[llvm-objcopy] Add --strip-unneeded option

Differential Revision: https://reviews.llvm.org/D46896

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@333267 91177308-0d34-0410-b5e6-96231b3b80d8

4 files changed, 43 insertions, 3 deletions

diff --git a/tools/llvm-objcopy/ObjcopyOpts.td b/tools/llvm-objcopy/ObjcopyOpts.td
index 9d1fb19c40c..e7c7541a32c 100644
--- a/tools/llvm-objcopy/ObjcopyOpts.td
+++ b/tools/llvm-objcopy/ObjcopyOpts.td
@@ -90,3 +90,5 @@ def K : JoinedOrSeparate<["-"], "K">,
         Alias<keep_symbol>;
 def only_keep_debug : Flag<["-", "--"], "only-keep-debug">,
                           HelpText<"Currently ignored. Only for compaitability with GNU objcopy.">;
+def strip_unneeded : Flag<["-", "--"], "strip-unneeded">,
+                      HelpText<"Remove all symbols not needed by relocations">;
diff --git a/tools/llvm-objcopy/Object.cpp b/tools/llvm-objcopy/Object.cpp
index efb3207aa8d..c00477a1980 100644
--- a/tools/llvm-objcopy/Object.cpp
+++ b/tools/llvm-objcopy/Object.cpp
@@ -50,6 +50,7 @@ void SectionBase::removeSectionReferences(const SectionBase *Sec) {}
 void SectionBase::removeSymbols(function_ref<bool(const Symbol &)> ToRemove) {}
 void SectionBase::initialize(SectionTableRef SecTable) {}
 void SectionBase::finalize() {}
+void SectionBase::markSymbols() {}
 
 template <class ELFT> void ELFWriter<ELFT>::writeShdr(const SectionBase &Sec) {
   uint8_t *Buf = BufPtr->getBufferStart();
@@ -255,6 +256,11 @@ const Symbol *SymbolTableSection::getSymbolByIndex(uint32_t Index) const {
   return Symbols[Index].get();
 }
 
+Symbol *SymbolTableSection::getSymbolByIndex(uint32_t Index) {
+  return const_cast<Symbol *>(
+      static_cast<const SymbolTableSection *>(this)->getSymbolByIndex(Index));
+}
+
 template <class ELFT>
 void ELFSectionWriter<ELFT>::visit(const SymbolTableSection &Sec) {
   uint8_t *Buf = Out.getBufferStart();
@@ -352,6 +358,11 @@ void RelocationSection::removeSymbols(
             "' because it is named in a relocation");
 }
 
+void RelocationSection::markSymbols() {
+  for (const Relocation &Reloc : Relocations)
+    Reloc.RelocSymbol->Referenced = true;
+}
+
 void SectionWriter::visit(const DynamicRelocationSection &Sec) {
   std::copy(std::begin(Sec.Contents), std::end(Sec.Contents),
             Out.getBufferStart() + Sec.Offset);
@@ -384,6 +395,11 @@ void GroupSection::removeSymbols(function_ref<bool(const Symbol &)> ToRemove) {
   }
 }
 
+void GroupSection::markSymbols() {
+  if (Sym)
+    Sym->Referenced = true;
+}
+
 void Section::initialize(SectionTableRef SecTable) {
   if (Link != ELF::SHN_UNDEF)
     LinkSection =
diff --git a/tools/llvm-objcopy/Object.h b/tools/llvm-objcopy/Object.h
index e4307794456..d005f0d4902 100644
--- a/tools/llvm-objcopy/Object.h
+++ b/tools/llvm-objcopy/Object.h
@@ -212,6 +212,7 @@ public:
   virtual void removeSectionReferences(const SectionBase *Sec);
   virtual void removeSymbols(function_ref<bool(const Symbol &)> ToRemove);
   virtual void accept(SectionVisitor &Visitor) const = 0;
+  virtual void markSymbols();
 };
 
 class Segment {
@@ -344,6 +345,7 @@ struct Symbol {
   uint8_t Type;
   uint64_t Value;
   uint8_t Visibility;
+  bool Referenced = false;
 
   uint16_t getShndx() const;
 };
@@ -368,6 +370,7 @@ public:
   bool empty() const { return Symbols.empty(); }
   const SectionBase *getStrTab() const { return SymbolNames; }
   const Symbol *getSymbolByIndex(uint32_t Index) const;
+  Symbol *getSymbolByIndex(uint32_t Index);
   void updateSymbols(function_ref<void(Symbol &)> Callable);
 
   void removeSectionReferences(const SectionBase *Sec) override;
@@ -382,7 +385,7 @@ public:
 };
 
