela ? SHT_RELA : SHT_REL,
-                       Config->Wordsize, Name),
-      Sort(Sort) {
-  this->Entsize = Config->IsRela ? sizeof(Elf_Rela) : sizeof(Elf_Rel);
-}
+RelocationBaseSection::RelocationBaseSection(StringRef Name, uint32_t Type,
+                                             int32_t DynamicTag,
+                                             int32_t SizeDynamicTag)
+    : SyntheticSection(SHF_ALLOC, Type, Config->Wordsize, Name),
+      DynamicTag(DynamicTag), SizeDynamicTag(SizeDynamicTag) {}
 
-template <class ELFT>
-void RelocationSection<ELFT>::addReloc(const DynamicReloc &Reloc) {
+void RelocationBaseSection::addReloc(const DynamicReloc &Reloc) {
   if (Reloc.Type == Target->RelativeRel)
     ++NumRelativeRelocs;
   Relocs.push_back(Reloc);
 }
 
+void RelocationBaseSection::finalizeContents() {
+  // If all relocations are R_*_RELATIVE they don't refer to any
+  // dynamic symbol and we don't need a dynamic symbol table. If that
+  // is the case, just use 0 as the link.
+  this->Link = InX::DynSymTab ? InX::DynSymTab->getParent()->SectionIndex : 0;
+
+  // Set required output section properties.
+  getParent()->Link = this->Link;
+}
+
+template <class ELFT>
+static void encodeDynamicReloc(typename ELFT::Rela *P,
+                               const DynamicReloc &Rel) {
+  if (Config->IsRela)
+    P->r_addend = Rel.getAddend();
+  P->r_offset = Rel.getOffset();
+  if (Config->EMachine == EM_MIPS && Rel.getInputSec() == InX::MipsGot)
+    // The MIPS GOT section contains dynamic relocations that correspond to TLS
+    // entries. These entries are placed after the global and local sections of
+    // the GOT. At the point when we create these relocations, the size of the
+    // global and local sections is unknown, so the offset that we store in the
+    // TLS entry's DynamicReloc is relative to the start of the TLS section of
+    // the GOT, rather than being relative to the start of the GOT. This line of
+    // code adds the size of the global and local sections to the virtual
+    // address computed by getOffset() in order to adjust it into the TLS
+    // section.
+    P->r_offset += InX::MipsGot->getTlsOffset();
+  P->setSymbolAndType(Rel.getSymIndex(), Rel.Type, Config->IsMips64EL);
+}
+
+template <class ELFT>
+RelocationSection<ELFT>::RelocationSection(StringRef Name, bool Sort)
+    : RelocationBaseSection(Name, Config->IsRela ? SHT_RELA : SHT_REL,
+                            Config->IsRela ? DT_RELA : DT_REL,
+                            Config->IsRela ? DT_RELASZ : DT_RELSZ),
+      Sort(Sort) {
+  this->Entsize = Config->IsRela ? sizeof(Elf_Rela) : sizeof(Elf_Rel);
+}
+
 template <class ELFT, class RelTy>
 static bool compRelocations(const RelTy &A, const RelTy &B) {
   bool AIsRel = A.getType(Config->IsMips64EL) == Target->RelativeRel;
@@ -1230,18 +1269,8 @@ static bool compRelocations(const RelTy &A, const RelTy &B) {
 template <class ELFT> void RelocationSection<ELFT>::writeTo(uint8_t *Buf) {
   uint8_t *BufBegin = Buf;
   for (const DynamicReloc &Rel : Relocs) {
-    auto *P = reinterpret_cast<Elf_Rela *>(Buf);
+    encodeDynamicReloc<ELFT>(reinterpret_cast<Elf_Rela *>(Buf), Rel);
     Buf += Config->IsRela ? sizeof(Elf_Rela) : sizeof(Elf_Rel);
-
-    if (Config->IsRela)
-      P->r_addend = Rel.getAddend();
-    P->r_offset = Rel.getOffset();
-    if (Config->EMachine == EM_MIPS && Rel.getInputSec() == InX::MipsGot)
-      // Dynamic relocation against MIPS GOT section make deal TLS entries
-      // allocated in the end of the GOT. We need to adjust the offset to take
-      // in account 'local' and 'global' GOT entries.
-      P->r_offset += InX::MipsGot->getTlsOffset();
-    P->setSymbolAndType(Rel.getSymIndex(), Rel.Type, Config->IsMips64EL);
   }
 
   if (Sort) {
@@ -1259,14 +1288,193 @@ template <class ELFT> unsigned RelocationSection<ELFT>::getRelocOffset() {
   return this->Entsize * Relocs.size();
 }
 
