Alternative fix for empty sections

[• rafael]

Summary

This is an alternative to D24230.

[evgeny777]

On some occasions this may create empty special sections like .tbss, .eh_frame_hdr and others. Are you sure this is really harmless?

[evgeny777]

On the second thought I'd rather push this. If there are any problems they can be fixed later on. LGTM

[• rafael]

r282159

ELF/LinkerScript.h

 template <class ELFT> class LinkerScript final : public LinkerScriptBase {
   typedef typename ELFT::uint uintX_t;
 
 public:
   LinkerScript();
   ~LinkerScript();
   void processCommands(OutputSectionFactory<ELFT> &Factory);
   void createSections(OutputSectionFactory<ELFT> &Factory);
+  void adjustSectionsBeforeSorting();
 
   std::vector<PhdrEntry<ELFT>> createPhdrs();
   bool ignoreInterpSection();
 
   ArrayRef<uint8_t> getFiller(StringRef Name);
   Expr getLma(StringRef Name);
   bool shouldKeep(InputSectionBase<ELFT> *S);
   void assignOffsets(OutputSectionCommand *Cmd);

ELF/LinkerScript.cpp

if (!AlreadyOutputOS.insert(Base).second)
       continue;
     switchTo(Base);
     Dot += CurOutSec->getSize();
   }
   std::for_each(E, Cmd->Commands.end(),
                 [this](std::unique_ptr<BaseCommand> &B) { process(*B.get()); });
 }
 
-template <class ELFT> void LinkerScript<ELFT>::assignAddresses() {
+template <class ELFT> void LinkerScript<ELFT>::adjustSectionsBeforeSorting() {
   // It is common practice to use very generic linker scripts. So for any
   // given run some of the output sections in the script will be empty.
   // We could create corresponding empty output sections, but that would
   // clutter the output.
   // We instead remove trivially empty sections. The bfd linker seems even
   // more aggressive at removing them.
   auto Pos = std::remove_if(
       Opt.Commands.begin(), Opt.Commands.end(),
       [&](const std::unique_ptr<BaseCommand> &Base) {
         auto *Cmd = dyn_cast<OutputSectionCommand>(Base.get());
         if (!Cmd)
           return false;
         std::vector<OutputSectionBase<ELFT> *> Secs =
             findSections(*Cmd, *OutputSections);
         if (!Secs.empty())
           return false;
         for (const std::unique_ptr<BaseCommand> &I : Cmd->Commands)
           if (!isa<InputSectionDescription>(I.get()))
             return false;
         return true;
       });
   Opt.Commands.erase(Pos, Opt.Commands.end());
 
+  // If the output section contains only symbol assignments, create a
+  // corresponding output section. The bfd linker seems to only create them if
+  // '.' is assigned to, but creating these section should not have any bad
+  // consequeces and gives us a section to put the symbol in.
+  uintX_t Flags = SHF_ALLOC;
+  for (const std::unique_ptr<BaseCommand> &Base : Opt.Commands) {
+    auto *Cmd = dyn_cast<OutputSectionCommand>(Base.get());
+    if (!Cmd)
+      continue;
+    std::vector<OutputSectionBase<ELFT> *> Secs =
+        findSections(*Cmd, *OutputSections);
+    if (!Secs.empty()) {
+      Flags = Secs[0]->getFlags();
+      continue;
+    }
+
+    auto *OutSec = new OutputSection<ELFT>(Cmd->Name, /*Type*/ 0, Flags);
+    Out<ELFT>::Pool.emplace_back(OutSec);
+    OutputSections->push_back(OutSec);
+  }
+}
+
+template <class ELFT> void LinkerScript<ELFT>::assignAddresses() {
   // Orphan sections are sections present in the input files which
   // are not explicitly placed into the output file by the linker script.
   // We place orphan sections at end of file.
   // Other linkers places them using some heuristics as described in
   // https://sourceware.org/binutils/docs/ld/Orphan-Sections.html#Orphan-Sections.
 
   // The OutputSections are already in the correct order.
   // This loops creates or moves commands as needed so that they are in the

ELF/Writer.cpp

if (!CommonInputSection<ELFT>::X->OutSec) {
     Out<ELFT>::Bss->addSection(CommonInputSection<ELFT>::X);
     Out<ELFT>::Bss->assignOffsets();
   }
 
   // So far we have added sections from input object files.
   // This function adds linker-created Out<ELFT>::* sections.
   addPredefinedSections();
 
+  if (ScriptConfig->HasSections)
+    Script<ELFT>::X->adjustSectionsBeforeSorting();
+
   std::stable_sort(OutputSections.begin(), OutputSections.end(),
                    compareSections<ELFT>);
 
   unsigned I = 1;
   for (OutputSectionBase<ELFT> *Sec : OutputSections) {
     Sec->SectionIndex = I++;
     Sec->setSHName(Out<ELFT>::ShStrTab->addString(Sec->getName()));
   }

test/ELF/linkerscript/symbol-only.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+
+# RUN: echo "SECTIONS { \
+# RUN:  abc : { foo = .; } \
+# RUN:  . = ALIGN(0x1000); \
+# RUN:  .text : { *(.text) } \
+# RUN: }" > %t.script
+# RUN: ld.lld -o %t1 --script %t.script %t -shared
+# RUN: llvm-objdump -section-headers -t %t1 | FileCheck %s
+# CHECK:      Sections:
+# CHECK-NEXT: Idx Name          Size      Address
+# CHECK-NEXT:   0               00000000 0000000000000000
+# CHECK-NEXT:   1 abc           00000000 0000000000000200
+# CHECK-NEXT:   2 .text         00000000 0000000000001000 TEXT DATA
+
+# CHECK: SYMBOL TABLE:
+# CHECK:     0000000000000200         abc                00000000 foo