[llvm-objcopy][MachO] Fix MachO::relocation_info use after 386f1c1.

Use shift/mask operations to access r_symbolnum rather than relying on
MachO::relocation_info. This should fix the big-endian bot failures that were
caused by 386f1c1.

Author: lhames

llvm/tools/llvm-objcopy/MachO/MachOReader.cpp

@@ -204,10 +204,9 @@ void MachOReader::setSymbolInRelocationInfo(Object &O) const {
   for (LoadCommand &LC : O.LoadCommands)
     for (std::unique_ptr<Section> &Sec : LC.Sections)
       for (auto &Reloc : Sec->Relocations)
-        if (!Reloc.Scattered) {
-          auto *Info = reinterpret_cast<MachO::relocation_info *>(&Reloc.Info);
-          Reloc.Symbol = O.SymTable.getSymbolByIndex(Info->r_symbolnum);
-        }
+        if (!Reloc.Scattered)
+          Reloc.Symbol = O.SymTable.getSymbolByIndex(
+              Reloc.getPlainRelocationSymbolNum(O.isLittleEndian()));
 }
 
 void MachOReader::readRebaseInfo(Object &O) const {

llvm/tools/llvm-objcopy/MachO/MachOWriter.cpp

@@ -240,11 +240,9 @@ void MachOWriter::writeSections() {
              Sec->Content.size());
       for (size_t Index = 0; Index < Sec->Relocations.size(); ++Index) {
         auto RelocInfo = Sec->Relocations[Index];
-        if (!RelocInfo.Scattered) {
-          auto *Info =
-              reinterpret_cast<MachO::relocation_info *>(&RelocInfo.Info);
-          Info->r_symbolnum = RelocInfo.Symbol->Index;
-        }
+        if (!RelocInfo.Scattered)
+          RelocInfo.setPlainRelocationSymbolNum(RelocInfo.Symbol->Index,
+                                                O.isLittleEndian());
 
         if (IsLittleEndian != sys::IsLittleEndianHost)
           MachO::swapStruct(

llvm/tools/llvm-objcopy/MachO/Object.h

@@ -161,6 +161,20 @@ struct RelocationInfo {
   // True if Info is a scattered_relocation_info.
   bool Scattered;
   MachO::any_relocation_info Info;
+
+  unsigned getPlainRelocationSymbolNum(bool IsLittleEndian) {
+    if (IsLittleEndian)
+      return Info.r_word1 & 0xffffff;
+    return Info.r_word1 >> 8;
+  }
+
+  void setPlainRelocationSymbolNum(unsigned SymbolNum, bool IsLittleEndian) {
+    assert(SymbolNum < (1 << 24) && "SymbolNum out of range");
+    if (IsLittleEndian)
+      Info.r_word1 = (Info.r_word1 & ~0x00ffffff) | SymbolNum;
+    else
+      Info.r_word1 = (Info.r_word1 & ~0xffffff00) | (SymbolNum << 8);
+  }
 };
 
 /// The ___location of the rebase info inside the binary is described by
@@ -300,6 +314,11 @@ struct Object {
   /// is not too long (SegName.size() should be less than or equal to 16).
   LoadCommand &addSegment(StringRef SegName);
 
+  bool isLittleEndian() const {
+    StringRef Magic(reinterpret_cast<const char *>(&Header.Magic), 4);
+    return Magic == "\xCE\xFA\xED\xFE" || Magic == "\xCF\xFA\xED\xFE";
+  }
+
   bool is64Bit() const {
     return Header.Magic == MachO::MH_MAGIC_64 ||
            Header.Magic == MachO::MH_CIGAM_64;