[llvm-mc] - Teach llvm-mc to generate compressed debug sections in zlib style.

Before this patch llvm-mc generated zlib-gnu styled sections. 
That means no SHF_COMPRESSED flag was set, magic 'zlib' signature
was used in combination with full size field. Sections were renamed to "*.z*".
This patch reimplements the compression style to zlib one as zlib-gnu looks
to be depricated everywhere.

Differential revision: http://reviews.llvm.org/D20331

llvm-svn: 270070

llvm/include/llvm/MC/MCContext.h

@@ -385,8 +385,6 @@ namespace llvm {
                                       const MCSymbolELF *Group,
                                       const MCSectionELF *Associated);
 
-    void renameELFSection(MCSectionELF *Section, StringRef Name);
-
     MCSectionELF *createELFGroupSection(const MCSymbolELF *Group);
 
     MCSectionCOFF *getCOFFSection(StringRef Section, unsigned Characteristics,

llvm/include/llvm/MC/MCSectionELF.h

@@ -62,8 +62,6 @@ private:
       Group->setIsSignature();
   }
 
-  void setSectionName(StringRef Name) { SectionName = Name; }
-
 public:
   ~MCSectionELF();
 
@@ -75,6 +73,7 @@ public:
   unsigned getType() const { return Type; }
   unsigned getFlags() const { return Flags; }
   unsigned getEntrySize() const { return EntrySize; }
+  void setFlags(unsigned F) { Flags = F; }
   const MCSymbolELF *getGroup() const { return Group; }
 
   void PrintSwitchToSection(const MCAsmInfo &MAI, raw_ostream &OS,

llvm/lib/MC/ELFObjectWriter.cpp

@@ -979,26 +979,6 @@ ELFObjectWriter::createRelocationSection(MCContext &Ctx,
   return RelaSection;
 }
 
-// Include the debug info compression header:
-// "ZLIB" followed by 8 bytes representing the uncompressed size of the section,
-// useful for consumers to preallocate a buffer to decompress into.
-static bool
-prependCompressionHeader(uint64_t Size,
-                         SmallVectorImpl<char> &CompressedContents) {
-  const StringRef Magic = "ZLIB";
-  if (Size <= Magic.size() + sizeof(Size) + CompressedContents.size())
-    return false;
-  if (sys::IsLittleEndianHost)
-    sys::swapByteOrder(Size);
-  CompressedContents.insert(CompressedContents.begin(),
-                            Magic.size() + sizeof(Size), 0);
-  std::copy(Magic.begin(), Magic.end(), CompressedContents.begin());
-  std::copy(reinterpret_cast<char *>(&Size),
-            reinterpret_cast<char *>(&Size + 1),
-            CompressedContents.begin() + Magic.size());
-  return true;
-}
-
 void ELFObjectWriter::writeSectionData(const MCAssembler &Asm, MCSection &Sec,
                                        const MCAsmLayout &Layout) {
   MCSectionELF &Section = static_cast<MCSectionELF &>(Sec);
@@ -1029,12 +1009,30 @@ void ELFObjectWriter::writeSectionData(const MCAssembler &Asm, MCSection &Sec,
     return;
   }
 
-  if (!prependCompressionHeader(UncompressedData.size(), CompressedContents)) {
+  uint64_t HdrSize =
+      is64Bit() ? sizeof(ELF::Elf32_Chdr) : sizeof(ELF::Elf64_Chdr);
+  if (UncompressedData.size() <= HdrSize + CompressedContents.size()) {
     getStream() << UncompressedData;
     return;
   }
-  Asm.getContext().renameELFSection(&Section,
-                                    (".z" + SectionName.drop_front(1)).str());
+
+  // Set the compressed flag. That is zlib style.
+  Section.setFlags(Section.getFlags() | ELF::SHF_COMPRESSED);
+
+  // Platform specific header is followed by compressed data.
+  if (is64Bit()) {
+    // Write Elf64_Chdr header.
+    write(static_cast<ELF::Elf64_Word>(ELF::ELFCOMPRESS_ZLIB));
+    write(static_cast<ELF::Elf64_Word>(0)); // ch_reserved field.
+    write(static_cast<ELF::Elf64_Xword>(UncompressedData.size()));
+    write(static_cast<ELF::Elf64_Xword>(Sec.getAlignment()));
+  } else {
+    // Write Elf32_Chdr header otherwise.
+    write(static_cast<ELF::Elf32_Word>(ELF::ELFCOMPRESS_ZLIB));
+    write(static_cast<ELF::Elf32_Word>(UncompressedData.size()));
+    write(static_cast<ELF::Elf32_Word>(Sec.getAlignment()));
+  }
+
   getStream() << CompressedContents;
 }
 

llvm/lib/MC/MCContext.cpp

@@ -285,22 +285,6 @@ MCSectionMachO *MCContext::getMachOSection(StringRef Segment, StringRef Section,
              Segment, Section, TypeAndAttributes, Reserved2, Kind, Begin);
 }
 
-void MCContext::renameELFSection(MCSectionELF *Section, StringRef Name) {
-  StringRef GroupName;
-  if (const MCSymbol *Group = Section->getGroup())
-    GroupName = Group->getName();
-
-  unsigned UniqueID = Section->getUniqueID();
-  ELFUniquingMap.erase(
-      ELFSectionKey{Section->getSectionName(), GroupName, UniqueID});
-  auto I = ELFUniquingMap.insert(std::make_pair(
-                                     ELFSectionKey{Name, GroupName, UniqueID},
-                                     Section))
-               .first;
-  StringRef CachedName = I->first.SectionName;
-  const_cast<MCSectionELF *>(Section)->setSectionName(CachedName);
-}
-
 MCSectionELF *MCContext::createELFRelSection(StringRef Name, unsigned Type,
                                              unsigned Flags, unsigned EntrySize,
                                              const MCSymbolELF *Group,

llvm/test/MC/ELF/compression.s

@@ -3,13 +3,19 @@
 // RUN: llvm-dwarfdump -debug-dump=info %t | FileCheck --check-prefix=INFO %s
 // RUN: llvm-mc -filetype=obj -compress-debug-sections -triple i386-pc-linux-gnu < %s \
 // RUN:     | llvm-readobj -symbols - | FileCheck --check-prefix=386-SYMBOLS %s
+// RUN: llvm-readobj -sections %t | FileCheck --check-prefix=ZLIB %s
 
 // REQUIRES: zlib
 
-// CHECK: Contents of section .zdebug_line:
-// Check for the 'ZLIB' file magic at the start of the section only
-// CHECK-NEXT: ZLIB
-// CHECK-NOT: ZLIB
+// Check that debug_line section was not renamed, so it is
+// zlib-style, not zlib-gnu one. Check that SHF_COMPRESSED was set.
+// ZLIB:      Section {
+// ZLIB:        Index:
+// ZLIB:        Name: .debug_line
+// ZLIB-NEXT:   Type: SHT_PROGBITS
+// ZLIB-NEXT:   Flags [
+// ZLIB-NEXT:     SHF_COMPRESSED
+// ZLIB-NEXT:   ]
 
 // Don't compress small sections, such as this simple debug_abbrev example
 // CHECK: Contents of section .debug_abbrev:
@@ -30,7 +36,7 @@
 // sections, so make sure we handle symbols inside compressed sections
 // 386-SYMBOLS: Name: .Linfo_string0
 // 386-SYMBOLS-NOT: }
-// 386-SYMBOLS: Section: .zdebug_str
+// 386-SYMBOLS: Section: .debug_str
 
 	.section	.debug_line,"",@progbits