[Object][Archive][NFC] Create all symbolic files objects before calculating offsets. (#85229)

This is refactoring preparing to move UseECMap computation to the
archive writer. We currently require writeArchive caller to pass that.
This is not practical for llvm-ar, which currently interprets at most
one passed object. For a reliable UseECMap, we need to interpret all
symbolic objects: we may have, for example, a list of x86_64 files
followed by aarch64 file, which indicates that we should use EC map for
x86_64 objects.

This commit interprets symbolic files in a separated pass, which will be
a convenient place to implement UseECMap computation in the follow up.
It also makes accessing the next member for AIX big archive offset
computation a bit easier.

Author: cjacek

llvm/lib/Object/ArchiveWriter.cpp

@@ -795,23 +795,31 @@ computeMemberData(raw_ostream &StringTable, raw_ostream &SymNames,
       Entry.second = Entry.second > 1 ? 1 : 0;
   }
 
+  std::vector<std::unique_ptr<SymbolicFile>> SymFiles;
+
+  if (NeedSymbols != SymtabWritingMode::NoSymtab || isAIXBigArchive(Kind)) {
+    for (const NewArchiveMember &M : NewMembers) {
+      Expected<std::unique_ptr<SymbolicFile>> SymFileOrErr =
+          getSymbolicFile(M.Buf->getMemBufferRef(), Context);
+      if (!SymFileOrErr)
+        return createFileError(M.MemberName, SymFileOrErr.takeError());
+      SymFiles.push_back(std::move(*SymFileOrErr));
+    }
+  }
+
   // The big archive format needs to know the offset of the previous member
   // header.
   uint64_t PrevOffset = 0;
   uint64_t NextMemHeadPadSize = 0;
-  std::unique_ptr<SymbolicFile> CurSymFile;
-  std::unique_ptr<SymbolicFile> NextSymFile;
-  uint16_t Index = 0;
 
-  for (auto M = NewMembers.begin(); M < NewMembers.end(); ++M) {
+  for (uint32_t Index = 0; Index < NewMembers.size(); ++Index) {
+    const NewArchiveMember *M = &NewMembers[Index];
     std::string Header;
     raw_string_ostream Out(Header);
 
     MemoryBufferRef Buf = M->Buf->getMemBufferRef();
     StringRef Data = Thin ? "" : Buf.getBuffer();
 
-    Index++;
-
     // ld64 expects the members to be 8-byte aligned for 64-bit content and at
     // least 4-byte aligned for 32-bit content.  Opt for the larger encoding
     // uniformly.  This matches the behaviour with cctools and ensures that ld64
@@ -837,29 +845,9 @@ computeMemberData(raw_ostream &StringTable, raw_ostream &SymNames,
           std::move(StringMsg), object::object_error::parse_failed);
     }
 
-    if (NeedSymbols != SymtabWritingMode::NoSymtab || isAIXBigArchive(Kind)) {
-      auto SetNextSymFile = [&NextSymFile,
-                             &Context](MemoryBufferRef Buf,
-                                       StringRef MemberName) -> Error {
-        Expected<std::unique_ptr<SymbolicFile>> SymFileOrErr =
-            getSymbolicFile(Buf, Context);
-        if (!SymFileOrErr)
-          return createFileError(MemberName, SymFileOrErr.takeError());
-        NextSymFile = std::move(*SymFileOrErr);
-        return Error::success();
-      };
-
-      if (M == NewMembers.begin())
-        if (Error Err = SetNextSymFile(Buf, M->MemberName))
-          return std::move(Err);
-
-      CurSymFile = std::move(NextSymFile);
-
-      if ((M + 1) != NewMembers.end())
-        if (Error Err = SetNextSymFile((M + 1)->Buf->getMemBufferRef(),
-                                       (M + 1)->MemberName))
-          return std::move(Err);
-    }
+    std::unique_ptr<SymbolicFile> CurSymFile;
+    if (!SymFiles.empty())
+      CurSymFile = std::move(SymFiles[Index]);
 
     // In the big archive file format, we need to calculate and include the next
     // member offset and previous member offset in the file member header.
@@ -880,13 +868,13 @@ computeMemberData(raw_ostream &StringTable, raw_ostream &SymNames,
 
       // If there is another member file after this, we need to calculate the
       // padding before the header.
-      if ((M + 1) != NewMembers.end()) {
-        uint64_t OffsetToNextMemData = NextOffset +
-                                       sizeof(object::BigArMemHdrType) +
-                                       alignTo((M + 1)->MemberName.size(), 2);
+      if (Index + 1 != SymFiles.size()) {
+        uint64_t OffsetToNextMemData =
+            NextOffset + sizeof(object::BigArMemHdrType) +
+            alignTo(NewMembers[Index + 1].MemberName.size(), 2);
         NextMemHeadPadSize =
             alignToPowerOf2(OffsetToNextMemData,
-                            getMemberAlignment(NextSymFile.get())) -
+                            getMemberAlignment(SymFiles[Index + 1].get())) -
             OffsetToNextMemData;
         NextOffset += NextMemHeadPadSize;
       }
@@ -902,7 +890,7 @@ computeMemberData(raw_ostream &StringTable, raw_ostream &SymNames,
     std::vector<unsigned> Symbols;
     if (NeedSymbols != SymtabWritingMode::NoSymtab) {
       Expected<std::vector<unsigned>> SymbolsOrErr =
-          getSymbols(CurSymFile.get(), Index, SymNames, SymMap);
+          getSymbols(CurSymFile.get(), Index + 1, SymNames, SymMap);
       if (!SymbolsOrErr)
         return createFileError(M->MemberName, SymbolsOrErr.takeError());
       Symbols = std::move(*SymbolsOrErr);