Make gc a bit more aggressive.

We were not gcing any section whose name was a C identifier. Both gold
and bfd only keep those if they are used.

To avoid having to create the __start/__stop symbols early or doing
string lookups in resolvedReloc, this patch just looks for undefined
symbols __start/__stop to decide if a section is needed or not.

1 files changed, 15 insertions, 8 deletions

diff --git a/lld/ELF/MarkLive.cpp b/lld/ELF/MarkLive.cpp
index 8f75de6dc30..9528708b584 100644
--- a/lld/ELF/MarkLive.cpp
+++ b/lld/ELF/MarkLive.cpp
@@ -170,12 +170,7 @@ template <class ELFT> static bool isReserved(InputSectionBase *Sec) {
     if (!(Sec->Flags & SHF_ALLOC))
       return true;
 
-    // We do not want to reclaim sections if they can be referred
-    // by __start_* and __stop_* symbols.
     StringRef S = Sec->Name;
-    if (isValidCIdentifier(S))
-      return true;
-
     return S.startswith(".ctors") || S.startswith(".dtors") ||
            S.startswith(".init") || S.startswith(".fini") ||
            S.startswith(".jcr");
@@ -226,11 +221,22 @@ template <class ELFT> void elf::markLive() {
   for (StringRef S : Config->Undefined)
     MarkSymbol(Symtab<ELFT>::X->find(S));
 
+  // Remember which __start_* or __stop_* symbols are used so that we don't gc
+  // those sections.
+  DenseSet<StringRef> UsedStartStopNames;
+
   // Preserve externally-visible symbols if the symbols defined by this
   // file can interrupt other ELF file's symbols at runtime.
-  for (const Symbol *S : Symtab<ELFT>::X->getSymbols())
-    if (S->includeInDynsym())
+  for (const Symbol *S : Symtab<ELFT>::X->getSymbols()) {
+    if (auto *U = dyn_cast_or_null<Undefined>(S->body())) {
+      StringRef Name = U->getName();
+      for (StringRef Prefix : {"__start_", "__stop_"})
+        if (Name.startswith(Prefix))
+          UsedStartStopNames.insert(Name.substr(Prefix.size()));
+    } else if (S->includeInDynsym()) {
       MarkSymbol(S->body());
+    }
+  }
 
   // Preserve special sections and those which are specified in linker
   // script KEEP command.
@@ -240,7 +246,8 @@ template <class ELFT> void elf::markLive() {
     // referred by .eh_frame here.
     if (auto *EH = dyn_cast_or_null<EhInputSection<ELFT>>(Sec))
       scanEhFrameSection<ELFT>(*EH, Enqueue);
-    if (isReserved<ELFT>(Sec) || Script<ELFT>::X->shouldKeep(Sec))
+    if (isReserved<ELFT>(Sec) || Script<ELFT>::X->shouldKeep(Sec) ||
+        UsedStartStopNames.count(Sec->Name))
       Enqueue({Sec, 0});
   }