[Object][Archive][NFC] Create all symbolic files objects before calculating offsets.

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. I think 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);