Revert "[lld-macho] Support .subsections_via_symbols"

Due to build breakage mentioned in https://reviews.llvm.org/D79926.

This reverts commit e270b2f172.

lld/MachO/Driver.cpp

@@ -326,14 +326,9 @@ bool macho::link(llvm::ArrayRef<const char *> argsArr, bool canExitEarly,
   createSyntheticSections();
 
   // Initialize InputSections.
-  for (InputFile *file : inputFiles) {
+  for (InputFile *file : inputFiles)
-    for (SubsectionMap &map : file->subsections) {
+    for (InputSection *sec : file->sections)
-      for (auto &p : map) {
+      inputSections.push_back(sec);
-        InputSection *isec = p.second;
-        inputSections.push_back(isec);
-      }
-    }
-  }
 
   // Write to an output file.
   writeResult();

lld/MachO/InputFiles.cpp

@@ -127,13 +127,17 @@ static const load_command *findCommand(const mach_header_64 *hdr,
   return nullptr;
 }
 
-void InputFile::parseSections(ArrayRef<section_64> sections) {
+std::vector<InputSection *>
-  subsections.reserve(sections.size());
+InputFile::parseSections(ArrayRef<section_64> sections) {
+  std::vector<InputSection *> ret;
+  ret.reserve(sections.size());
+
   auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
 
   for (const section_64 &sec : sections) {
     InputSection *isec = make<InputSection>();
     isec->file = this;
+    isec->header = &sec;
     isec->name = StringRef(sec.sectname, strnlen(sec.sectname, 16));
     isec->segname = StringRef(sec.segname, strnlen(sec.segname, 16));
     isec->data = {buf + sec.offset, static_cast<size_t>(sec.size)};
@@ -143,185 +147,96 @@ void InputFile::parseSections(ArrayRef<section_64> sections) {
     else
       isec->align = 1 << sec.align;
     isec->flags = sec.flags;
-    subsections.push_back({{0, isec}});
+    ret.push_back(isec);
   }
-}
 
-// Find the subsection corresponding to the greatest section offset that is <=
+  return ret;
-// that of the given offset.
-//
-// offset: an offset relative to the start of the original InputSection (before
-// any subsection splitting has occurred). It will be updated to represent the
-// same location as an offset relative to the start of the containing
-// subsection.
-static InputSection *findContainingSubsection(SubsectionMap &map,
-                                              uint32_t *offset) {
-  auto it = std::prev(map.upper_bound(*offset));
-  *offset -= it->first;
-  return it->second;
 }
 
 void InputFile::parseRelocations(const section_64 &sec,
-                                 SubsectionMap &subsecMap) {
+                                 std::vector<Reloc> &relocs) {
   auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
   ArrayRef<any_relocation_info> relInfos(
       reinterpret_cast<const any_relocation_info *>(buf + sec.reloff),
       sec.nreloc);
 