-template <class ELFT> void RelocationSection<ELFT>::finalizeContents() {
-  // If all relocations are *RELATIVE they don't refer to any
-  // dynamic symbol and we don't need a dynamic symbol table. If that
-  // is the case, just use 0 as the link.
-  this->Link = InX::DynSymTab ? InX::DynSymTab->getParent()->SectionIndex : 0;
+template <class ELFT>
+AndroidPackedRelocationSection<ELFT>::AndroidPackedRelocationSection(
+    StringRef Name)
+    : RelocationBaseSection(
+          Name, Config->IsRela ? SHT_ANDROID_RELA : SHT_ANDROID_REL,
+          Config->IsRela ? DT_ANDROID_RELA : DT_ANDROID_REL,
+          Config->IsRela ? DT_ANDROID_RELASZ : DT_ANDROID_RELSZ) {
+  this->Entsize = 1;
+}
 
-  // Set required output section properties.
-  getParent()->Link = this->Link;
+template <class ELFT>
+bool AndroidPackedRelocationSection<ELFT>::updateAllocSize() {
+  // This function computes the contents of an Android-format packed relocation
+  // section.
+  //
+  // This format compresses relocations by using relocation groups to factor out
+  // fields that are common between relocations and storing deltas from previous
+  // relocations in SLEB128 format (which has a short representation for small
+  // numbers). A good example of a relocation type with common fields is
+  // R_*_RELATIVE, which is normally used to represent function pointers in
+  // vtables. In the REL format, each relative relocation has the same r_info
+  // field, and is only different from other relative relocations in terms of
+  // the r_offset field. By sorting relocations by offset, grouping them by
+  // r_info and representing each relocation with only the delta from the
+  // previous offset, each 8-byte relocation can be compressed to as little as 1
+  // byte (or less with run-length encoding). This relocation packer was able to
+  // reduce the size of the relocation section in an Android Chromium DSO from
+  // 2,911,184 bytes to 174,693 bytes, or 6% of the original size.
+  //
+  // A relocation section consists of a header containing the literal bytes
+  // 'APS2' followed by a sequence of SLEB128-encoded integers. The first two
+  // elements are the total number of relocations in the section and an initial
+  // r_offset value. The remaining elements define a sequence of relocation
+  // groups. Each relocation group starts with a header consisting of the
+  // following elements:
+  //
+  // - the number of relocations in the relocation group
+  // - flags for the relocation group
+  // - (if RELOCATION_GROUPED_BY_OFFSET_DELTA_FLAG is set) the r_offset delta
+  //   for each relocation in the group.
+  // - (if RELOCATION_GROUPED_BY_INFO_FLAG is set) the value of the r_info
+  //   field for each relocation in the group.
+  // - (if RELOCATION_GROUP_HAS_ADDEND_FLAG and
+  //   RELOCATION_GROUPED_BY_ADDEND_FLAG are set) the r_addend delta for
+  //   each relocation in the group.
+  //
+  // Following the relocation group header are descriptions of each of the
+  // relocations in the group. They consist of the following elements:
+  //
+  // - (if RELOCATION_GROUPED_BY_OFFSET_DELTA_FLAG is not set) the r_offset
+  //   delta for this relocation.
+  // - (if RELOCATION_GROUPED_BY_INFO_FLAG is not set) the value of the r_info
+  //   field for this relocation.
+  // - (if RELOCATION_GROUP_HAS_ADDEND_FLAG is set and
+  //   RELOCATION_GROUPED_BY_ADDEND_FLAG is not set) the r_addend delta for
+  //   this relocation.
+
+  size_t OldSize = RelocData.size();
+
+  RelocData = {'A', 'P', 'S', '2'};
+  raw_svector_ostream OS(RelocData);
+
+  // The format header includes the number of relocations and the initial
+  // offset (we set this to zero because the first relocation group will
+  // perform the initial adjustment).
+  encodeSLEB128(Relocs.size(), OS);
+  encodeSLEB128(0, OS);
+
+  std::vector<Elf_Rela> Relatives, NonRelatives;
+
+  for (const DynamicReloc &Rel : Relocs) {
+    Elf_Rela R;
+    encodeDynamicReloc<ELFT>(&R, Rel);
+
+    if (R.getType(Config->IsMips64EL) == Target->RelativeRel)
+      Relatives.push_back(R);
+    else
+      NonRelatives.push_back(R);
+  }
+
+  std::sort(Relatives.begin(), Relatives.end(),
+            [](const Elf_Rel &A, const Elf_Rel &B) {
+              return A.r_offset < B.r_offset;
+            });
+
+  // Try to find groups of relative relocations which are spaced one word
+  // apart from one another. These generally correspond to vtable entries. The
