Basic GDB JIT Interface integration.

src/gdbjit.cc

Index: src/gdbjit.cc
diff --git a/src/gdbjit.cc b/src/gdbjit.cc
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+++ b/src/gdbjit.cc
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+// Copyright 2010 the V8 project authors. All rights reserved.
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+//       copyright notice, this list of conditions and the following
+//       disclaimer in the documentation and/or other materials provided
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifdef ENABLE_GDBJIT_INTERFACE
+#include "gdbjit.h"
+
+#include "bootstrapper.h"
+#include "compiler.h"
+#include "global-handles.h"
+#include "messages.h"
+#include "natives.h"
+
+namespace v8 {
+namespace internal {
+
+class ELF;
+
+class Writer BASE_EMBEDDED {
+ public:
+  explicit Writer(ELF* elf)
+      : elf_(elf),
+        position_(0),
+        capacity_(1024),
+        buffer_(reinterpret_cast<byte*>(malloc(capacity_))) {
+  }
+
+  ~Writer() {
+    free(buffer_);
+  }
+
+  uintptr_t position() const {
+    return position_;
+  }
+
+  template<typename T>
+  class Slot {
+   public:
+    Slot(Writer* w, uintptr_t offset) : w_(w), offset_(offset) { }
+
+    T* operator-> () {
+      return w_->RawSlotAt<T>(offset_);
+    }
+
+    void set(const T& value) {
+      *w_->RawSlotAt<T>(offset_) = value;
+    }
+
+    Slot<T> at(int i) {
+      return Slot<T>(w_, offset_ + sizeof(T) * i);
+    }
+
+   private:
+    Writer* w_;
+    uintptr_t offset_;
+  };
+
+  template<typename T>
+  void Write(const T& val) {
+    Ensure(position_ + sizeof(T));
+    *RawSlotAt<T>(position_) = val;
+    position_ += sizeof(T);
+  }
+
+  template<typename T>
+  Slot<T> SlotAt(uintptr_t offset) {
+    Ensure(offset + sizeof(T));
+    return Slot<T>(this, offset);
+  }
+
+  template<typename T>
+  Slot<T> SlotHere() {

[Erik Corry, 2011/01/04 14:21:48]

The name should reflect the fact that this function (and the next) create slots.
 eg CreateSlotHere()

[Vyacheslav Egorov (Chromium), 2011/01/18 16:10:42]

Done.

+    return SlotsHere<T>(1);
+  }
+
+  template<typename T>
+  Slot<T> SlotsHere(uint32_t count) {
+    uintptr_t slot_position = position_;
+    Ensure(position_ += sizeof(T) * count);

[Erik Corry, 2011/01/04 14:21:48]

no no no no

[Vyacheslav Egorov (Chromium), 2011/01/18 16:10:42]

Done.

