MachO/UnwindInfoSection.cpp - llvm-project/lld - Git at Google

 //===- UnwindInfoSection.cpp ----------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//

 #include "UnwindInfoSection.h"
 #include "Config.h"
 #include "InputSection.h"
 #include "MergedOutputSection.h"
 #include "OutputSection.h"
 #include "OutputSegment.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
 #include "SyntheticSections.h"
 #include "Target.h"

 #include "lld/Common/ErrorHandler.h"
 #include "lld/Common/Memory.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/BinaryFormat/MachO.h"

 using namespace llvm;
 using namespace llvm::MachO;
 using namespace lld;
 using namespace lld::macho;

 #define COMMON_ENCODINGS_MAX 127
 #define COMPACT_ENCODINGS_MAX 256

 #define SECOND_LEVEL_PAGE_BYTES 4096
 #define SECOND_LEVEL_PAGE_WORDS (SECOND_LEVEL_PAGE_BYTES / sizeof(uint32_t))
 #define REGULAR_SECOND_LEVEL_ENTRIES_MAX                                       \
   ((SECOND_LEVEL_PAGE_BYTES -                                                  \
     sizeof(unwind_info_regular_second_level_page_header)) /                    \
    sizeof(unwind_info_regular_second_level_entry))
 #define COMPRESSED_SECOND_LEVEL_ENTRIES_MAX                                    \
   ((SECOND_LEVEL_PAGE_BYTES -                                                  \
     sizeof(unwind_info_compressed_second_level_page_header)) /                 \
    sizeof(uint32_t))

 #define COMPRESSED_ENTRY_FUNC_OFFSET_BITS 24
 #define COMPRESSED_ENTRY_FUNC_OFFSET_MASK                                      \
   UNWIND_INFO_COMPRESSED_ENTRY_FUNC_OFFSET(~0)

 // Compact Unwind format is a Mach-O evolution of DWARF Unwind that
 // optimizes space and exception-time lookup.  Most DWARF unwind
 // entries can be replaced with Compact Unwind entries, but the ones
 // that cannot are retained in DWARF form.
 //
 // This comment will address macro-level organization of the pre-link
 // and post-link compact unwind tables. For micro-level organization
 // pertaining to the bitfield layout of the 32-bit compact unwind
 // entries, see libunwind/include/mach-o/compact_unwind_encoding.h
 //
 // Important clarifying factoids:
 //
 // * __LD,__compact_unwind is the compact unwind format for compiler
 // output and linker input. It is never a final output. It could be
 // an intermediate output with the `-r` option which retains relocs.
 //
 // * __TEXT,__unwind_info is the compact unwind format for final
 // linker output. It is never an input.
 //
 // * __TEXT,__eh_frame is the DWARF format for both linker input and output.
 //
 // * __TEXT,__unwind_info entries are divided into 4 KiB pages (2nd
 // level) by ascending address, and the pages are referenced by an
 // index (1st level) in the section header.
 //
 // * Following the headers in __TEXT,__unwind_info, the bulk of the
 // section contains a vector of compact unwind entries
 // `{functionOffset, encoding}` sorted by ascending `functionOffset`.
 // Adjacent entries with the same encoding can be folded to great
 // advantage, achieving a 3-order-of-magnitude reduction in the
 // number of entries.
 //
 // * The __TEXT,__unwind_info format can accommodate up to 127 unique
 // encodings for the space-efficient compressed format. In practice,
 // fewer than a dozen unique encodings are used by C++ programs of
 // all sizes. Therefore, we don't even bother implementing the regular
 // non-compressed format. Time will tell if anyone in the field ever
 // overflows the 127-encodings limit.
 //
 // Refer to the definition of unwind_info_section_header in
 // compact_unwind_encoding.h for an overview of the format we are encoding
 // here.

 // TODO(gkm): prune __eh_frame entries superseded by __unwind_info
 // TODO(gkm): how do we align the 2nd-level pages?

 UnwindInfoSection::UnwindInfoSection()
     : SyntheticSection(segment_names::text, section_names::unwindInfo) {
   align = 4; // mimic ld64
 }

 bool UnwindInfoSection::isNeeded() const {
   return (compactUnwindSection != nullptr);
 }

 SmallDenseMap<std::pair<InputSection *, uint64_t /* addend */>, macho::Symbol *>
     personalityTable;

 // Compact unwind relocations have different semantics, so we handle them in a
 // separate code path from regular relocations. First, we do not wish to add
 // rebase opcodes for __LD,__compact_unwind, because that section doesn't
 // actually end up in the final binary. Second, personality pointers always
 // reside in the GOT and must be treated specially.
 void macho::prepareCompactUnwind(InputSection *isec) {
   assert(isec->segname == segment_names::ld &&
          isec->name == section_names::compactUnwind);

   for (Reloc &r : isec->relocs) {
     assert(target->hasAttr(r.type, RelocAttrBits::UNSIGNED));
     if (r.offset % sizeof(CompactUnwindEntry64) !=
         offsetof(struct CompactUnwindEntry64, personality))
       continue;

     if (auto *s = r.referent.dyn_cast<lld::macho::Symbol *>()) {
       if (auto *undefined = dyn_cast<Undefined>(s)) {
         treatUndefinedSymbol(*undefined);
         // treatUndefinedSymbol() can replace s with a DylibSymbol; re-check.
         if (isa<Undefined>(s))
           continue;
       }
       if (auto *defined = dyn_cast<Defined>(s)) {
         // Check if we have created a synthetic symbol at the same address.
         macho::Symbol *&personality =
             personalityTable[{defined->isec, defined->value}];
         if (personality == nullptr) {
           personality = defined;
           in.got->addEntry(defined);
         } else if (personality != defined) {
           r.referent = personality;
         }
         continue;
       }
       assert(isa<DylibSymbol>(s));
       in.got->addEntry(s);
       continue;
     }