   for (const any_relocation_info &anyRel : relInfos) {
-    if (anyRel.r_word0 & R_SCATTERED)
-      fatal("TODO: Scattered relocations not supported");
-
-    auto rel = reinterpret_cast<const relocation_info &>(anyRel);
-    if (!rel.r_pcrel)
-      fatal("TODO: Only pcrel relocations are supported");
-
     Reloc r;
-    r.type = rel.r_type;
+    if (anyRel.r_word0 & R_SCATTERED) {
-    uint32_t secRelOffset = rel.r_address;
+      error("TODO: Scattered relocations not supported");
-    uint64_t rawAddend =
-        target->getImplicitAddend(buf + sec.offset + secRelOffset, r.type);
-
-    if (rel.r_extern) {
-      r.target = symbols[rel.r_symbolnum];
-      r.addend = rawAddend;
     } else {
-      if (rel.r_symbolnum == 0 || rel.r_symbolnum > subsections.size())
+      auto rel = reinterpret_cast<const relocation_info &>(anyRel);
-        fatal("invalid section index in relocation for offset " +
+      r.type = rel.r_type;
-              std::to_string(r.offset) + " in section " + sec.sectname +
+      r.offset = rel.r_address;
-              " of " + getName());
+      r.addend = target->getImplicitAddend(buf + sec.offset + r.offset, r.type);
-
+      if (rel.r_extern) {
-      SubsectionMap &targetSubsecMap = subsections[rel.r_symbolnum - 1];
+        r.target = symbols[rel.r_symbolnum];
-      const section_64 &targetSec = sectionHeaders[rel.r_symbolnum - 1];
+      } else {
-      // The implicit addend for pcrel section relocations is the pcrel offset
+        if (rel.r_symbolnum == 0 || rel.r_symbolnum > sections.size())
-      // in terms of the addresses in the input file. Here we adjust it so that
+          fatal("invalid section index in relocation for offset " +
-      // it describes the offset from the start of the target section.
+                std::to_string(r.offset) + " in section " + sec.sectname +
-      // TODO: Figure out what to do for non-pcrel section relocations.
+                " of " + getName());
-      // TODO: The offset of 4 is probably not right for ARM64, nor for
+        r.target = sections[rel.r_symbolnum - 1];
-      //       relocations with r_length != 2.
+      }
-      uint32_t targetOffset =
-          sec.addr + secRelOffset + 4 + rawAddend - targetSec.addr;
-      r.target = findContainingSubsection(targetSubsecMap, &targetOffset);
-      r.addend = targetOffset;
     }
-
+    relocs.push_back(r);
-    InputSection *subsec = findContainingSubsection(subsecMap, &secRelOffset);
-    r.offset = secRelOffset;
-    subsec->relocs.push_back(r);
-  }
-}
-
-void InputFile::parseSymbols(ArrayRef<const nlist_64> nList, const char *strtab,
-                             bool subsectionsViaSymbols) {
-  // resize(), not reserve(), because we are going to create N_ALT_ENTRY symbols
-  // out-of-sequence.
-  symbols.resize(nList.size());
-  std::vector<size_t> altEntrySymIdxs;
-
-  auto createDefined = [&](const nlist_64 &sym, InputSection *isec,
-                           uint32_t value) -> Symbol * {
-    StringRef name = strtab + sym.n_strx;
-    if (sym.n_type & N_EXT)
-      // Global defined symbol
-      return symtab->addDefined(name, isec, value);
-    else
-      // Local defined symbol
-      return make<Defined>(name, isec, value);
-  };
-
-  for (size_t i = 0, n = nList.size(); i < n; ++i) {
-    const nlist_64 &sym = nList[i];
-
-    // Undefined symbol
-    if (!sym.n_sect) {
-      StringRef name = strtab + sym.n_strx;
-      symbols[i] = symtab->addUndefined(name);
-      continue;
-    }
-
-    const section_64 &sec = sectionHeaders[sym.n_sect - 1];
-    SubsectionMap &subsecMap = subsections[sym.n_sect - 1];
-    uint64_t offset = sym.n_value - sec.addr;
-
-    // If the input file does not use subsections-via-symbols, all symbols can
-    // use the same subsection. Otherwise, we must split the sections along
-    // symbol boundaries.
-    if (!subsectionsViaSymbols) {
-      symbols[i] = createDefined(sym, subsecMap[0], offset);
-      continue;
-    }
-
-    // nList entries aren't necessarily arranged in address order. Therefore,
-    // we can't create alt-entry symbols at this point because a later symbol
-    // may split its section, which may affect which subsection the alt-entry
-    // symbol is assigned to. So we need to handle them in a second pass below.
-    if (sym.n_desc & N_ALT_ENTRY) {
-      altEntrySymIdxs.push_back(i);
-      continue;
-    }
-
-    // Find the subsection corresponding to the greatest section offset that is
-    // <= that of the current symbol. The subsection that we find either needs
-    // to be used directly or split in two.
-    uint32_t firstSize = offset;
-    InputSection *firstIsec = findContainingSubsection(subsecMap, &firstSize);
-
-    if (firstSize == 0) {
-      // Alias of an existing symbol, or the first symbol in the section. These
-      // are handled by reusing the existing section.
-      symbols[i] = createDefined(sym, firstIsec, 0);
-      continue;
-    }
-
-    // We saw a symbol definition at a new offset. Split the section into two
-    // subsections. The new symbol uses the second subsection.
-    auto *secondIsec = make<InputSection>(*firstIsec);
-    secondIsec->data = firstIsec->data.slice(firstSize);
-    firstIsec->data = firstIsec->data.slice(0, firstSize);
-    // TODO: ld64 appears to preserve the original alignment as well as each
-    // subsection's offset from the last aligned address. We should consider
-    // emulating that behavior.
-    secondIsec->align = MinAlign(firstIsec->align, offset);
-
-    subsecMap[offset] = secondIsec;
-    // By construction, the symbol will be at offset zero in the new section.
-    symbols[i] = createDefined(sym, secondIsec, 0);
-  }
-
-  for (size_t idx : altEntrySymIdxs) {
-    const nlist_64 &sym = nList[idx];
-    SubsectionMap &subsecMap = subsections[sym.n_sect - 1];
-    uint32_t off = sym.n_value - sectionHeaders[sym.n_sect - 1].addr;
-    InputSection *subsec = findContainingSubsection(subsecMap, &off);
-    symbols[idx] = createDefined(sym, subsec, off);
   }
 }
 