+    return SlotAt<T>(slot_position);
+  }
+
+  void Ensure(uintptr_t pos) {
+    if (capacity_ < pos) {
+      while (capacity_ < pos) capacity_ *= 2;
+      buffer_ = reinterpret_cast<byte*>(realloc(buffer_, capacity_));
+    }
+  }
+
+  ELF* elf() { return elf_; }
+
+  byte* buffer() { return buffer_; }
+
+  void Align(uintptr_t align) {
+    uintptr_t delta = position_ % align;
+    if (delta == 0) return;
+    uintptr_t padding = align - delta;
+    Ensure(position_ += padding);
+    ASSERT((position_ % align) == 0);
+  }
+
+  void WriteULEB128(uintptr_t value) {
+    do {
+      uint8_t byte = value & 0x7F;
+      value >>= 7;
+      if (value != 0) byte |= 0x80;
+      Write<uint8_t>(byte);
+    } while (value != 0);
+  }
+
+  void WriteSLEB128(intptr_t value) {
+    bool more = true;
+    while (more) {
+      int8_t byte = value & 0x7F;
+      bool byte_sign = byte & 0x40;
+      value >>= 7;
+
+      if ((value == 0 && !byte_sign) || (value == -1 && byte_sign)) {
+        more = false;
+      } else {
+        byte |= 0x80;
+      }
+
+      Write<int8_t>(byte);
+    }
+  }
+
+  void WriteString(const char* str) {
+    do {
+      Write<char>(*str);
+    } while (*str++);
+  }
+
+ private:
+  template<typename T> friend class Slot;
+
+  template<typename T>
+  T* RawSlotAt(uintptr_t offset) {
+    ASSERT(offset < capacity_ && offset + sizeof(T) <= capacity_);
+    return reinterpret_cast<T*>(&buffer_[offset]);
+  }
+
+  ELF* elf_;
+  uintptr_t position_;
+  uintptr_t capacity_;
+  byte* buffer_;
+};
+
+class StringTable;
+
+class ELFSection : public ZoneObject {
+ public:
+  struct Header {
+    uint32_t name;
+    uint32_t type;
+    uintptr_t flags;
+    uintptr_t address;
+    uintptr_t offset;
+    uintptr_t size;
+    uint32_t link;
+    uint32_t info;
+    uintptr_t alignment;
+    uintptr_t entry_size;
+  };
+
+  enum Type {
+    TYPE_NULL = 0,
+    TYPE_PROGBITS = 1,
+    TYPE_SYMTAB = 2,
+    TYPE_STRTAB = 3,
+    TYPE_RELA = 4,
+    TYPE_HASH = 5,
+    TYPE_DYNAMIC = 6,
+    TYPE_NOTE = 7,
+    TYPE_NOBITS = 8,
+    TYPE_REL = 9,
+    TYPE_SHLIB = 10,
+    TYPE_DYNSYM = 11,
+    TYPE_LOPROC = 0x70000000,
+    TYPE_HIPROC = 0x7fffffff,
+    TYPE_LOUSER = 0x80000000,
+    TYPE_HIUSER = 0xffffffff
+  };
+
+  enum Flags {
+    FLAG_WRITE = 1,
+    FLAG_ALLOC = 2,
+    FLAG_EXEC = 4
+  };
+
+  enum SpecialIndexes {
+    INDEX_ABSOLUTE = 0xfff1
+  };
+
+  ELFSection(const char* name, Type type, uintptr_t align)
+      : name_(name), type_(type), align_(align) { }
+
+  virtual ~ELFSection() { }
+
+  void PopulateHeader(Writer::Slot<Header> header, StringTable* strtab);
+
+  virtual void WriteBody(Writer::Slot<Header> header, Writer* w) {
+    uintptr_t start = w->position();
+    if (WriteBody(w)) {
+      uintptr_t end = w->position();
+      header->offset = start;
+      header->size = end - start;
+    }
+  }
+
+  virtual bool WriteBody(Writer* w) {
+    return false;
+  }
+
+  uint16_t index() const { return index_; }
+  void set_index(uint16_t index) { index_ = index; }
+
+ protected:
+  virtual void PopulateHeader(Writer::Slot<Header> header) {
+    header->flags = 0;
+    header->address = 0;
+    header->offset = 0;
+    header->size = 0;
+    header->link = 0;
+    header->info = 0;
+    header->entry_size = 0;
+  }
+
+
+ private:
+  const char* name_;
+  Type type_;
+  uintptr_t align_;
+  uint16_t index_;
+};
+
+
+class FullHeaderELFSection : public ELFSection {
+ public:
+  FullHeaderELFSection(const char* name,
+                       Type type,
+                       uintptr_t align,
+                       uintptr_t addr,
+                       uintptr_t offset,
+                       uintptr_t size,
+                       uintptr_t flags)
+      : ELFSection(name, type, align),
+        addr_(addr),
+        offset_(offset),
+        size_(size),
+        flags_(flags) { }
+
+ protected:
+  virtual void PopulateHeader(Writer::Slot<Header> header) {
+    ELFSection::PopulateHeader(header);
+    header->address = addr_;
+    header->offset = offset_;
+    header->size = size_;
+    header->flags = flags_;
+  }
+
+ private:
+  uintptr_t addr_;
+  uintptr_t offset_;
+  uintptr_t size_;
+  uintptr_t flags_;
+};
+
+
+class StringTable : public ELFSection {
+ public:
+  explicit StringTable(const char* name)
+      : ELFSection(name, TYPE_STRTAB, 1), writer_(NULL), offset_(0), size_(0) {
+  }
+
+  uintptr_t Add(const char* str) {
+    if (*str == '\0') return 0;
+
+    uintptr_t offset = size_;
+    WriteString(str);
+    return offset;
+  }
+
+  void AttachWriter(Writer* w) {
+    writer_ = w;
+    offset_ = writer_->position();
+
+    WriteString("");

[Erik Corry, 2011/01/04 14:21:48]

I'm sure there is a good reason why we do this which we could put in a comment.

