DebugInfo: Support for compressed debug info sections

1) When creating a .debug_* section and instead create a .zdebug_
   section.
2) When creating a fragment in a .zdebug_* section, make it a compressed
   fragment.
3) When computing the size of a compressed section, compress the data
   and use the size of the compressed data.
4) Emit the compressed bytes.

Also, check that only if a section has a compressed fragment, then that
is the only fragment in the section.

Assert-fail if the fragment's data is modified after it is compressed.

Initial review on llvm-commits by Eric Christopher and Rafael Espindola.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204958 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/MC/MCAssembler.h

@@ -51,6 +51,7 @@ public:
   enum FragmentType {
     FT_Align,
     FT_Data,
+    FT_Compressed,
     FT_CompactEncodedInst,
     FT_Fill,
     FT_Relaxable,
@@ -160,6 +161,7 @@ public:
         return false;
       case MCFragment::FT_Relaxable:
       case MCFragment::FT_CompactEncodedInst:
+      case MCFragment::FT_Compressed:
       case MCFragment::FT_Data:
         return true;
     }
@@ -194,7 +196,8 @@ public:
 
   static bool classof(const MCFragment *F) {
     MCFragment::FragmentType Kind = F->getKind();
-    return Kind == MCFragment::FT_Relaxable || Kind == MCFragment::FT_Data;
+    return Kind == MCFragment::FT_Relaxable || Kind == MCFragment::FT_Data ||
+           Kind == MCFragment::FT_Compressed;
   }
 };
 
@@ -213,6 +216,11 @@ class MCDataFragment : public MCEncodedFragmentWithFixups {
 
   /// Fixups - The list of fixups in this fragment.
   SmallVector<MCFixup, 4> Fixups;
+protected:
+  MCDataFragment(MCFragment::FragmentType FType, MCSectionData *SD = 0)
+      : MCEncodedFragmentWithFixups(FType, SD), HasInstructions(false),
+        AlignToBundleEnd(false) {}
+
 public:
   MCDataFragment(MCSectionData *SD = 0)
     : MCEncodedFragmentWithFixups(FT_Data, SD),
@@ -246,10 +254,21 @@ public:
   const_fixup_iterator fixup_end() const override {return Fixups.end();}
 
   static bool classof(const MCFragment *F) {
-    return F->getKind() == MCFragment::FT_Data;
+    return F->getKind() == MCFragment::FT_Data ||
+           F->getKind() == MCFragment::FT_Compressed;
   }
 };
 
+class MCCompressedFragment: public MCDataFragment {
+  mutable SmallVector<char, 32> CompressedContents;
+public:
+  MCCompressedFragment(MCSectionData *SD = nullptr)
+      : MCDataFragment(FT_Compressed, SD) {}
+  const SmallVectorImpl<char> &getCompressedContents() const;
+  using MCDataFragment::getContents;
+  SmallVectorImpl<char> &getContents() override;
+};
+
 /// This is a compact (memory-size-wise) fragment for holding an encoded
 /// instruction (non-relaxable) that has no fixups registered. When applicable,
 /// it can be used instead of MCDataFragment and lead to lower memory

lib/MC/MCAsmInfo.cpp

@@ -100,6 +100,8 @@ MCAsmInfo::MCAsmInfo() {
   //   architecture basis.
   //   - The target subclasses for AArch64, ARM, and X86  handle these cases
   UseIntegratedAssembler = false;
+
+  CompressDebugSections = false;
 }
 
 MCAsmInfo::~MCAsmInfo() {

lib/MC/MCAssembler.cpp

@@ -28,6 +28,9 @@
 #include "llvm/Support/LEB128.h"
 #include "llvm/Support/TargetRegistry.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/Compression.h"
+#include "llvm/Support/Host.h"
 
 using namespace llvm;
 
@@ -230,6 +233,39 @@ MCEncodedFragmentWithFixups::~MCEncodedFragmentWithFixups() {
 
