[ELF] - Simplify output section creation.

When there is no SECTION commands given, all sections are
technically orphans, but now we handle script orphans sections
and regular "orphans" sections for non-scripted case differently,
though we can handle them at one place.

Patch do that change.

Differential revision: https://reviews.llvm.org/D39045

llvm-svn: 316984

lld/ELF/LinkerScript.cpp

@@ -426,25 +426,6 @@ void LinkerScript::processSectionCommands() {
   }
 }
 
-// If no SECTIONS command was given, we create simple SectionCommands
-// as if a minimum SECTIONS command were given. This function does that.
-void LinkerScript::fabricateDefaultCommands() {
-  // Define start address
-  uint64_t StartAddr = UINT64_MAX;
-
-  // The Sections with -T<section> have been sorted in order of ascending
-  // address. We must lower StartAddr if the lowest -T<section address> as
-  // calls to setDot() must be monotonically increasing.
-  for (auto &KV : Config->SectionStartMap)
-    StartAddr = std::min(StartAddr, KV.second);
-
-  auto Expr = [=] {
-    return std::min(StartAddr, Target->getImageBase() + elf::getHeaderSize());
-  };
-  SectionCommands.insert(SectionCommands.begin(),
-                         make<SymbolAssignment>(".", Expr, ""));
-}
-
 static OutputSection *findByName(ArrayRef<BaseCommand *> Vec,
                                  StringRef Name) {
   for (BaseCommand *Base : Vec)
@@ -465,6 +446,7 @@ static void reportOrphan(InputSectionBase *IS, StringRef Name) {
 void LinkerScript::addOrphanSections(OutputSectionFactory &Factory) {
   unsigned End = SectionCommands.size();
 
+  std::vector<OutputSection *> V;
   for (InputSectionBase *S : InputSections) {
     if (!S->Live || S->Parent)
       continue;
@@ -479,9 +461,18 @@ void LinkerScript::addOrphanSections(OutputSectionFactory &Factory) {
     }
 
     if (OutputSection *OS = Factory.addInputSec(S, Name))
-      SectionCommands.push_back(OS);
+      V.push_back(OS);
     assert(S->getOutputSection()->SectionIndex == INT_MAX);
   }
+
+  // If no SECTIONS command was given, we should insert sections commands
+  // before others, so that we can handle scripts which refers them,
+  // for example: "foo = ABSOLUTE(ADDR(.text)));".
+  // When SECTIONS command is present we just add all orphans to the end.
+  if (HasSectionsCommand)
+    SectionCommands.insert(SectionCommands.end(), V.begin(), V.end());
+  else
+    SectionCommands.insert(SectionCommands.begin(), V.begin(), V.end());
 }
 
 uint64_t LinkerScript::advance(uint64_t Size, unsigned Alignment) {
@@ -802,11 +793,26 @@ LinkerScript::AddressState::AddressState() {
   }
 }
 
-// Assign addresses as instructed by linker script SECTIONS sub-commands.
+static uint64_t getInitialDot() {
+  // By default linker scripts use an initial value of 0 for '.',
+  // but prefer -image-base if set.
+  if (Script->HasSectionsCommand)
+    return Config->ImageBase ? *Config->ImageBase : 0;
+
+  uint64_t StartAddr = UINT64_MAX;
+  // The Sections with -T<section> have been sorted in order of ascending
+  // address. We must lower StartAddr if the lowest -T<section address> as
+  // calls to setDot() must be monotonically increasing.
+  for (auto &KV : Config->SectionStartMap)
+    StartAddr = std::min(StartAddr, KV.second);
+  return std::min(StartAddr, Target->getImageBase() + elf::getHeaderSize());
+}
+
+// Here we assign addresses as instructed by linker script SECTIONS
+// sub-commands. Doing that allows us to use final VA values, so here
+// we also handle rest commands like symbol assignments and ASSERTs.
 void LinkerScript::assignAddresses() {
-  // By default linker scripts use an initial value of 0 for '.', but prefer
-  // -image-base if set.
-  Dot = Config->ImageBase ? *Config->ImageBase : 0;
+  Dot = getInitialDot();
 
   auto Deleter = make_unique<AddressState>();
   Ctx = Deleter.get();

lld/ELF/LinkerScript.h

@@ -250,7 +250,6 @@ class LinkerScript final {
 
   ExprValue getSymbolValue(StringRef Name, const Twine &Loc);
 
-  void fabricateDefaultCommands();
   void addOrphanSections(OutputSectionFactory &Factory);
   void removeEmptyCommands();
   void adjustSectionsBeforeSorting();

lld/ELF/Writer.cpp

@@ -49,7 +49,6 @@ template <class ELFT> class Writer {
   void copyLocalSymbols();
   void addSectionSymbols();
   void addReservedSymbols();
-  void createSections();
   void forEachRelSec(std::function<void(InputSectionBase &)> Fn);
   void sortSections();
   void sortInputSections();
@@ -169,22 +168,14 @@ template <class ELFT> void Writer<ELFT>::run() {
   if (!Config->Relocatable)
     addReservedSymbols();
 
-  // Create output sections.
-  if (Script->HasSectionsCommand) {
-    // If linker script contains SECTIONS commands, let it create sections.
-    Script->processSectionCommands();
+  // We want to process linker script commands. When SECTIONS command
+  // is given we let it create sections.
+  Script->processSectionCommands();
 
-    // Linker scripts may have left some input sections unassigned.
-    // Assign such sections using the default rule.
-    Script->addOrphanSections(Factory);
-  } else {
-    // If linker script does not contain SECTIONS commands, create
-    // output sections by default rules. We still need to give the
-    // linker script a chance to run, because it might contain
-    // non-SECTIONS commands such as ASSERT.
-    Script->processSectionCommands();
-    createSections();
-  }
+  // Linker scripts controls how input sections are assigned to output sections.
+  // Input sections that were not handled by scripts are called "orphans", and
+  // they are assigned to output sections by the default rule. Process that.
+  Script->addOrphanSections(Factory);
 
   if (Config->Discard != DiscardPolicy::All)
     copyLocalSymbols();
@@ -852,20 +843,6 @@ void Writer<ELFT>::forEachRelSec(std::function<void(InputSectionBase &)> Fn) {
     Fn(*ES);
 }
 
-template <class ELFT> void Writer<ELFT>::createSections() {
-  std::vector<OutputSection *> Vec;
-  for (InputSectionBase *IS : InputSections)
-    if (IS && IS->Live)
-      if (OutputSection *Sec =
-              Factory.addInputSec(IS, getOutputSectionName(IS->Name)))
-        Vec.push_back(Sec);
-
-  Script->SectionCommands.insert(Script->SectionCommands.begin(), Vec.begin(),
-                                 Vec.end());
-
-  Script->fabricateDefaultCommands();
-}
-
 // This function generates assignments for predefined symbols (e.g. _end or
 // _etext) and inserts them into the commands sequence to be processed at the
 // appropriate time. This ensures that the value is going to be correct by the