[Vyacheslav Egorov (Chromium), 2011/01/18 16:10:42]

Done.

+  }
+
+  void DetachWriter() {
+    writer_ = NULL;
+  }
+
+  virtual void WriteBody(Writer::Slot<Header> header, Writer* w) {
+    ASSERT(writer_ == NULL);
+    header->offset = offset_;
+    header->size = size_;
+  }
+
+ private:
+  void WriteString(const char* str) {
+    uintptr_t written = 0;
+    do {
+      writer_->Write(*str);
+      written++;
+    } while (*str++);
+    size_ += written;
+  }
+
+  Writer* writer_;
+
+  uintptr_t offset_;
+  uintptr_t size_;
+};
+
+
+void ELFSection::PopulateHeader(Writer::Slot<ELFSection::Header> header,
+                                StringTable* strtab) {
+  header->name = strtab->Add(name_);
+  header->type = type_;
+  header->alignment = align_;
+  PopulateHeader(header);
+}
+
+
+class ELF BASE_EMBEDDED {
+ public:
+  ELF() : sections_(6) {
+    sections_.Add(new ELFSection("", ELFSection::TYPE_NULL, 0));
+    sections_.Add(new StringTable(".shstrtab"));
+  }
+
+  void Write(Writer* w) {
+    WriteHeader(w);
+    WriteSectionTable(w);
+    WriteSections(w);
+  }
+
+  ELFSection* SectionAt(uint32_t index) {
+    return sections_[index];
+  }
+
+  uint32_t AddSection(ELFSection* section) {
+    sections_.Add(section);
+    section->set_index(sections_.length() - 1);
+    return sections_.length() - 1;
+  }
+
+ private:
+  struct ELFHeader {
+    uint8_t ident[16];
+    uint16_t type;
+    uint16_t machine;
+    uint32_t version;
+    uintptr_t entry;
+    uintptr_t pht_offset;
+    uintptr_t sht_offset;
+    uint32_t flags;
+    uint16_t header_size;
+    uint16_t pht_entry_size;
+    uint16_t pht_entry_num;
+    uint16_t sht_entry_size;
+    uint16_t sht_entry_num;
+    uint16_t sht_strtab_index;
+  };
+
+
+  void WriteHeader(Writer* w) {
+    ASSERT(w->position() == 0);
+    Writer::Slot<ELFHeader> header = w->SlotHere<ELFHeader>();
+#if defined(V8_TARGET_ARCH_IA32)
+    const uint8_t ident[16] =
+        { 0x7f, 'E', 'L', 'F', 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0};
+#elif defined(V8_TARGET_ARCH_X64)
+    const uint8_t ident[16] =
+        { 0x7f, 'E', 'L', 'F', 2, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0};
+#endif

[Erik Corry, 2011/01/04 14:21:48]

At least one place in this file should have an else and a #error to catch the
ARM case and give a sensible error message.

[Vyacheslav Egorov (Chromium), 2011/01/18 16:10:42]

Done.