     if (auto *referentIsec = r.referent.dyn_cast<InputSection *>()) {
       // Personality functions can be referenced via section relocations
       // if they live in the same object file. Create placeholder synthetic
       // symbols for them in the GOT.
       macho::Symbol *&s = personalityTable[{referentIsec, r.addend}];
       if (s == nullptr) {
         s = make<Defined>("<internal>", nullptr, referentIsec, r.addend, false,
                           false, false);
         in.got->addEntry(s);
       }
       r.referent = s;
       r.addend = 0;
     }
   }
 }

 // Unwind info lives in __DATA, and finalization of __TEXT will occur before
 // finalization of __DATA. Moreover, the finalization of unwind info depends on
 // the exact addresses that it references. So it is safe for compact unwind to
 // reference addresses in __TEXT, but not addresses in any other segment.
 static void checkTextSegment(InputSection *isec) {
   if (isec->segname != segment_names::text)
     error("compact unwind references address in " + toString(isec) +
           " which is not in segment __TEXT");
 }

 // We need to apply the relocations to the pre-link compact unwind section
 // before converting it to post-link form. There should only be absolute
 // relocations here: since we are not emitting the pre-link CU section, there
 // is no source address to make a relative location meaningful.
 static void relocateCompactUnwind(MergedOutputSection *compactUnwindSection,
                                   std::vector<CompactUnwindEntry64> &cuVector) {
   for (const InputSection *isec : compactUnwindSection->inputs) {
     uint8_t *buf =
         reinterpret_cast<uint8_t *>(cuVector.data()) + isec->outSecFileOff;
     memcpy(buf, isec->data.data(), isec->data.size());

     for (const Reloc &r : isec->relocs) {
       uint64_t referentVA = 0;
       if (auto *referentSym = r.referent.dyn_cast<macho::Symbol *>()) {
         if (!isa<Undefined>(referentSym)) {
           assert(referentSym->isInGot());
           if (auto *defined = dyn_cast<Defined>(referentSym))
             checkTextSegment(defined->isec);
           // At this point in the link, we may not yet know the final address of
           // the GOT, so we just encode the index. We make it a 1-based index so
           // that we can distinguish the null pointer case.
           referentVA = referentSym->gotIndex + 1;
         }
       } else if (auto *referentIsec = r.referent.dyn_cast<InputSection *>()) {
         checkTextSegment(referentIsec);
         referentVA = referentIsec->getVA() + r.addend;
       }
       support::endian::write64le(buf + r.offset, referentVA);
     }
   }
 }

 // There should only be a handful of unique personality pointers, so we can
 // encode them as 2-bit indices into a small array.
 void encodePersonalities(const std::vector<CompactUnwindEntry64 *> &cuPtrVector,
                          std::vector<uint32_t> &personalities) {
   for (CompactUnwindEntry64 *cu : cuPtrVector) {
     if (cu->personality == 0)
       continue;
     uint32_t personalityOffset = cu->personality - in.header->addr;
     // Linear search is fast enough for a small array.
     auto it = find(personalities, personalityOffset);
     uint32_t personalityIndex; // 1-based index
     if (it != personalities.end()) {
       personalityIndex = std::distance(personalities.begin(), it) + 1;
     } else {
       personalities.push_back(cu->personality);
       personalityIndex = personalities.size();
     }
     cu->encoding |=
         personalityIndex << countTrailingZeros(
             static_cast<compact_unwind_encoding_t>(UNWIND_PERSONALITY_MASK));
   }
   if (personalities.size() > 3)
     error("too many personalities (" + std::to_string(personalities.size()) +
           ") for compact unwind to encode");
 }

 // Scan the __LD,__compact_unwind entries and compute the space needs of
 // __TEXT,__unwind_info and __TEXT,__eh_frame
 void UnwindInfoSection::finalize() {
   if (compactUnwindSection == nullptr)
     return;

   // At this point, the address space for __TEXT,__text has been
   // assigned, so we can relocate the __LD,__compact_unwind entries
   // into a temporary buffer. Relocation is necessary in order to sort
   // the CU entries by function address. Sorting is necessary so that
   // we can fold adjacent CU entries with identical
   // encoding+personality+lsda. Folding is necessary because it reduces
   // the number of CU entries by as much as 3 orders of magnitude!
   compactUnwindSection->finalize();
   assert(compactUnwindSection->getSize() % sizeof(CompactUnwindEntry64) == 0);
   size_t cuCount =
       compactUnwindSection->getSize() / sizeof(CompactUnwindEntry64);
   cuVector.resize(cuCount);
   relocateCompactUnwind(compactUnwindSection, cuVector);

   // Rather than sort & fold the 32-byte entries directly, we create a
   // vector of pointers to entries and sort & fold that instead.
   cuPtrVector.reserve(cuCount);
   for (CompactUnwindEntry64 &cuEntry : cuVector)
     cuPtrVector.emplace_back(&cuEntry);
   std::sort(cuPtrVector.begin(), cuPtrVector.end(),
             [](const CompactUnwindEntry64 *a, const CompactUnwindEntry64 *b) {
               return a->functionAddress < b->functionAddress;
             });

