[ELF][ARM] Garbage collection support for .ARM.exidx sections

.ARM.exidx sections have a reverse dependency on the section they have
a SHF_LINK_ORDER dependency on. In other words a .ARM.exidx section is
live only if the executable section it describes is live. We implement
this with a reverse dependency field in InputSection.

Adding the dependency to InputSection is the simplest implementation
but it could be moved out to a separate map if it were found to decrease
performance for non ARM targets.

Differential revision: https://reviews.llvm.org/D25234

llvm-svn: 283734

1 files changed, 20 insertions, 0 deletions

diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index d9630790e2c..aa6a53cdbd4 100644
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -145,6 +145,8 @@ void elf::ObjectFile<ELFT>::parse(DenseSet<StringRef> &ComdatGroups) {
   // Read section and symbol tables.
   initializeSections(ComdatGroups);
   initializeSymbols();
+  if (Config->GcSections && Config->EMachine == EM_ARM )
+    initializeReverseDependencies();
 }
 
 // Sections with SHT_GROUP and comdat bits define comdat section groups.
@@ -270,6 +272,24 @@ void elf::ObjectFile<ELFT>::initializeSections(
   }
 }
 
+// .ARM.exidx sections have a reverse dependency on the InputSection they
+// have a SHF_LINK_ORDER dependency, this is identified by the sh_link.
+template <class ELFT>
+void elf::ObjectFile<ELFT>::initializeReverseDependencies() {
+  unsigned I = -1;
+  for (const Elf_Shdr &Sec : this->ELFObj.sections()) {
+    ++I;
+    if ((Sections[I] == &InputSection<ELFT>::Discarded) ||
+        !(Sec.sh_flags & SHF_LINK_ORDER))
+      continue;
+    if (Sec.sh_link >= Sections.size())
+      fatal(getFilename(this) + ": invalid sh_link index: " +
+            Twine(Sec.sh_link));
+    auto *IS = cast<InputSection<ELFT>>(Sections[Sec.sh_link]);
+    IS->DependentSection = Sections[I];
+  }
+}
+
 template <class ELFT>
 InputSectionBase<ELFT> *
 elf::ObjectFile<ELFT>::getRelocTarget(const Elf_Shdr &Sec) {