+    memcpy(header->ident, ident, 16);
+    header->type = 1;
+#if defined(V8_TARGET_ARCH_IA32)
+    header->machine = 3;
+#elif defined(V8_TARGET_ARCH_X64)
+    // Processor identification value for x64 is 62 as defined in
+    //    System V ABI, AMD64 Supplement
+    //    http://www.x86-64.org/documentation/abi.pdf
+    header->machine = 62;
+#endif
+    header->version = 1;
+    header->entry = 0;
+    header->pht_offset = 0;
+    header->sht_offset = sizeof(ELFHeader);  // Section table follows header.
+    header->flags = 0;
+    header->header_size = sizeof(ELFHeader);
+    header->pht_entry_size = 0;
+    header->pht_entry_num = 0;
+    header->sht_entry_size = sizeof(ELFSection::Header);
+    header->sht_entry_num = sections_.length();
+    header->sht_strtab_index = 1;
+  }
+
+  void WriteSectionTable(Writer* w) {
+    // Section headers table immediately follows file header.
+    ASSERT(w->position() == sizeof(ELFHeader));
+
+    Writer::Slot<ELFSection::Header> headers =
+        w->SlotsHere<ELFSection::Header>(sections_.length());
+
+    // String table for section table is the first section.
+    StringTable* strtab = static_cast<StringTable*>(SectionAt(1));
+    strtab->AttachWriter(w);
+    for (int i = 0, length = sections_.length();
+         i < length;
+         i++) {
+      sections_[i]->PopulateHeader(headers.at(i), strtab);
+    }
+    strtab->DetachWriter();
+  }
+
+  int SectionHeaderPosition(uint32_t section_index) {
+    return sizeof(ELFHeader) + sizeof(ELFSection::Header) * section_index;
+  }
+
+  void WriteSections(Writer* w) {
+    Writer::Slot<ELFSection::Header> headers =
+        w->SlotAt<ELFSection::Header>(sizeof(ELFHeader));
+
+    for (int i = 0, length = sections_.length();
+         i < length;
+         i++) {
+      sections_[i]->WriteBody(headers.at(i), w);
+    }
+  }
+
+  ZoneList<ELFSection*> sections_;
+};
+
+
+class ELFSymbol BASE_EMBEDDED {
+ public:
+  enum Type {
+    TYPE_NOTYPE = 0,
+    TYPE_OBJECT = 1,
+    TYPE_FUNC = 2,
+    TYPE_SECTION = 3,
+    TYPE_FILE = 4,
+    TYPE_LOPROC = 13,
+    TYPE_HIPROC = 15
+  };
+
+  enum Binding {
+    BIND_LOCAL = 0,
+    BIND_GLOBAL = 1,
+    BIND_WEAK = 2,
+    BIND_LOPROC = 13,
+    BIND_HIPROC = 15
+  };
+
+  ELFSymbol(const char* name,
+            uintptr_t value,
+            uintptr_t size,
+            Binding binding,
+            Type type,
+            uint16_t section)
+      : name(name),
+        value(value),
+        size(size),
+        info((binding << 4) | type),
+        other(0),
+        section(section) {
+  }
+
+  Binding binding() const {
+    return static_cast<Binding>(info >> 4);
+  }
+
+#if defined(V8_TARGET_ARCH_IA32)
+  struct SerializedLayout {
+    SerializedLayout(uint32_t name,
+                     uintptr_t value,
+                     uintptr_t size,
+                     Binding binding,
+                     Type type,
+                     uint16_t section)
+        : name(name),
+          value(value),
+          size(size),
+          info((binding << 4) | type),
+          other(0),
+          section(section) {
+    }
+
+    uint32_t name;
+    uintptr_t value;
+    uintptr_t size;
+    uint8_t info;
+    uint8_t other;
+    uint16_t section;
+  };
+#elif defined(V8_TARGET_ARCH_X64)
+  struct SerializedLayout {
+    SerializedLayout(uint32_t name,
+                     uintptr_t value,