   // Fold adjacent entries with matching encoding+personality+lsda
   // We use three iterators on the same cuPtrVector to fold in-situ:
   // (1) `foldBegin` is the first of a potential sequence of matching entries
   // (2) `foldEnd` is the first non-matching entry after `foldBegin`.
   // The semi-open interval [ foldBegin .. foldEnd ) contains a range
   // entries that can be folded into a single entry and written to ...
   // (3) `foldWrite`
   auto foldWrite = cuPtrVector.begin();
   for (auto foldBegin = cuPtrVector.begin(); foldBegin < cuPtrVector.end();) {
     auto foldEnd = foldBegin;
     while (++foldEnd < cuPtrVector.end() &&
            (*foldBegin)->encoding == (*foldEnd)->encoding &&
            (*foldBegin)->personality == (*foldEnd)->personality &&
            (*foldBegin)->lsda == (*foldEnd)->lsda)
       ;
     *foldWrite++ = *foldBegin;
     foldBegin = foldEnd;
   }
   cuPtrVector.erase(foldWrite, cuPtrVector.end());

   encodePersonalities(cuPtrVector, personalities);

   // Count frequencies of the folded encodings
   EncodingMap encodingFrequencies;
   for (const CompactUnwindEntry64 *cuPtrEntry : cuPtrVector)
     encodingFrequencies[cuPtrEntry->encoding]++;

   // Make a vector of encodings, sorted by descending frequency
   for (const auto &frequency : encodingFrequencies)
     commonEncodings.emplace_back(frequency);
   std::sort(commonEncodings.begin(), commonEncodings.end(),
             [](const std::pair<compact_unwind_encoding_t, size_t> &a,
                const std::pair<compact_unwind_encoding_t, size_t> &b) {
               if (a.second == b.second)
                 // When frequencies match, secondarily sort on encoding
                 // to maintain parity with validate-unwind-info.py
                 return a.first > b.first;
               return a.second > b.second;
             });

   // Truncate the vector to 127 elements.
   // Common encoding indexes are limited to 0..126, while encoding
   // indexes 127..255 are local to each second-level page
   if (commonEncodings.size() > COMMON_ENCODINGS_MAX)
     commonEncodings.resize(COMMON_ENCODINGS_MAX);

   // Create a map from encoding to common-encoding-table index
   for (size_t i = 0; i < commonEncodings.size(); i++)
     commonEncodingIndexes[commonEncodings[i].first] = i;

   // Split folded encodings into pages, where each page is limited by ...
   // (a) 4 KiB capacity
   // (b) 24-bit difference between first & final function address
   // (c) 8-bit compact-encoding-table index,
   //     for which 0..126 references the global common-encodings table,
   //     and 127..255 references a local per-second-level-page table.
   // First we try the compact format and determine how many entries fit.
   // If more entries fit in the regular format, we use that.
   for (size_t i = 0; i < cuPtrVector.size();) {
     secondLevelPages.emplace_back();
     UnwindInfoSection::SecondLevelPage &page = secondLevelPages.back();
     page.entryIndex = i;
     uintptr_t functionAddressMax =
         cuPtrVector[i]->functionAddress + COMPRESSED_ENTRY_FUNC_OFFSET_MASK;
     size_t n = commonEncodings.size();
     size_t wordsRemaining =
         SECOND_LEVEL_PAGE_WORDS -
         sizeof(unwind_info_compressed_second_level_page_header) /
             sizeof(uint32_t);
     while (wordsRemaining >= 1 && i < cuPtrVector.size()) {
       const CompactUnwindEntry64 *cuPtr = cuPtrVector[i];
       if (cuPtr->functionAddress >= functionAddressMax) {
         break;
       } else if (commonEncodingIndexes.count(cuPtr->encoding) ||
                  page.localEncodingIndexes.count(cuPtr->encoding)) {
         i++;
         wordsRemaining--;
       } else if (wordsRemaining >= 2 && n < COMPACT_ENCODINGS_MAX) {
         page.localEncodings.emplace_back(cuPtr->encoding);
         page.localEncodingIndexes[cuPtr->encoding] = n++;
         i++;
         wordsRemaining -= 2;
       } else {
         break;
       }
     }
     page.entryCount = i - page.entryIndex;

     // If this is not the final page, see if it's possible to fit more
     // entries by using the regular format. This can happen when there
     // are many unique encodings, and we we saturated the local
     // encoding table early.
     if (i < cuPtrVector.size() &&
         page.entryCount < REGULAR_SECOND_LEVEL_ENTRIES_MAX) {
       page.kind = UNWIND_SECOND_LEVEL_REGULAR;
       page.entryCount = std::min(REGULAR_SECOND_LEVEL_ENTRIES_MAX,
                                  cuPtrVector.size() - page.entryIndex);
       i = page.entryIndex + page.entryCount;
     } else {
       page.kind = UNWIND_SECOND_LEVEL_COMPRESSED;
     }
   }

   for (const CompactUnwindEntry64 *cu : cuPtrVector) {
     uint32_t functionOffset = cu->functionAddress - in.header->addr;
     functionToLsdaIndex[functionOffset] = lsdaEntries.size();
     if (cu->lsda != 0)
       lsdaEntries.push_back(
           {functionOffset, static_cast<uint32_t>(cu->lsda - in.header->addr)});
   }

