Overwrite contents of .debug_line section.

Summary:
Overwrite contents of .debug_line section since we don't reference
the original contents anymore. This saves ~100MB of HHVM binary.

(cherry picked from FBD3314917)

bolt/RewriteInstance.cpp

@@ -268,6 +268,7 @@ uint8_t *ExecutableFileMemoryManager::allocateSection(intptr_t Size,
   return ret;
 }
 
+/// Notifier for non-allocatable (note) section.
 uint8_t *ExecutableFileMemoryManager::recordNoteSection(
     const uint8_t *Data,
     uintptr_t Size,
@@ -758,8 +759,6 @@ void RewriteInstance::readSpecialSections() {
       FrameHdrAddress = Section.getAddress();
       FrameHdrContents = SectionContents;
       FrameHdrAlign = Section.getAlignment();
-    } else if (SectionName == ".debug_line") {
-      DebugLineSize = Section.getSize();
     } else if (SectionName == ".debug_ranges") {
       DebugRangesSize = Section.getSize();
     } else if (SectionName == ".debug_loc") {
@@ -1697,11 +1696,11 @@ void RewriteInstance::rewriteNoteSections() {
     ErrorOr<StringRef> SectionName = Obj->getSectionName(&Section);
     check_error(SectionName.getError(), "cannot get section name");
 
-    // Copy over section contents unless it's .debug_aranges, which shall be
-    // overwritten if -update-debug-sections is passed.
+    // New section size.
     uint64_t Size = 0;
 
-    if (*SectionName != ".debug_aranges" || !opts::UpdateDebugSections) {
+    // Copy over section contents unless it's one of the sections we ovewrite.
+    if (!shouldOverwriteSection(*SectionName)) {
       Size = Section.sh_size;
       std::string Data = InputFile->getData().substr(Section.sh_offset, Size);
       auto SectionPatchersIt = SectionPatchers.find(*SectionName);
@@ -1725,7 +1724,8 @@ void RewriteInstance::rewriteNoteSections() {
       // Write section extension.
       Address = SI.AllocAddress;
       if (Address) {
-        DEBUG(dbgs() << "BOLT: appending contents to section "
+        DEBUG(dbgs() << "BOLT: " << (Size ? "appending" : "writing")
+                     << " contents to section "
                      << *SectionName << '\n');
         OS.write(reinterpret_cast<const char *>(Address), SI.Size);
         Size += SI.Size;
@@ -2081,7 +2081,6 @@ void RewriteInstance::computeLineTableOffsets() {
       continue;
 
     auto Fragment = Label->getFragment();
-
     while (&*CurrentFragment != Fragment) {
       switch (CurrentFragment->getKind()) {
       case MCFragment::FT_Dwarf:
@@ -2096,7 +2095,6 @@ void RewriteInstance::computeLineTableOffsets() {
         llvm_unreachable(".debug_line section shouldn't contain other types "
                          "of fragments.");
       }
-
       ++CurrentFragment;
       CurrentOffset = 0;
     }
@@ -2113,7 +2111,7 @@ void RewriteInstance::computeLineTableOffsets() {
                    << "in .debug_info\n");
       auto &SI = SectionMM->NoteSectionInfo[".debug_info"];
       SI.PendingRelocs.emplace_back(
-          SectionInfo::Reloc{LTOI->second, 4, 0, Offset + DebugLineSize});
+          SectionInfo::Reloc{LTOI->second, 4, 0, Offset});
     }
     DEBUG(dbgs() << "BOLT-DEBUG: CU " << CUIDLineTablePair.first
                 << " has line table at " << Offset << "\n");
@@ -2326,3 +2324,14 @@ void RewriteInstance::updateDebugLineInfoForNonSimpleFunctions() {
     }
   }
 }
+
+bool RewriteInstance::shouldOverwriteSection(StringRef SectionName) {
+  if (opts::UpdateDebugSections) {
+    for (auto &OverwriteName : DebugSectionsToOverwrite) {
+      if (SectionName == OverwriteName)
+        return true;
+    }
+  }
+
+  return false;
+}

bolt/RewriteInstance.h

@@ -179,9 +179,6 @@ class RewriteInstance {
 
 private:
 
-  /// Huge page size used for alignment.
-  static constexpr unsigned PageAlign = 0x200000;
-
   /// Detect addresses and offsets available in the binary for allocating
   /// new sections.
   void discoverStorage();
@@ -247,7 +244,20 @@ class RewriteInstance {
     return Address - NewTextSegmentAddress + NewTextSegmentOffset;
   }
 
+  /// Return true if we should overwrite contents of the section instead
+  /// of appending contents to it.
+  bool shouldOverwriteSection(StringRef SectionName);
+
 private:
+
+  /// If we are updating debug info, these are the section we need to overwrite.
+  static constexpr const char *DebugSectionsToOverwrite[] = {
+    ".debug_aranges",
+    ".debug_line"};
+
+  /// Huge page size used for alignment.
+  static constexpr unsigned PageAlign = 0x200000;
+
   /// An instance of the input binary we are processing, externally owned.
   llvm::object::ELFObjectFileBase *InputFile;
 
@@ -306,9 +316,6 @@ class RewriteInstance {
   /// rewriting CFI info for these functions.
   std::vector<uint64_t> FailedAddresses;
 
-  /// Size of the .debug_line section on input.
-  uint32_t DebugLineSize{0};
-
   /// Size of the .debug_loc section in input.
   uint32_t DebugLocSize{0};