+                     uintptr_t size,
+                     Binding binding,
+                     Type type,
+                     uint16_t section)
+        : name(name),
+          info((binding << 4) | type),
+          other(0),
+          section(section),
+          value(value),
+          size(size) {
+    }
+
+    uint32_t name;
+    uint8_t info;
+    uint8_t other;
+    uint16_t section;
+    uintptr_t value;
+    uintptr_t size;
+  };
+#endif
+
+  void Write(Writer::Slot<SerializedLayout> s, StringTable* t) {
+    // Convert symbol names from strings to indexes in the string table.
+    s->name = t->Add(name);
+    s->value = value;
+    s->size = size;
+    s->info = info;
+    s->other = other;
+    s->section = section;
+  }
+
+ private:
+  const char* name;
+  uintptr_t value;
+  uintptr_t size;
+  uint8_t info;
+  uint8_t other;
+  uint16_t section;
+};
+
+
+class ELFSymbolTable : public ELFSection {
+ public:
+  explicit ELFSymbolTable(const char* name)
+      : ELFSection(name, TYPE_SYMTAB, sizeof(uintptr_t)),
+        locals_(1),
+        globals_(1) {
+  }
+
+  virtual void WriteBody(Writer::Slot<Header> header, Writer* w) {
+    w->Align(header->alignment);
+    int total_symbols = locals_.length() + globals_.length() + 1;
+    header->offset = w->position();
+
+    Writer::Slot<ELFSymbol::SerializedLayout> symbols =
+        w->SlotsHere<ELFSymbol::SerializedLayout>(total_symbols);
+
+    header->size = w->position() - header->offset;
+
+    // String table for this symbol table should follow it in the section table.
+    StringTable* strtab =
+        static_cast<StringTable*>(w->elf()->SectionAt(index() + 1));
+    strtab->AttachWriter(w);
+    symbols.at(0).set(ELFSymbol::SerializedLayout(0,
+                                                  0,
+                                                  0,
+                                                  ELFSymbol::BIND_LOCAL,
+                                                  ELFSymbol::TYPE_NOTYPE,
+                                                  0));
+    WriteSymbolsList(&locals_, symbols.at(1), strtab);
+    WriteSymbolsList(&globals_, symbols.at(locals_.length() + 1), strtab);
+    strtab->DetachWriter();
+  }
+
+  void Add(const ELFSymbol& symbol) {
+    if (symbol.binding() == ELFSymbol::BIND_LOCAL) {
+      locals_.Add(symbol);
+    } else {
+      globals_.Add(symbol);
+    }
+  }
+
+ protected:
+  virtual void PopulateHeader(Writer::Slot<Header> header) {
+    ELFSection::PopulateHeader(header);
+    // We are assuming that string table will follow symbol table.
+    header->link = index() + 1;
+    header->info = locals_.length() + 1;
+    header->entry_size = sizeof(ELFSymbol::SerializedLayout);
+  }
+
+ private:
+  void WriteSymbolsList(const ZoneList<ELFSymbol>* src,
+                        Writer::Slot<ELFSymbol::SerializedLayout> dst,
+                        StringTable* strtab) {
+    for (int i = 0, len = src->length();
+         i < len;
+         i++) {
+      src->at(i).Write(dst.at(i), strtab);
+    }
+  }
+
+  ZoneList<ELFSymbol> locals_;
+  ZoneList<ELFSymbol> globals_;
+};
+
+
+class CodeDescription BASE_EMBEDDED {
+ public:
+  CodeDescription(const char* name,
+                  Code* code,
+                  Handle<Script> script,
+                  GDBJITLineInfo* lineinfo)