   // compute size of __TEXT,__unwind_info section
   level2PagesOffset =
       sizeof(unwind_info_section_header) +
       commonEncodings.size() * sizeof(uint32_t) +
       personalities.size() * sizeof(uint32_t) +
       // The extra second-level-page entry is for the sentinel
       (secondLevelPages.size() + 1) *
           sizeof(unwind_info_section_header_index_entry) +
       lsdaEntries.size() * sizeof(unwind_info_section_header_lsda_index_entry);
   unwindInfoSize =
       level2PagesOffset + secondLevelPages.size() * SECOND_LEVEL_PAGE_BYTES;
 }

 // All inputs are relocated and output addresses are known, so write!

 void UnwindInfoSection::writeTo(uint8_t *buf) const {
   // section header
   auto *uip = reinterpret_cast<unwind_info_section_header *>(buf);
   uip->version = 1;
   uip->commonEncodingsArraySectionOffset = sizeof(unwind_info_section_header);
   uip->commonEncodingsArrayCount = commonEncodings.size();
   uip->personalityArraySectionOffset =
       uip->commonEncodingsArraySectionOffset +
       (uip->commonEncodingsArrayCount * sizeof(uint32_t));
   uip->personalityArrayCount = personalities.size();
   uip->indexSectionOffset = uip->personalityArraySectionOffset +
                             (uip->personalityArrayCount * sizeof(uint32_t));
   uip->indexCount = secondLevelPages.size() + 1;

   // Common encodings
   auto *i32p = reinterpret_cast<uint32_t *>(&uip[1]);
   for (const auto &encoding : commonEncodings)
     *i32p++ = encoding.first;

   // Personalities
   for (const uint32_t &personality : personalities)
     *i32p++ = in.got->addr + (personality - 1) * WordSize;

   // Level-1 index
   uint32_t lsdaOffset =
       uip->indexSectionOffset +
       uip->indexCount * sizeof(unwind_info_section_header_index_entry);
   uint64_t l2PagesOffset = level2PagesOffset;
   auto *iep = reinterpret_cast<unwind_info_section_header_index_entry *>(i32p);
   for (const SecondLevelPage &page : secondLevelPages) {
     iep->functionOffset =
         cuPtrVector[page.entryIndex]->functionAddress - in.header->addr;
     iep->secondLevelPagesSectionOffset = l2PagesOffset;
     iep->lsdaIndexArraySectionOffset =
         lsdaOffset + functionToLsdaIndex.lookup(iep->functionOffset) *
                          sizeof(unwind_info_section_header_lsda_index_entry);
     iep++;
     l2PagesOffset += SECOND_LEVEL_PAGE_BYTES;
   }
   // Level-1 sentinel
   const CompactUnwindEntry64 &cuEnd = cuVector.back();
   iep->functionOffset = cuEnd.functionAddress + cuEnd.functionLength;
   iep->secondLevelPagesSectionOffset = 0;
   iep->lsdaIndexArraySectionOffset =
       lsdaOffset +
       lsdaEntries.size() * sizeof(unwind_info_section_header_lsda_index_entry);
   iep++;

   // LSDAs
   size_t lsdaBytes =
       lsdaEntries.size() * sizeof(unwind_info_section_header_lsda_index_entry);
   if (lsdaBytes > 0)
     memcpy(iep, lsdaEntries.data(), lsdaBytes);

   // Level-2 pages
   auto *pp = reinterpret_cast<uint32_t *>(reinterpret_cast<uint8_t *>(iep) +
                                           lsdaBytes);
   for (const SecondLevelPage &page : secondLevelPages) {
     if (page.kind == UNWIND_SECOND_LEVEL_COMPRESSED) {
       uintptr_t functionAddressBase =
           cuPtrVector[page.entryIndex]->functionAddress;
       auto *p2p =
           reinterpret_cast<unwind_info_compressed_second_level_page_header *>(
               pp);
       p2p->kind = page.kind;
       p2p->entryPageOffset =
           sizeof(unwind_info_compressed_second_level_page_header);
       p2p->entryCount = page.entryCount;
       p2p->encodingsPageOffset =
           p2p->entryPageOffset + p2p->entryCount * sizeof(uint32_t);
       p2p->encodingsCount = page.localEncodings.size();
       auto *ep = reinterpret_cast<uint32_t *>(&p2p[1]);
       for (size_t i = 0; i < page.entryCount; i++) {
         const CompactUnwindEntry64 *cuep = cuPtrVector[page.entryIndex + i];
         auto it = commonEncodingIndexes.find(cuep->encoding);
         if (it == commonEncodingIndexes.end())
           it = page.localEncodingIndexes.find(cuep->encoding);
         *ep++ = (it->second << COMPRESSED_ENTRY_FUNC_OFFSET_BITS) |
                 (cuep->functionAddress - functionAddressBase);
       }
       if (page.localEncodings.size() != 0)
         memcpy(ep, page.localEncodings.data(),
                page.localEncodings.size() * sizeof(uint32_t));
     } else {
       auto *p2p =
           reinterpret_cast<unwind_info_regular_second_level_page_header *>(pp);
       p2p->kind = page.kind;
       p2p->entryPageOffset =
           sizeof(unwind_info_regular_second_level_page_header);
       p2p->entryCount = page.entryCount;
       auto *ep = reinterpret_cast<uint32_t *>(&p2p[1]);
       for (size_t i = 0; i < page.entryCount; i++) {
         const CompactUnwindEntry64 *cuep = cuPtrVector[page.entryIndex + i];
         *ep++ = cuep->functionAddress;
         *ep++ = cuep->encoding;
       }
     }
     pp += SECOND_LEVEL_PAGE_WORDS;
   }
 }
	//===- UnwindInfoSection.cpp ----------------------------------------------===//
	//
	// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
	// See https://llvm.org/LICENSE.txt for license information.
	// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
	//
	//===----------------------------------------------------------------------===//