 ObjFile::ObjFile(MemoryBufferRef mb) : InputFile(ObjKind, mb) {
   auto *buf = reinterpret_cast<const uint8_t *>(mb.getBufferStart());
   auto *hdr = reinterpret_cast<const mach_header_64 *>(mb.getBufferStart());
+  ArrayRef<section_64> objSections;
 
   if (const load_command *cmd = findCommand(hdr, LC_SEGMENT_64)) {
     auto *c = reinterpret_cast<const segment_command_64 *>(cmd);
-    sectionHeaders = ArrayRef<section_64>{
+    objSections = ArrayRef<section_64>{
         reinterpret_cast<const section_64 *>(c + 1), c->nsects};
-    parseSections(sectionHeaders);
+    sections = parseSections(objSections);
   }
 
   // TODO: Error on missing LC_SYMTAB?
   if (const load_command *cmd = findCommand(hdr, LC_SYMTAB)) {
     auto *c = reinterpret_cast<const symtab_command *>(cmd);
+    const char *strtab = reinterpret_cast<const char *>(buf) + c->stroff;
     ArrayRef<const nlist_64> nList(
         reinterpret_cast<const nlist_64 *>(buf + c->symoff), c->nsyms);
-    const char *strtab = reinterpret_cast<const char *>(buf) + c->stroff;
+
-    bool subsectionsViaSymbols = hdr->flags & MH_SUBSECTIONS_VIA_SYMBOLS;
+    symbols.reserve(c->nsyms);
-    parseSymbols(nList, strtab, subsectionsViaSymbols);
+
+    for (const nlist_64 &sym : nList) {
+      StringRef name = strtab + sym.n_strx;
+
+      // Undefined symbol
+      if (!sym.n_sect) {
+        symbols.push_back(symtab->addUndefined(name));
+        continue;
+      }
+
+      InputSection *isec = sections[sym.n_sect - 1];
+      const section_64 &objSec = objSections[sym.n_sect - 1];
+      uint64_t value = sym.n_value - objSec.addr;
+
+      // Global defined symbol
+      if (sym.n_type & N_EXT) {
+        symbols.push_back(symtab->addDefined(name, isec, value));
+        continue;
+      }
+
+      // Local defined symbol
+      symbols.push_back(make<Defined>(name, isec, value));
+    }
   }
 
   // The relocations may refer to the symbols, so we parse them after we have
-  // parsed all the symbols.
+  // the symbols loaded.
-  for (size_t i = 0, n = subsections.size(); i < n; ++i)
+  if (!sections.empty()) {
-    parseRelocations(sectionHeaders[i], subsections[i]);
+    auto it = sections.begin();
+    for (const section_64 &sec : objSections) {
+      parseRelocations(sec, (*it)->relocs);
+      ++it;
+    }
+  }
 }
 
 DylibFile::DylibFile(MemoryBufferRef mb, DylibFile *umbrella)
@@ -409,8 +324,7 @@ void ArchiveFile::fetch(const object::Archive::Symbol &sym) {
                 sym.getName());
   auto file = make<ObjFile>(mb);
   symbols.insert(symbols.end(), file->symbols.begin(), file->symbols.end());
-  subsections.insert(subsections.end(), file->subsections.begin(),
+  sections.insert(sections.end(), file->sections.begin(), file->sections.end());
-                     file->subsections.end());
 }
 