+      : name_(name), code_(code), script_(script), lineinfo_(lineinfo)
+  { }
+
+  const char* code_name() const {
+    return name_;
+  }
+
+  uintptr_t code_size() const {
+    return code_->instruction_end() - code_->instruction_start();
+  }
+
+  uintptr_t code_start() const {
+    return (uintptr_t)code_->instruction_start();
+  }
+
+  bool is_line_info_available() {
+    return !script_.is_null() &&
+        script_->source()->IsString() &&
+        script_->HasValidSource() &&
+        script_->name()->IsString() &&
+        lineinfo_ != NULL;
+  }
+
+  GDBJITLineInfo* lineinfo() const { return lineinfo_; }
+
+  SmartPointer<char> filename() {
+    return String::cast(script_->name())->ToCString();
+  }
+
+  int GetScriptLineNumber(int pos) {
+    return GetScriptLineNumberSafe(script_, pos) + 1;
+  }
+
+ private:
+  const char* name_;
+  Code* code_;
+  Handle<Script> script_;
+  GDBJITLineInfo* lineinfo_;
+};
+
+
+static void CreateSymbolsTable(CodeDescription* desc,
+                               ELF* elf,
+                               int text_section_index) {
+  ELFSymbolTable* symtab = new ELFSymbolTable(".symtab");
+  StringTable* strtab = new StringTable(".strtab");
+
+  // Symbol table should be followed by the linked string table.
+  elf->AddSection(symtab);
+  elf->AddSection(strtab);
+
+  symtab->Add(ELFSymbol("V8 Code",
+                        0,
+                        0,
+                        ELFSymbol::BIND_LOCAL,
+                        ELFSymbol::TYPE_FILE,
+                        ELFSection::INDEX_ABSOLUTE));
+
+  symtab->Add(ELFSymbol(desc->code_name(),
+                        0,
+                        desc->code_size(),
+                        ELFSymbol::BIND_GLOBAL,
+                        ELFSymbol::TYPE_FUNC,
+                        text_section_index));
+}
+
+
+class DebugInfoSection : public ELFSection {
+ public:
+  explicit DebugInfoSection(CodeDescription* desc)
+      : ELFSection(".debug_info", TYPE_PROGBITS, 1), desc_(desc) { }
+
+  bool WriteBody(Writer* w) {
+    Writer::Slot<uint32_t> size = w->SlotHere<uint32_t>();
+    uintptr_t start = w->position();
+    w->Write<uint16_t>(2);  // DWARF version.
+    w->Write<uint32_t>(0);  // Abbreviation table offset.
+    w->Write<uint8_t>(sizeof(intptr_t));
+
+    w->WriteULEB128(1);  // Abbreviation code.
+    w->WriteString(*desc_->filename());
+    w->Write<intptr_t>(desc_->code_start());
+    w->Write<intptr_t>(desc_->code_start() + desc_->code_size());
+    w->Write<uint32_t>(0);
+    size.set(static_cast<uint32_t>(w->position() - start));
+    return true;
+  }
+
+ private:
+  CodeDescription* desc_;
+};
+
+
+class DebugAbbrevSection : public ELFSection {
+ public:
+  DebugAbbrevSection() : ELFSection(".debug_abbrev", TYPE_PROGBITS, 1) { }
+
+  // DWARF2 standard, figure 14.
+  enum DWARF2Tags {
+    DW_TAG_COMPILE_UNIT = 0x11
+  };
+
+  // DWARF2 standard, figure 16.
+  enum DWARF2ChildrenDetermination {
+    DW_CHILDREN_NO = 0,
+    DW_CHILDREN_YES = 1
+  };
+
+  // DWARF standard, figure 17.
+  enum DWARF2Attribute {
+    DW_AT_NAME = 0x3,
+    DW_AT_STMT_LIST = 0x10,
+    DW_AT_LOW_PC = 0x11,
+    DW_AT_HIGH_PC = 0x12
+  };
+
+  // DWARF2 standard, figure 19.
+  enum DWARF2AttributeForm {
+    DW_FORM_ADDR = 0x1,
+    DW_FORM_STRING = 0x8,
+    DW_FORM_DATA4 = 0x6
+  };
+
+  bool WriteBody(Writer* w) {
+    w->WriteULEB128(1);