	#include "UnwindInfoSection.h"
	#include "Config.h"
	#include "InputSection.h"
	#include "MergedOutputSection.h"
	#include "OutputSection.h"
	#include "OutputSegment.h"
	#include "SymbolTable.h"
	#include "Symbols.h"
	#include "SyntheticSections.h"
	#include "Target.h"

	#include "lld/Common/ErrorHandler.h"
	#include "lld/Common/Memory.h"
	#include "llvm/ADT/SmallVector.h"
	#include "llvm/BinaryFormat/MachO.h"

	using namespace llvm;
	using namespace llvm::MachO;
	using namespace lld;
	using namespace lld::macho;

	#define COMMON_ENCODINGS_MAX 127
	#define COMPACT_ENCODINGS_MAX 256

	#define SECOND_LEVEL_PAGE_BYTES 4096
	#define SECOND_LEVEL_PAGE_WORDS (SECOND_LEVEL_PAGE_BYTES / sizeof(uint32_t))
	#define REGULAR_SECOND_LEVEL_ENTRIES_MAX \
	((SECOND_LEVEL_PAGE_BYTES - \
	sizeof(unwind_info_regular_second_level_page_header)) / \
	sizeof(unwind_info_regular_second_level_entry))
	#define COMPRESSED_SECOND_LEVEL_ENTRIES_MAX \
	((SECOND_LEVEL_PAGE_BYTES - \
	sizeof(unwind_info_compressed_second_level_page_header)) / \
	sizeof(uint32_t))

	#define COMPRESSED_ENTRY_FUNC_OFFSET_BITS 24
	#define COMPRESSED_ENTRY_FUNC_OFFSET_MASK \
	UNWIND_INFO_COMPRESSED_ENTRY_FUNC_OFFSET(~0)

	// Compact Unwind format is a Mach-O evolution of DWARF Unwind that
	// optimizes space and exception-time lookup. Most DWARF unwind
	// entries can be replaced with Compact Unwind entries, but the ones
	// that cannot are retained in DWARF form.
	//
	// This comment will address macro-level organization of the pre-link
	// and post-link compact unwind tables. For micro-level organization
	// pertaining to the bitfield layout of the 32-bit compact unwind
	// entries, see libunwind/include/mach-o/compact_unwind_encoding.h
	//
	// Important clarifying factoids:
	//
	// * __LD,__compact_unwind is the compact unwind format for compiler
	// output and linker input. It is never a final output. It could be
	// an intermediate output with the `-r` option which retains relocs.
	//
	// * __TEXT,__unwind_info is the compact unwind format for final
	// linker output. It is never an input.
	//
	// * __TEXT,__eh_frame is the DWARF format for both linker input and output.
	//
	// * __TEXT,__unwind_info entries are divided into 4 KiB pages (2nd
	// level) by ascending address, and the pages are referenced by an
	// index (1st level) in the section header.
	//
	// * Following the headers in __TEXT,__unwind_info, the bulk of the
	// section contains a vector of compact unwind entries
	// `{functionOffset, encoding}` sorted by ascending `functionOffset`.
	// Adjacent entries with the same encoding can be folded to great
	// advantage, achieving a 3-order-of-magnitude reduction in the
	// number of entries.
	//
	// * The __TEXT,__unwind_info format can accommodate up to 127 unique
	// encodings for the space-efficient compressed format. In practice,
	// fewer than a dozen unique encodings are used by C++ programs of
	// all sizes. Therefore, we don't even bother implementing the regular
	// non-compressed format. Time will tell if anyone in the field ever
	// overflows the 127-encodings limit.
	//
	// Refer to the definition of unwind_info_section_header in
	// compact_unwind_encoding.h for an overview of the format we are encoding
	// here.

	// TODO(gkm): prune __eh_frame entries superseded by __unwind_info
	// TODO(gkm): how do we align the 2nd-level pages?

	UnwindInfoSection::UnwindInfoSection()
	: SyntheticSection(segment_names::text, section_names::unwindInfo) {
	align = 4; // mimic ld64
	}

	bool UnwindInfoSection::isNeeded() const {
	return (compactUnwindSection != nullptr);
	}

	SmallDenseMap<std::pair<InputSection , uint64_t / addend />, macho::Symbol >
	personalityTable;

	// Compact unwind relocations have different semantics, so we handle them in a
	// separate code path from regular relocations. First, we do not wish to add
	// rebase opcodes for __LD,__compact_unwind, because that section doesn't
	// actually end up in the final binary. Second, personality pointers always
	// reside in the GOT and must be treated specially.
	void macho::prepareCompactUnwind(InputSection *isec) {
	assert(isec->segname == segment_names::ld &&
	isec->name == section_names::compactUnwind);

	for (Reloc &r : isec->relocs) {
	assert(target->hasAttr(r.type, RelocAttrBits::UNSIGNED));
	if (r.offset % sizeof(CompactUnwindEntry64) !=
	offsetof(struct CompactUnwindEntry64, personality))
	continue;