 struct Relocation {
-  const Symbol *RelocSymbol = nullptr;
+  Symbol *RelocSymbol = nullptr;
   uint64_t Offset;
   uint64_t Addend;
   uint32_t Type;
@@ -436,6 +439,7 @@ public:
   void addRelocation(Relocation Rel) { Relocations.push_back(Rel); }
   void accept(SectionVisitor &Visitor) const override;
   void removeSymbols(function_ref<bool(const Symbol &)> ToRemove) override;
+  void markSymbols() override;
 
   static bool classof(const SectionBase *S) {
     if (S->Flags & ELF::SHF_ALLOC)
@@ -450,7 +454,7 @@ public:
 class GroupSection : public SectionBase {
   MAKE_SEC_WRITER_FRIEND
   const SymbolTableSection *SymTab = nullptr;
-  const Symbol *Sym = nullptr;
+  Symbol *Sym = nullptr;
   ELF::Elf32_Word FlagWord;
   SmallVector<SectionBase *, 3> GroupMembers;
 
@@ -462,7 +466,7 @@ public:
   explicit GroupSection(ArrayRef<uint8_t> Data) : Contents(Data) {}
 
   void setSymTab(const SymbolTableSection *SymTabSec) { SymTab = SymTabSec; }
-  void setSymbol(const Symbol *S) { Sym = S; }
+  void setSymbol(Symbol *S) { Sym = S; }
   void setFlagWord(ELF::Elf32_Word W) { FlagWord = W; }
   void addMember(SectionBase *Sec) { GroupMembers.push_back(Sec); }
 
@@ -470,6 +474,7 @@ public:
   void accept(SectionVisitor &) const override;
   void finalize() override;
   void removeSymbols(function_ref<bool(const Symbol &)> ToRemove) override;
+  void markSymbols() override;
 
   static bool classof(const SectionBase *S) {
     return S->Type == ELF::SHT_GROUP;
diff --git a/tools/llvm-objcopy/llvm-objcopy.cpp b/tools/llvm-objcopy/llvm-objcopy.cpp
index 2c2c9926447..0a5b5b0d066 100644
--- a/tools/llvm-objcopy/llvm-objcopy.cpp
+++ b/tools/llvm-objcopy/llvm-objcopy.cpp
@@ -168,6 +168,7 @@ struct CopyConfig {
   bool StripSections = false;
   bool StripNonAlloc = false;
   bool StripDWO = false;
+  bool StripUnneeded = false;
   bool ExtractDWO = false;
   bool LocalizeHidden = false;
   bool Weaken = false;
@@ -263,6 +264,14 @@ void HandleArgs(const CopyConfig &Config, Object &Obj, const Reader &Reader,
         Sym.Name = I->getValue();
     });
 
+    // The purpose of this loop is to mark symbols referenced by sections
+    // (like GroupSection or RelocationSection). This way, we know which
+    // symbols are still 'needed' and wich are not.
+    if (Config.StripUnneeded) {
+      for (auto &Section : Obj.sections())
+        Section.markSymbols();
+    }
+
     Obj.removeSymbols([&](const Symbol &Sym) {
       if (!Config.SymbolsToKeep.empty() &&
           is_contained(Config.SymbolsToKeep, Sym.Name))
@@ -281,6 +290,13 @@ void HandleArgs(const CopyConfig &Config, Object &Obj, const Reader &Reader,
         return true;
       }
 
+      // TODO: We might handle the 'null symbol' in a different way
+      // by probably handling it the same way as we handle 'null section' ?
+      if (Config.StripUnneeded && !Sym.Referenced && Sym.Index != 0 &&
+          (Sym.Binding == STB_LOCAL || Sym.getShndx() == SHN_UNDEF) &&
+          Sym.Type != STT_FILE && Sym.Type != STT_SECTION)
+        return true;
+
       return false;
     });
   }
@@ -509,6 +525,7 @@ CopyConfig ParseObjcopyOptions(ArrayRef<const char *> ArgsArr) {
   Config.StripDWO = InputArgs.hasArg(OBJCOPY_strip_dwo);
   Config.StripSections = InputArgs.hasArg(OBJCOPY_strip_sections);
   Config.StripNonAlloc = InputArgs.hasArg(OBJCOPY_strip_non_alloc);
+  Config.StripUnneeded = InputArgs.hasArg(OBJCOPY_strip_unneeded);
   Config.ExtractDWO = InputArgs.hasArg(OBJCOPY_extract_dwo);
   Config.LocalizeHidden = InputArgs.hasArg(OBJCOPY_localize_hidden);
   Config.Weaken = InputArgs.hasArg(OBJCOPY_weaken);