 // Returns "<internal>" or "baz.o".

lld/MachO/InputFiles.h

@@ -14,8 +14,6 @@
 #include "llvm/BinaryFormat/MachO.h"
 #include "llvm/Object/Archive.h"
 #include "llvm/Support/MemoryBuffer.h"
-
-#include <map>
 #include <vector>
 
 namespace lld {
@@ -25,11 +23,6 @@ class InputSection;
 class Symbol;
 struct Reloc;
 
-// If .subsections_via_symbols is set, each InputSection will be split along
-// symbol boundaries. The keys of a SubsectionMap represent the offsets of
-// each subsection from the start of the original pre-split InputSection.
-using SubsectionMap = std::map<uint32_t, InputSection *>;
-
 class InputFile {
 public:
   enum Kind {
@@ -44,18 +37,15 @@ public:
 
   MemoryBufferRef mb;
   std::vector<Symbol *> symbols;
-  ArrayRef<llvm::MachO::section_64> sectionHeaders;
+  std::vector<InputSection *> sections;
-  std::vector<SubsectionMap> subsections;
 
 protected:
   InputFile(Kind kind, MemoryBufferRef mb) : mb(mb), fileKind(kind) {}
 
-  void parseSections(ArrayRef<llvm::MachO::section_64>);
+  std::vector<InputSection *> parseSections(ArrayRef<llvm::MachO::section_64>);
 
-  void parseSymbols(ArrayRef<const llvm::MachO::nlist_64> nList,
+  void parseRelocations(const llvm::MachO::section_64 &,
-                    const char *strtab, bool subsectionsViaSymbols);
+                        std::vector<Reloc> &relocs);
-
-  void parseRelocations(const llvm::MachO::section_64 &, SubsectionMap &);
 
 private:
   const Kind fileKind;

lld/MachO/InputSection.cpp

@@ -32,6 +32,7 @@ void InputSection::writeTo(uint8_t *buf) {
 
   for (Reloc &r : relocs) {
     uint64_t va = 0;
+    uint64_t addend = r.addend;
     if (auto *s = r.target.dyn_cast<Symbol *>()) {
       if (auto *dylibSymbol = dyn_cast<DylibSymbol>(s)) {
         va = target->getDylibSymbolVA(*dylibSymbol, r.type);
@@ -40,9 +41,15 @@ void InputSection::writeTo(uint8_t *buf) {
       }
     } else if (auto *isec = r.target.dyn_cast<InputSection *>()) {
       va = isec->getVA();
+      // The implicit addend for pcrel section relocations is the pcrel offset
+      // in terms of the addresses in the input file. Here we adjust it so that
+      // it describes the offset from the start of the target section.
+      // TODO: Figure out what to do for non-pcrel section relocations.
+      // TODO: The offset of 4 is probably not right for ARM64.
+      addend -= isec->header->addr - (header->addr + r.offset + 4);
     }
 
-    uint64_t val = va + r.addend;
+    uint64_t val = va + addend;
     if (1) // TODO: handle non-pcrel relocations
       val -= getVA() + r.offset;
     target->relocateOne(buf + r.offset, r.type, val);

lld/MachO/InputSection.h

@@ -24,11 +24,7 @@ class Symbol;
 
 struct Reloc {
   uint8_t type;
-  // Adding this offset to the address of the target symbol or subsection gives
-  // the destination that this relocation refers to.
   uint32_t addend;
-  // The offset from the start of the subsection that this relocation belongs
-  // to.
   uint32_t offset;
   llvm::PointerUnion<Symbol *, InputSection *> target;
 };
@@ -46,6 +42,8 @@ public:
   InputFile *file = nullptr;
   StringRef name;
   StringRef segname;
+  // This provides access to the address of the section in the input file.
+  const llvm::MachO::section_64 *header;
 
