[ELF] - add predefined sections to output sections list in one place.

[grimar]

Summary

Minor cleanup.
Currently it looks wierd that having method addPredefinedSections()
we still add 2 sections outside it without real reasons.

[ruiu]

Naturally this change made me wonder whether this is too inefficient or not, but it is probably okay since the number of output sections is not very large.

ELF/Writer.cpp

1470–1474

Move this after the code to add BuildId to remove the variable I.

if (needsInterpSection())
  OutputSections.insert(OutputSections.begin(), Out<ELFT>::Interp);

1477

now -> to beginning of the file

[grimar]

Addressed review comments

[grimar]

In D19981#423197, @ruiu wrote:

Naturally this change made me wonder whether this is too inefficient or not, but it is probably okay since the number of output sections is not very large.

While this is not large vector of pointers, it should be fine.

ELF/Writer.cpp

1470–1474

Done.

1477

Done.

[grimar]

ping ?

[grimar]

• Rebased

[• rafael]

LGTM

ELF/Writer.cpp

 // Sort input sections by the special rule for .ctors and .dtors.
 template <class ELFT> static void sortCtorsDtors(OutputSectionBase<ELFT> *S) {
   if (S)
     reinterpret_cast<OutputSection<ELFT> *>(S)->sortCtorsDtors();
 }
 
 // Create output section objects and add them to OutputSections.
 template <class ELFT> void Writer<ELFT>::createSections() {
-  // Add .interp first because some loaders want to see that section
-  // on the first page of the executable file when loaded into memory.
-  if (needsInterpSection())
-    OutputSections.push_back(Out<ELFT>::Interp);
-
-  // A core file does not usually contain unmodified segments except
-  // the first page of the executable. Add the build ID section now
-  // so that the section is included in the first page.
-  if (Out<ELFT>::BuildId)
-    OutputSections.push_back(Out<ELFT>::BuildId);
-
   // Create output sections for input object file sections.
   std::vector<OutputSectionBase<ELFT> *> RegularSections;
   OutputSectionFactory<ELFT> Factory;
   for (const std::unique_ptr<elf::ObjectFile<ELFT>> &F :
        Symtab.getObjectFiles()) {
     for (InputSectionBase<ELFT> *C : F->getSections()) {
       if (isDiscarded(C)) {
         reportDiscarded(C, F);

 
 // This function add Out<ELFT>::* sections to OutputSections.
 template <class ELFT> void Writer<ELFT>::addPredefinedSections() {
   auto Add = [&](OutputSectionBase<ELFT> *C) {
     if (C)
       OutputSections.push_back(C);
   };
 
+  // Add .interp at first because some loaders want to see that section
+  // on the first page of the executable file when loaded into memory.
+  auto I = OutputSections.begin();
+  if (needsInterpSection())
+    I = OutputSections.insert(I, Out<ELFT>::Interp);
+
+  // A core file does not usually contain unmodified segments except
+  // the first page of the executable. Add the build ID section now
+  // so that the section is included in the first page.
+  if (Out<ELFT>::BuildId)
+    OutputSections.insert(I, Out<ELFT>::BuildId);
+
   // This order is not the same as the final output order
   // because we sort the sections using their attributes below.
   Add(Out<ELFT>::SymTab);
   Add(Out<ELFT>::ShStrTab);
   Add(Out<ELFT>::StrTab);
   if (isOutputDynamic()) {
     Add(Out<ELFT>::DynSymTab);
     if (Out<ELFT>::VerNeed->getNeedNum() != 0) {