+    w->WriteULEB128(DW_TAG_COMPILE_UNIT);
+    w->Write<uint8_t>(DW_CHILDREN_NO);
+    w->WriteULEB128(DW_AT_NAME);
+    w->WriteULEB128(DW_FORM_STRING);
+    w->WriteULEB128(DW_AT_LOW_PC);
+    w->WriteULEB128(DW_FORM_ADDR);
+    w->WriteULEB128(DW_AT_HIGH_PC);
+    w->WriteULEB128(DW_FORM_ADDR);
+    w->WriteULEB128(DW_AT_STMT_LIST);
+    w->WriteULEB128(DW_FORM_DATA4);
+    w->WriteULEB128(0);
+    w->WriteULEB128(0);
+    w->WriteULEB128(0);
+    return true;
+  }
+};
+
+
+class DebugLineSection : public ELFSection {
+ public:
+  explicit DebugLineSection(CodeDescription* desc)
+      : ELFSection(".debug_line", TYPE_PROGBITS, 1),
+        desc_(desc) { }
+
+  // DWARF2 standard, figure 34.
+  enum DWARF2Opcodes {
+    DW_LNS_COPY = 1,
+    DW_LNS_ADVANCE_PC = 2,
+    DW_LNS_ADVANCE_LINE = 3,
+    DW_LNS_SET_FILE = 4,
+    DW_LNS_SET_COLUMN = 5,
+    DW_LNS_NEGATE_STMT = 6
+  };
+
+  // DWARF2 standard, figure 35.
+  enum DWARF2ExtendedOpcode {
+    DW_LNE_END_SEQUENCE = 1,
+    DW_LNE_SET_ADDRESS = 2,
+    DW_LNE_DEFINE_FILE = 3
+  };
+
+  bool WriteBody(Writer* w) {
+    // Write prologue.
+    Writer::Slot<uint32_t> total_length = w->SlotHere<uint32_t>();
+    uintptr_t start = w->position();
+
+    w->Write<uint16_t>(2);  // Field version.
+    Writer::Slot<uint32_t> prologue_length = w->SlotHere<uint32_t>();
+    uintptr_t prologue_start = w->position();
+    w->Write<uint8_t>(1);  // Field minimum_instruction_length.
+    w->Write<uint8_t>(1);  // Field default_is_stmt.
+    w->Write<int8_t>(0);  // Field line_base.
+    w->Write<uint8_t>(2);  // Field line_range.
+    w->Write<uint8_t>(DW_LNS_NEGATE_STMT + 1);  // Field opcode_base.
+    w->Write<uint8_t>(0);  // DW_LNS_COPY operands count.
+    w->Write<uint8_t>(1);  // DW_LNS_ADVANCE_PC operands count.
+    w->Write<uint8_t>(1);  // DW_LNS_ADVANCE_LINE operands count.
+    w->Write<uint8_t>(1);  // DW_LNS_SET_FILE operands count.
+    w->Write<uint8_t>(1);  // DW_LNS_SET_COLUMN operands count.
+    w->Write<uint8_t>(0);  // DW_LNS_NEGATE_STMT operands count.
+    w->Write<uint8_t>(0);  // Empty include_directories sequence.
+    w->WriteString(*desc_->filename());  // File name.
+    w->WriteULEB128(0);  // Current directory.
+    w->WriteULEB128(0);  // Unknown modification time.
+    w->WriteULEB128(0);  // Unknown file size.
+    w->Write<uint8_t>(0);
+    prologue_length.set(static_cast<uint32_t>(w->position() - prologue_start));
+
+    WriteExtendedOpcode(w, DW_LNE_SET_ADDRESS, sizeof(intptr_t));
+    w->Write<intptr_t>(desc_->code_start());
+
+    intptr_t pc = 0;
+    intptr_t line = 1;
+    bool is_statement = true;
+
+    List<GDBJITLineInfo::PCInfo>* pc_info = desc_->lineinfo()->pc_info();
+    for (int i = 0; i < pc_info->length(); i++) {
+      GDBJITLineInfo::PCInfo* info = &pc_info->at(i);
+      uintptr_t pc_diff = info->pc_ - pc;
+      ASSERT(info->pc_ >= pc);
+      if (pc_diff != 0) {
+        w->Write<uint8_t>(DW_LNS_ADVANCE_PC);
+        w->WriteSLEB128(pc_diff);
+        pc += pc_diff;
+      }
+      intptr_t line_diff = desc_->GetScriptLineNumber(info->pos_) - line;
+      if (line_diff != 0) {
+        w->Write<uint8_t>(DW_LNS_ADVANCE_LINE);
+        w->WriteSLEB128(line_diff);
+        line += line_diff;
+      }
+      if (is_statement != info->is_statement_) {
+        w->Write<uint8_t>(DW_LNS_NEGATE_STMT);
+        is_statement = !is_statement;