   OutputSection *parent = nullptr;
   uint64_t outSecOff = 0;

lld/test/MachO/subsections-section-relocs.s

@@ -1,47 +0,0 @@
-# REQUIRES: x86
-# RUN: mkdir -p %t
-# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t/test.o
-
-# RUN: echo "_bar_str" > %t/order-file
-# RUN: echo "_foo_str" >> %t/order-file
-
-# RUN: lld -flavor darwinnew -o %t/test %t/test.o -order_file %t/order-file
-# RUN: llvm-objdump --section-headers -d --no-show-raw-insn %t/test | FileCheck %s
-# CHECK-LABEL: Sections:
-# CHECK:       __cstring {{[^ ]*}} {{0*}}[[#%x, CSTRING_ADDR:]]
-# CHECK-LABEL: Disassembly of section __TEXT,__text:
-## L._str should end up at CSTRING_ADDR + 4, and leaq is 7 bytes long so we
-## have RIP = ADDR + 7
-# CHECK:       [[#%x, ADDR:]]: leaq
-# CHECK-SAME:    [[#%u, CSTRING_ADDR + 4 - ADDR - 7]](%rip), %rsi {{.*}} <_bar_str+0x4>
-
-# RUN: llvm-readobj --string-dump=__cstring %t/test | FileCheck %s --check-prefix=STRINGS
-# STRINGS: bar
-# STRINGS: Private symbol
-# STRINGS: foo
-
-.text
-.globl _main, _foo_str, _bar_str
-
-_main:
-  leaq L_.str(%rip), %rsi
-  mov $0, %rax
-  ret
-
-.section __TEXT,__cstring
-_foo_str:
-  .asciz "foo"
-
-_bar_str:
-  .asciz "bar"
-
-## References to this generate a section relocation
-## N.B.: ld64 doesn't actually reorder symbols in __cstring based on the order
-##       file. Only our implementation does. However, I'm not sure how else to
-##       test section relocations that target an address inside a relocated
-##       symbol: using a non-__cstring section would cause llvm-mc to emit a
-##       symbol relocation instead using the nearest symbol.
-L_.str:
-  .asciz "Private symbol"
-
-.subsections_via_symbols

lld/test/MachO/subsections-symbol-relocs.s

@@ -1,55 +0,0 @@
-# REQUIRES: x86
-# RUN: mkdir -p %t
-# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t/test.o
-
-# RUN: echo "_bar" > %t/order-file-1
-# RUN: echo "_foo" >> %t/order-file-1
-# RUN: echo "_main" >> %t/order-file-1
-## _qux is marked as .alt_entry, so it should not create a new subsection and
-## its contents should move with _bar to the start of the output despite the
-## order file listing it at the end.
-# RUN: echo "_qux" >> %t/order-file-1
-
-## _bar and _baz point to the same address, so both order files should achieve
-## the same result.
-# RUN: echo "_baz" > %t/order-file-2
-# RUN: echo "_foo" >> %t/order-file-2
-# RUN: echo "_main" >> %t/order-file-2
-# RUN: echo "_qux" >> %t/order-file-2
-
-# RUN: lld -flavor darwinnew -o %t/test-1 %t/test.o -order_file %t/order-file-1
-# RUN: llvm-objdump -d --no-show-raw-insn %t/test-1 | FileCheck %s
-# RUN: lld -flavor darwinnew -o %t/test-2 %t/test.o -order_file %t/order-file-2
-# RUN: llvm-objdump -d --no-show-raw-insn %t/test-2 | FileCheck %s
-# CHECK-LABEL: Disassembly of section __TEXT,__text:
-# CHECK:       <_bar>:
-# CHECK-NEXT:    callq {{.*}} <_foo>
-# CHECK-EMPTY:
-# CHECK-NEXT:  <_qux>:
-# CHECK-NEXT:    retq
-# CHECK:       <_foo>:
-# CHECK-NEXT:    retq
-# CHECK:       <_main>:
-# CHECK-NEXT:    callq {{.*}} <_bar>
-# CHECK-NEXT:    movq $0, %rax
-# CHECK-NEXT:    retq
-
-.text
-.globl _main, _foo, _bar, _qux
-.alt_entry _qux
-
-_foo:
-  retq
-
-_main:
-  callq _bar
-  movq $0, %rax
-  retq
-
-_bar:
-_baz:
-  callq _foo
-_qux:
-  retq
-
-.subsections_via_symbols