	if (auto s = r.referent.dyn_cast<lld::macho::Symbol >()) {
	if (auto *undefined = dyn_cast<Undefined>(s)) {
	treatUndefinedSymbol(*undefined);
	// treatUndefinedSymbol() can replace s with a DylibSymbol; re-check.
	if (isa<Undefined>(s))
	continue;
	}
	if (auto *defined = dyn_cast<Defined>(s)) {
	// Check if we have created a synthetic symbol at the same address.
	macho::Symbol *&personality =
	personalityTable[{defined->isec, defined->value}];
	if (personality == nullptr) {
	personality = defined;
	in.got->addEntry(defined);
	} else if (personality != defined) {
	r.referent = personality;
	}
	continue;
	}
	assert(isa<DylibSymbol>(s));
	in.got->addEntry(s);
	continue;
	}

	if (auto referentIsec = r.referent.dyn_cast<InputSection >()) {
	// Personality functions can be referenced via section relocations
	// if they live in the same object file. Create placeholder synthetic
	// symbols for them in the GOT.
	macho::Symbol *&s = personalityTable[{referentIsec, r.addend}];
	if (s == nullptr) {
	s = make<Defined>("<internal>", nullptr, referentIsec, r.addend, false,
	false, false);
	in.got->addEntry(s);
	}
	r.referent = s;
	r.addend = 0;
	}
	}
	}

	// Unwind info lives in __DATA, and finalization of __TEXT will occur before
	// finalization of __DATA. Moreover, the finalization of unwind info depends on
	// the exact addresses that it references. So it is safe for compact unwind to
	// reference addresses in __TEXT, but not addresses in any other segment.
	static void checkTextSegment(InputSection *isec) {
	if (isec->segname != segment_names::text)
	error("compact unwind references address in " + toString(isec) +
	" which is not in segment __TEXT");
	}

	// We need to apply the relocations to the pre-link compact unwind section
	// before converting it to post-link form. There should only be absolute
	// relocations here: since we are not emitting the pre-link CU section, there
	// is no source address to make a relative location meaningful.
	static void relocateCompactUnwind(MergedOutputSection *compactUnwindSection,
	std::vector<CompactUnwindEntry64> &cuVector) {
	for (const InputSection *isec : compactUnwindSection->inputs) {
	uint8_t *buf =
	reinterpret_cast<uint8_t *>(cuVector.data()) + isec->outSecFileOff;
	memcpy(buf, isec->data.data(), isec->data.size());

	for (const Reloc &r : isec->relocs) {
	uint64_t referentVA = 0;
	if (auto referentSym = r.referent.dyn_cast<macho::Symbol >()) {
	if (!isa<Undefined>(referentSym)) {
	assert(referentSym->isInGot());
	if (auto *defined = dyn_cast<Defined>(referentSym))
	checkTextSegment(defined->isec);
	// At this point in the link, we may not yet know the final address of
	// the GOT, so we just encode the index. We make it a 1-based index so
	// that we can distinguish the null pointer case.
	referentVA = referentSym->gotIndex + 1;
	}
	} else if (auto referentIsec = r.referent.dyn_cast<InputSection >()) {
	checkTextSegment(referentIsec);
	referentVA = referentIsec->getVA() + r.addend;
	}
	support::endian::write64le(buf + r.offset, referentVA);
	}
	}
	}

	// There should only be a handful of unique personality pointers, so we can
	// encode them as 2-bit indices into a small array.
	void encodePersonalities(const std::vector<CompactUnwindEntry64 *> &cuPtrVector,
	std::vector<uint32_t> &personalities) {
	for (CompactUnwindEntry64 *cu : cuPtrVector) {
	if (cu->personality == 0)
	continue;
	uint32_t personalityOffset = cu->personality - in.header->addr;
	// Linear search is fast enough for a small array.
	auto it = find(personalities, personalityOffset);
	uint32_t personalityIndex; // 1-based index
	if (it != personalities.end()) {
	personalityIndex = std::distance(personalities.begin(), it) + 1;
	} else {
	personalities.push_back(cu->personality);
	personalityIndex = personalities.size();
	}
	cu->encoding \|=
	personalityIndex << countTrailingZeros(
	static_cast<compact_unwind_encoding_t>(UNWIND_PERSONALITY_MASK));
	}
	if (personalities.size() > 3)
	error("too many personalities (" + std::to_string(personalities.size()) +
	") for compact unwind to encode");
	}

	// Scan the __LD,__compact_unwind entries and compute the space needs of
	// __TEXT,__unwind_info and __TEXT,__eh_frame
	void UnwindInfoSection::finalize() {
	if (compactUnwindSection == nullptr)
	return;

	// At this point, the address space for __TEXT,__text has been
	// assigned, so we can relocate the __LD,__compact_unwind entries
	// into a temporary buffer. Relocation is necessary in order to sort
	// the CU entries by function address. Sorting is necessary so that
	// we can fold adjacent CU entries with identical
	// encoding+personality+lsda. Folding is necessary because it reduces
	// the number of CU entries by as much as 3 orders of magnitude!
	compactUnwindSection->finalize();
	assert(compactUnwindSection->getSize() % sizeof(CompactUnwindEntry64) == 0);
	size_t cuCount =
	compactUnwindSection->getSize() / sizeof(CompactUnwindEntry64);
	cuVector.resize(cuCount);
	relocateCompactUnwind(compactUnwindSection, cuVector);

	// Rather than sort & fold the 32-byte entries directly, we create a
	// vector of pointers to entries and sort & fold that instead.
	cuPtrVector.reserve(cuCount);
	for (CompactUnwindEntry64 &cuEntry : cuVector)
	cuPtrVector.emplace_back(&cuEntry);
	std::sort(cuPtrVector.begin(), cuPtrVector.end(),
	[](const CompactUnwindEntry64 a, const CompactUnwindEntry64 b) {
	return a->functionAddress < b->functionAddress;
	});