 /* *** */
 
+const SmallVectorImpl<char> &MCCompressedFragment::getCompressedContents() const {
+  assert(getParent()->size() == 1 &&
+         "Only compress sections containing a single fragment");
+  if (CompressedContents.empty()) {
+    std::unique_ptr<MemoryBuffer> CompressedSection;
+    zlib::Status Success =
+        zlib::compress(StringRef(getContents().data(), getContents().size()),
+                       CompressedSection);
+    (void)Success;
+    assert(Success == zlib::StatusOK);
+    CompressedContents.push_back('Z');
+    CompressedContents.push_back('L');
+    CompressedContents.push_back('I');
+    CompressedContents.push_back('B');
+    uint64_t Size = getContents().size();
+    if (sys::IsLittleEndianHost)
+      Size = sys::SwapByteOrder(Size);
+    CompressedContents.append(reinterpret_cast<char *>(&Size),
+                              reinterpret_cast<char *>(&Size + 1));
+    CompressedContents.append(CompressedSection->getBuffer().begin(),
+                              CompressedSection->getBuffer().end());
+  }
+  return CompressedContents;
+}
+
+SmallVectorImpl<char> &MCCompressedFragment::getContents() {
+  assert(CompressedContents.empty() &&
+         "Fragment contents should not be altered after compression");
+  return MCDataFragment::getContents();
+}
+
+/* *** */
+
 MCSectionData::MCSectionData() : Section(0) {}
 
 MCSectionData::MCSectionData(const MCSection &_Section, MCAssembler *A)
@@ -430,6 +466,8 @@ uint64_t MCAssembler::computeFragmentSize(const MCAsmLayout &Layout,
   case MCFragment::FT_Relaxable:
   case MCFragment::FT_CompactEncodedInst:
     return cast<MCEncodedFragment>(F).getContents().size();
+  case MCFragment::FT_Compressed:
+    return cast<MCCompressedFragment>(F).getCompressedContents().size();
   case MCFragment::FT_Fill:
     return cast<MCFillFragment>(F).getSize();
 
@@ -618,6 +656,11 @@ static void writeFragment(const MCAssembler &Asm, const MCAsmLayout &Layout,
     break;
   }
 
+  case MCFragment::FT_Compressed:
+    ++stats::EmittedDataFragments;
+    OW->WriteBytes(cast<MCCompressedFragment>(F).getCompressedContents());
+    break;
+
   case MCFragment::FT_Data: 
     ++stats::EmittedDataFragments;
     writeFragmentContents(F, OW);
@@ -694,6 +737,7 @@ void MCAssembler::writeSectionData(const MCSectionData *SD,
            ie = SD->end(); it != ie; ++it) {
       switch (it->getKind()) {
       default: llvm_unreachable("Invalid fragment in virtual section!");
+      case MCFragment::FT_Compressed:
       case MCFragment::FT_Data: {
         // Check that we aren't trying to write a non-zero contents (or fixups)
         // into a virtual section. This is to support clients which use standard
@@ -1021,6 +1065,8 @@ void MCFragment::dump() {
   switch (getKind()) {
   case MCFragment::FT_Align: OS << "MCAlignFragment"; break;
   case MCFragment::FT_Data:  OS << "MCDataFragment"; break;
+  case MCFragment::FT_Compressed:
+    OS << "MCCompressedFragment"; break;
   case MCFragment::FT_CompactEncodedInst:
     OS << "MCCompactEncodedInstFragment"; break;
   case MCFragment::FT_Fill:  OS << "MCFillFragment"; break;
@@ -1047,6 +1093,7 @@ void MCFragment::dump() {
        << " MaxBytesToEmit:" << AF->getMaxBytesToEmit() << ">";
     break;
   }
+  case MCFragment::FT_Compressed:
   case MCFragment::FT_Data:  {
     const MCDataFragment *DF = cast<MCDataFragment>(this);
     OS << "\n       ";

lib/MC/MCContext.cpp

@@ -37,13 +37,13 @@ typedef std::map<SectionGroupPair, const MCSectionCOFF *> COFFUniqueMapTy;
 
 MCContext::MCContext(const MCAsmInfo *mai, const MCRegisterInfo *mri,
                      const MCObjectFileInfo *mofi, const SourceMgr *mgr,
-                     bool DoAutoReset) :
-  SrcMgr(mgr), MAI(mai), MRI(mri), MOFI(mofi),
-  Allocator(), Symbols(Allocator), UsedNames(Allocator),
-  NextUniqueID(0),
-  CurrentDwarfLoc(0,0,0,DWARF2_FLAG_IS_STMT,0,0),
-  DwarfLocSeen(false), GenDwarfForAssembly(false), GenDwarfFileNumber(0),
-  AllowTemporaryLabels(true), DwarfCompileUnitID(0), AutoReset(DoAutoReset) {
+                     bool DoAutoReset)
+    : SrcMgr(mgr), MAI(mai), MRI(mri), MOFI(mofi), Allocator(),
+      Symbols(Allocator), UsedNames(Allocator), NextUniqueID(0),
+      CurrentDwarfLoc(0, 0, 0, DWARF2_FLAG_IS_STMT, 0, 0), DwarfLocSeen(false),
+      GenDwarfForAssembly(false), GenDwarfFileNumber(0),
+      AllowTemporaryLabels(true), DwarfCompileUnitID(0),
+      AutoReset(DoAutoReset) {
 
