[ELF] - Allow section to have multiple dependent sections.

[grimar]

Summary

That fixes a case when section has more than one metadata section.
Previously GC would collect one of such sections because we had implementation
that stored only last one as dependent.

[ruiu]

LGTM

lld/trunk/ELF/InputFiles.cpp

for (const Elf_Shdr &Sec : ObjSections) {
 
     // .ARM.exidx sections have a reverse dependency on the InputSection they
     // have a SHF_LINK_ORDER dependency, this is identified by the sh_link.
     if (Sec.sh_flags & SHF_LINK_ORDER) {
       if (Sec.sh_link >= Sections.size())
         fatal(toString(this) + ": invalid sh_link index: " +
               Twine(Sec.sh_link));
       auto *IS = cast<InputSection<ELFT>>(Sections[Sec.sh_link]);
-      IS->DependentSection = Sections[I];
+      IS->DependentSections.push_back(Sections[I]);
     }
   }
 }
 
 template <class ELFT>
 InputSectionBase<ELFT> *
 elf::ObjectFile<ELFT>::getRelocTarget(const Elf_Shdr &Sec) {
   uint32_t Idx = Sec.sh_info;

lld/trunk/ELF/InputSection.h

public:
   // Write this section to a mmap'ed file, assuming Buf is pointing to
   // beginning of the output section.
   void writeTo(uint8_t *Buf);
 
   // The offset from beginning of the output sections this section was assigned
   // to. The writer sets a value.
   uint64_t OutSecOff = 0;
 
-  // InputSection that is dependent on us (reverse dependency for GC)
-  InputSectionBase<ELFT> *DependentSection = nullptr;
+  // InputSections that are dependent on us (reverse dependency for GC)
+  llvm::TinyPtrVector<InputSectionBase<ELFT> *> DependentSections;
 
   static bool classof(const InputSectionData *S);
 
   InputSectionBase<ELFT> *getRelocatedSection();
 
   template <class RelTy>
   void relocateNonAlloc(uint8_t *Buf, llvm::ArrayRef<RelTy> Rels);
 

lld/trunk/ELF/MarkLive.cpp

static void forEachSuccessor(InputSection<ELFT> &Sec,
                              std::function<void(ResolvedReloc<ELFT>)> Fn) {
   if (Sec.AreRelocsRela) {
     for (const typename ELFT::Rela &Rel : Sec.relas())
       Fn(resolveReloc(Sec, Rel));
   } else {
     for (const typename ELFT::Rel &Rel : Sec.rels())
       Fn(resolveReloc(Sec, Rel));
   }
-  if (Sec.DependentSection)
-    Fn({Sec.DependentSection, 0});
+  for (InputSectionBase<ELFT> *IS : Sec.DependentSections)
+    Fn({IS, 0});
 }
 
 // The .eh_frame section is an unfortunate special case.
 // The section is divided in CIEs and FDEs and the relocations it can have are
 // * CIEs can refer to a personality function.
 // * FDEs can refer to a LSDA
 // * FDEs refer to the function they contain information about
 // The last kind of relocation cannot keep the referred section alive, or they

lld/trunk/test/ELF/gc-sections-metadata2.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: ld.lld --gc-sections %t.o -o %t
+# RUN: llvm-objdump -section-headers %t | FileCheck %s
+
+# CHECK: .foo
+# CHECK: .bar
+# CHECK: .zed
+
+.globl _start
+_start:
+.quad .foo
+
+.section .foo,"a"
+.quad 0
+.section .bar,"am",@progbits,.foo
+.quad 0
+.section .zed,"am",@progbits,.foo
+.quad 0