	// Fold adjacent entries with matching encoding+personality+lsda
	// We use three iterators on the same cuPtrVector to fold in-situ:
	// (1) `foldBegin` is the first of a potential sequence of matching entries
	// (2) `foldEnd` is the first non-matching entry after `foldBegin`.
	// The semi-open interval [ foldBegin .. foldEnd ) contains a range
	// entries that can be folded into a single entry and written to ...
	// (3) `foldWrite`
	auto foldWrite = cuPtrVector.begin();
	for (auto foldBegin = cuPtrVector.begin(); foldBegin < cuPtrVector.end();) {
	auto foldEnd = foldBegin;
	while (++foldEnd < cuPtrVector.end() &&
	(foldBegin)->encoding == (foldEnd)->encoding &&
	(foldBegin)->personality == (foldEnd)->personality &&
	(foldBegin)->lsda == (foldEnd)->lsda)
	;
	foldWrite++ = foldBegin;
	foldBegin = foldEnd;
	}
	cuPtrVector.erase(foldWrite, cuPtrVector.end());

	encodePersonalities(cuPtrVector, personalities);

	// Count frequencies of the folded encodings
	EncodingMap encodingFrequencies;
	for (const CompactUnwindEntry64 *cuPtrEntry : cuPtrVector)
	encodingFrequencies[cuPtrEntry->encoding]++;

	// Make a vector of encodings, sorted by descending frequency
	for (const auto &frequency : encodingFrequencies)
	commonEncodings.emplace_back(frequency);
	std::sort(commonEncodings.begin(), commonEncodings.end(),
	[](const std::pair<compact_unwind_encoding_t, size_t> &a,
	const std::pair<compact_unwind_encoding_t, size_t> &b) {
	if (a.second == b.second)
	// When frequencies match, secondarily sort on encoding
	// to maintain parity with validate-unwind-info.py
	return a.first > b.first;
	return a.second > b.second;
	});

	// Truncate the vector to 127 elements.
	// Common encoding indexes are limited to 0..126, while encoding
	// indexes 127..255 are local to each second-level page
	if (commonEncodings.size() > COMMON_ENCODINGS_MAX)
	commonEncodings.resize(COMMON_ENCODINGS_MAX);

	// Create a map from encoding to common-encoding-table index
	for (size_t i = 0; i < commonEncodings.size(); i++)
	commonEncodingIndexes[commonEncodings[i].first] = i;

	// Split folded encodings into pages, where each page is limited by ...
	// (a) 4 KiB capacity
	// (b) 24-bit difference between first & final function address
	// (c) 8-bit compact-encoding-table index,
	// for which 0..126 references the global common-encodings table,
	// and 127..255 references a local per-second-level-page table.
	// First we try the compact format and determine how many entries fit.
	// If more entries fit in the regular format, we use that.
	for (size_t i = 0; i < cuPtrVector.size();) {
	secondLevelPages.emplace_back();
	UnwindInfoSection::SecondLevelPage &page = secondLevelPages.back();
	page.entryIndex = i;
	uintptr_t functionAddressMax =
	cuPtrVector[i]->functionAddress + COMPRESSED_ENTRY_FUNC_OFFSET_MASK;
	size_t n = commonEncodings.size();
	size_t wordsRemaining =
	SECOND_LEVEL_PAGE_WORDS -
	sizeof(unwind_info_compressed_second_level_page_header) /
	sizeof(uint32_t);
	while (wordsRemaining >= 1 && i < cuPtrVector.size()) {
	const CompactUnwindEntry64 *cuPtr = cuPtrVector[i];
	if (cuPtr->functionAddress >= functionAddressMax) {
	break;
	} else if (commonEncodingIndexes.count(cuPtr->encoding) \|\|
	page.localEncodingIndexes.count(cuPtr->encoding)) {
	i++;
	wordsRemaining--;
	} else if (wordsRemaining >= 2 && n < COMPACT_ENCODINGS_MAX) {
	page.localEncodings.emplace_back(cuPtr->encoding);
	page.localEncodingIndexes[cuPtr->encoding] = n++;
	i++;
	wordsRemaining -= 2;
	} else {
	break;
	}
	}
	page.entryCount = i - page.entryIndex;

	// If this is not the final page, see if it's possible to fit more
	// entries by using the regular format. This can happen when there
	// are many unique encodings, and we we saturated the local
	// encoding table early.
	if (i < cuPtrVector.size() &&
	page.entryCount < REGULAR_SECOND_LEVEL_ENTRIES_MAX) {
	page.kind = UNWIND_SECOND_LEVEL_REGULAR;
	page.entryCount = std::min(REGULAR_SECOND_LEVEL_ENTRIES_MAX,
	cuPtrVector.size() - page.entryIndex);
	i = page.entryIndex + page.entryCount;
	} else {
	page.kind = UNWIND_SECOND_LEVEL_COMPRESSED;
	}
	}

	for (const CompactUnwindEntry64 *cu : cuPtrVector) {
	uint32_t functionOffset = cu->functionAddress - in.header->addr;
	functionToLsdaIndex[functionOffset] = lsdaEntries.size();
	if (cu->lsda != 0)
	lsdaEntries.push_back(
	{functionOffset, static_cast<uint32_t>(cu->lsda - in.header->addr)});
	}