+  // format allows these groups to be encoded using a sort of run-length
+  // encoding, but each group will cost 7 bytes in addition to the offset from
+  // the previous group, so it is only profitable to do this for groups of
+  // size 8 or larger.
+  std::vector<Elf_Rela> UngroupedRelatives;
+  std::vector<std::vector<Elf_Rela>> RelativeGroups;
+  for (auto I = Relatives.begin(), E = Relatives.end(); I != E;) {
+    std::vector<Elf_Rela> Group;
+    do {
+      Group.push_back(*I++);
+    } while (I != E && (I - 1)->r_offset + Config->Wordsize == I->r_offset);
+
+    if (Group.size() < 8)
+      UngroupedRelatives.insert(UngroupedRelatives.end(), Group.begin(),
+                                Group.end());
+    else
+      RelativeGroups.emplace_back(std::move(Group));
+  }
+
+  unsigned HasAddendIfRela =
+      Config->IsRela ? RELOCATION_GROUP_HAS_ADDEND_FLAG : 0;
+
+  uint64_t Offset = 0;
+  uint64_t Addend = 0;
+
+  // Emit the run-length encoding for the groups of adjacent relative
+  // relocations. Each group is represented using two groups in the packed
+  // format. The first is used to set the current offset to the start of the
+  // group (and also encodes the first relocation), and the second encodes the
+  // remaining relocations.
+  for (std::vector<Elf_Rela> &G : RelativeGroups) {
+    // The first relocation in the group.
+    encodeSLEB128(1, OS);
+    encodeSLEB128(RELOCATION_GROUPED_BY_OFFSET_DELTA_FLAG |
+                      RELOCATION_GROUPED_BY_INFO_FLAG | HasAddendIfRela,
+                  OS);
+    encodeSLEB128(G[0].r_offset - Offset, OS);
+    encodeSLEB128(Target->RelativeRel, OS);
+    if (Config->IsRela) {
+      encodeSLEB128(G[0].r_addend - Addend, OS);
+      Addend = G[0].r_addend;
+    }
+
+    // The remaining relocations.
+    encodeSLEB128(G.size() - 1, OS);
+    encodeSLEB128(RELOCATION_GROUPED_BY_OFFSET_DELTA_FLAG |
+                      RELOCATION_GROUPED_BY_INFO_FLAG | HasAddendIfRela,
+                  OS);
+    encodeSLEB128(Config->Wordsize, OS);
+    encodeSLEB128(Target->RelativeRel, OS);
+    if (Config->IsRela) {
+      for (auto I = G.begin() + 1, E = G.end(); I != E; ++I) {
+        encodeSLEB128(I->r_addend - Addend, OS);
+        Addend = I->r_addend;
+      }
+    }
+
+    Offset = G.back().r_offset;
+  }
+
+  // Now the ungrouped relatives.
+  if (!UngroupedRelatives.empty()) {
+    encodeSLEB128(UngroupedRelatives.size(), OS);
+    encodeSLEB128(RELOCATION_GROUPED_BY_INFO_FLAG | HasAddendIfRela, OS);
+    encodeSLEB128(Target->RelativeRel, OS);
+    for (Elf_Rela &R : UngroupedRelatives) {
+      encodeSLEB128(R.r_offset - Offset, OS);
+      Offset = R.r_offset;
+      if (Config->IsRela) {
+        encodeSLEB128(R.r_addend - Addend, OS);
+        Addend = R.r_addend;
+      }
+    }
+  }
+
+  // Finally the non-relative relocations.
+  std::sort(NonRelatives.begin(), NonRelatives.end(),
+            [](const Elf_Rela &A, const Elf_Rela &B) {
+              return A.r_offset < B.r_offset;
+            });
+  if (!NonRelatives.empty()) {
+    encodeSLEB128(NonRelatives.size(), OS);
+    encodeSLEB128(HasAddendIfRela, OS);
+    for (Elf_Rela &R : NonRelatives) {
+      encodeSLEB128(R.r_offset - Offset, OS);
+      Offset = R.r_offset;
+      encodeSLEB128(R.r_info, OS);
+      if (Config->IsRela) {
+        encodeSLEB128(R.r_addend - Addend, OS);
+        Addend = R.r_addend;
+      }
+    }
+  }
+
+  // Returns whether the section size changed. We need to keep recomputing both
+  // section layout and the contents of this section until the size converges
+  // because changing this section's size can affect section layout, which in
+  // turn can affect the sizes of the LEB-encoded integers stored in this
+  // section.
+  return RelocData.size() != OldSize;
 }
 
 SymbolTableBaseSection::SymbolTableBaseSection(StringTableSection &StrTabSec)
@@ -2471,6 +2679,11 @@ template class elf::RelocationSection<ELF32BE>;
 template class elf::RelocationSection<ELF64LE>;
 template class elf::RelocationSection<ELF64BE>;
 
+template class elf::AndroidPackedRelocationSection<ELF32LE>;
+template class elf::AndroidPackedRelocationSection<ELF32BE>;
+template class elf::AndroidPackedRelocationSection<ELF64LE>;
+template class elf::AndroidPackedRelocationSection<ELF64BE>;
+
 template class elf::SymbolTableSection<ELF32LE>;
 template class elf::SymbolTableSection<ELF32BE>;
 template class elf::SymbolTableSection<ELF64LE>;