   error_code EC = llvm::sys::fs::current_path(CompilationDir);
   if (EC)
@@ -251,6 +251,11 @@ getELFSection(StringRef Section, unsigned Type, unsigned Flags,
     ELFUniquingMap = new ELFUniqueMapTy();
   ELFUniqueMapTy &Map = *(ELFUniqueMapTy*)ELFUniquingMap;
 
+  SmallString<32> ZDebugName;
+  if (MAI->compressDebugSections() && Section.startswith(".debug_") &&
+      Section != ".debug_frame")
+    Section = (".z" + Section.drop_front(1)).toStringRef(ZDebugName);
+
   // Do the lookup, if we have a hit, return it.
   std::pair<ELFUniqueMapTy::iterator, bool> Entry = Map.insert(
       std::make_pair(SectionGroupPair(Section, Group), (MCSectionELF *)0));

lib/MC/MCObjectStreamer.cpp

@@ -20,6 +20,7 @@
 #include "llvm/MC/MCSection.h"
 #include "llvm/MC/MCSymbol.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/MC/MCSectionELF.h"
 using namespace llvm;
 
 MCObjectStreamer::MCObjectStreamer(MCContext &Context, MCAsmBackend &TAB,
@@ -63,7 +64,11 @@ MCDataFragment *MCObjectStreamer::getOrCreateDataFragment() const {
   // When bundling is enabled, we don't want to add data to a fragment that
   // already has instructions (see MCELFStreamer::EmitInstToData for details)
   if (!F || (Assembler->isBundlingEnabled() && F->hasInstructions())) {
-    F = new MCDataFragment();
+    const auto *Sec = dyn_cast<MCSectionELF>(&getCurrentSectionData()->getSection());
+    if (Sec && Sec->getSectionName().startswith(".zdebug_"))
+      F = new MCCompressedFragment();
+    else
+      F = new MCDataFragment();
     insert(F);
   }
   return F;

test/MC/ELF/compression.s

@@ -0,0 +1,18 @@
+// RUN: llvm-mc -filetype=obj -compress-debug-sections -triple x86_64-pc-linux-gnu %s -o - | llvm-objdump -s - | FileCheck %s
+
+// CHECK: Contents of section .zdebug_line:
+// Check for the 'ZLIB' file magic at the start of the section
+// CHECK-NEXT: ZLIB
+// We shouldn't compress the debug_frame section, since it can be relaxed
+// CHECK: Contents of section .debug_frame
+// CHECK-NOT: ZLIB
+
+	.section	.debug_line,"",@progbits
+	.text
+foo:
+	.cfi_startproc
+	.file 1 "Driver.ii"
+	.loc 1 2 0
+        nop
+	.cfi_endproc
+	.cfi_sections .debug_frame

tools/llvm-mc/llvm-mc.cpp

@@ -49,6 +49,9 @@ OutputFilename("o", cl::desc("Output filename"),
 static cl::opt<bool>
 ShowEncoding("show-encoding", cl::desc("Show instruction encodings"));
 
+static cl::opt<bool>
+CompressDebugSections("compress-debug-sections", cl::desc("Compress DWARF debug sections"));
+
 static cl::opt<bool>
 ShowInst("show-inst", cl::desc("Show internal instruction representation"));
 
@@ -381,6 +384,9 @@ int main(int argc, char **argv) {
   std::unique_ptr<MCAsmInfo> MAI(TheTarget->createMCAsmInfo(*MRI, TripleName));
   assert(MAI && "Unable to create target asm info!");
 
+  if (CompressDebugSections)
+    MAI->setCompressDebugSections(true);
+
   // FIXME: This is not pretty. MCContext has a ptr to MCObjectFileInfo and
   // MCObjectFileInfo needs a MCContext reference in order to initialize itself.
   std::unique_ptr<MCObjectFileInfo> MOFI(new MCObjectFileInfo());