	// compute size of __TEXT,__unwind_info section
	level2PagesOffset =
	sizeof(unwind_info_section_header) +
	commonEncodings.size() * sizeof(uint32_t) +
	personalities.size() * sizeof(uint32_t) +
	// The extra second-level-page entry is for the sentinel
	(secondLevelPages.size() + 1) *
	sizeof(unwind_info_section_header_index_entry) +
	lsdaEntries.size() * sizeof(unwind_info_section_header_lsda_index_entry);
	unwindInfoSize =
	level2PagesOffset + secondLevelPages.size() * SECOND_LEVEL_PAGE_BYTES;
	}

	// All inputs are relocated and output addresses are known, so write!

	void UnwindInfoSection::writeTo(uint8_t *buf) const {
	// section header
	auto uip = reinterpret_cast<unwind_info_section_header >(buf);
	uip->version = 1;
	uip->commonEncodingsArraySectionOffset = sizeof(unwind_info_section_header);
	uip->commonEncodingsArrayCount = commonEncodings.size();
	uip->personalityArraySectionOffset =
	uip->commonEncodingsArraySectionOffset +
	(uip->commonEncodingsArrayCount * sizeof(uint32_t));
	uip->personalityArrayCount = personalities.size();
	uip->indexSectionOffset = uip->personalityArraySectionOffset +
	(uip->personalityArrayCount * sizeof(uint32_t));
	uip->indexCount = secondLevelPages.size() + 1;

	// Common encodings
	auto i32p = reinterpret_cast<uint32_t >(&uip[1]);
	for (const auto &encoding : commonEncodings)
	*i32p++ = encoding.first;

	// Personalities
	for (const uint32_t &personality : personalities)
	i32p++ = in.got->addr + (personality - 1) WordSize;

	// Level-1 index
	uint32_t lsdaOffset =
	uip->indexSectionOffset +
	uip->indexCount * sizeof(unwind_info_section_header_index_entry);
	uint64_t l2PagesOffset = level2PagesOffset;
	auto iep = reinterpret_cast<unwind_info_section_header_index_entry >(i32p);
	for (const SecondLevelPage &page : secondLevelPages) {
	iep->functionOffset =
	cuPtrVector[page.entryIndex]->functionAddress - in.header->addr;
	iep->secondLevelPagesSectionOffset = l2PagesOffset;
	iep->lsdaIndexArraySectionOffset =
	lsdaOffset + functionToLsdaIndex.lookup(iep->functionOffset) *
	sizeof(unwind_info_section_header_lsda_index_entry);
	iep++;
	l2PagesOffset += SECOND_LEVEL_PAGE_BYTES;
	}
	// Level-1 sentinel
	const CompactUnwindEntry64 &cuEnd = cuVector.back();
	iep->functionOffset = cuEnd.functionAddress + cuEnd.functionLength;
	iep->secondLevelPagesSectionOffset = 0;
	iep->lsdaIndexArraySectionOffset =
	lsdaOffset +
	lsdaEntries.size() * sizeof(unwind_info_section_header_lsda_index_entry);
	iep++;

	// LSDAs
	size_t lsdaBytes =
	lsdaEntries.size() * sizeof(unwind_info_section_header_lsda_index_entry);
	if (lsdaBytes > 0)
	memcpy(iep, lsdaEntries.data(), lsdaBytes);

	// Level-2 pages
	auto pp = reinterpret_cast<uint32_t >(reinterpret_cast<uint8_t *>(iep) +
	lsdaBytes);
	for (const SecondLevelPage &page : secondLevelPages) {
	if (page.kind == UNWIND_SECOND_LEVEL_COMPRESSED) {
	uintptr_t functionAddressBase =
	cuPtrVector[page.entryIndex]->functionAddress;
	auto *p2p =
	reinterpret_cast<unwind_info_compressed_second_level_page_header *>(
	pp);
	p2p->kind = page.kind;
	p2p->entryPageOffset =
	sizeof(unwind_info_compressed_second_level_page_header);
	p2p->entryCount = page.entryCount;
	p2p->encodingsPageOffset =
	p2p->entryPageOffset + p2p->entryCount * sizeof(uint32_t);
	p2p->encodingsCount = page.localEncodings.size();
	auto ep = reinterpret_cast<uint32_t >(&p2p[1]);
	for (size_t i = 0; i < page.entryCount; i++) {
	const CompactUnwindEntry64 *cuep = cuPtrVector[page.entryIndex + i];
	auto it = commonEncodingIndexes.find(cuep->encoding);
	if (it == commonEncodingIndexes.end())
	it = page.localEncodingIndexes.find(cuep->encoding);
	*ep++ = (it->second << COMPRESSED_ENTRY_FUNC_OFFSET_BITS) \|
	(cuep->functionAddress - functionAddressBase);
	}
	if (page.localEncodings.size() != 0)
	memcpy(ep, page.localEncodings.data(),
	page.localEncodings.size() * sizeof(uint32_t));
	} else {
	auto *p2p =
	reinterpret_cast<unwind_info_regular_second_level_page_header *>(pp);
	p2p->kind = page.kind;
	p2p->entryPageOffset =
	sizeof(unwind_info_regular_second_level_page_header);
	p2p->entryCount = page.entryCount;
	auto ep = reinterpret_cast<uint32_t >(&p2p[1]);
	for (size_t i = 0; i < page.entryCount; i++) {
	const CompactUnwindEntry64 *cuep = cuPtrVector[page.entryIndex + i];
	*ep++ = cuep->functionAddress;
	*ep++ = cuep->encoding;
	}
	}
	pp += SECOND_LEVEL_PAGE_WORDS;
	}
	}