[Erik Corry, 2011/01/04 14:21:48]

How about is_statement = info->is_statement_;

[Vyacheslav Egorov (Chromium), 2011/01/18 16:10:42]

I want to reflect the meaning of an emitted operation: in this case emitted
operation is negation not an assignment.

+      }
+      if (pc_diff != 0 || i == 0) {
+        w->Write<uint8_t>(DW_LNS_COPY);
+      }
+    }
+    WriteExtendedOpcode(w, DW_LNE_END_SEQUENCE, 0);
+    total_length.set(static_cast<uint32_t>(w->position() - start));
+    return true;
+  }
+
+ private:
+  void WriteExtendedOpcode(Writer* w,
+                           DWARF2ExtendedOpcode op,
+                           size_t operands_size) {
+    w->Write<uint8_t>(0);
+    w->WriteULEB128(operands_size + 1);
+    w->Write<uint8_t>(op);
+  }
+
+  CodeDescription* desc_;
+};
+
+
+static void CreateDWARFSections(CodeDescription* desc, ELF* elf) {
+  if (desc->is_line_info_available()) {
+    elf->AddSection(new DebugInfoSection(desc));
+    elf->AddSection(new DebugAbbrevSection);
+    elf->AddSection(new DebugLineSection(desc));
+  }
+}
+
+
+// -------------------------------------------------------------------
+// GDB JIT Interface declarations copied as is from
+//   http://sourceware.org/gdb/onlinedocs/gdb/Declarations.html
+extern "C" {
+  typedef enum {
+    JIT_NOACTION = 0,
+    JIT_REGISTER_FN,
+    JIT_UNREGISTER_FN
+  } jit_actions_t;
+
+  struct jit_code_entry {
+    struct jit_code_entry *next_entry;
+    struct jit_code_entry *prev_entry;
+    char *symfile_addr;
+    uint64_t symfile_size;
+  };
+
+  struct jit_descriptor {
+    uint32_t version;
+    uint32_t action_flag;
+    struct jit_code_entry *relevant_entry;
+    struct jit_code_entry *first_entry;
+  };
+
+  void __attribute__((noinline)) __jit_debug_register_code() {
+    __asm__ ("");
+  }
+
+  struct jit_descriptor __jit_debug_descriptor = { 1, 0, 0, 0 };
+}
+
+
+static jit_code_entry* CreateCodeEntry(byte* symfile_addr,
+                                       uintptr_t symfile_size) {
+  jit_code_entry* entry = static_cast<jit_code_entry*>(
+      malloc(sizeof(jit_code_entry) + symfile_size));
+
+  entry->symfile_addr = reinterpret_cast<char*>(entry + 1);
+  entry->symfile_size = symfile_size;
+  memcpy(entry->symfile_addr, symfile_addr, symfile_size);
+
+  entry->prev_entry = entry->next_entry = NULL;
+
+  return entry;
+}
+
+
+static void DestroyCodeEntry(jit_code_entry* entry) {
+  free(entry);
+}
+
+
+static void RegisterCodeEntry(jit_code_entry* entry) {
+  entry->next_entry = __jit_debug_descriptor.first_entry;
+  if (entry->next_entry != NULL) entry->next_entry->prev_entry = entry;
+  __jit_debug_descriptor.first_entry =
+      __jit_debug_descriptor.relevant_entry = entry;
+
+  __jit_debug_descriptor.action_flag = JIT_REGISTER_FN;
+  __jit_debug_register_code();
+}
+
+
+static void UnregisterCodeEntry(jit_code_entry* entry) {
+  if (entry->prev_entry != NULL) {
+    entry->prev_entry->next_entry = entry->next_entry;
+  } else {
+    __jit_debug_descriptor.first_entry = entry->next_entry;
+  }
+
+  if (entry->next_entry != NULL) {
+    entry->next_entry->prev_entry = entry->prev_entry;
+  }
+
+  __jit_debug_descriptor.relevant_entry = entry;
+  __jit_debug_descriptor.action_flag = JIT_UNREGISTER_FN;
+  __jit_debug_register_code();
+}
+
+
+static jit_code_entry* CreateELFObject(CodeDescription* desc) {
+  ZoneScope zone_scope(DELETE_ON_EXIT);
+
+  ELF elf;
+  Writer w(&elf);
+
+  int text_section_index = elf.AddSection(
+      new FullHeaderELFSection(".text",
+                               ELFSection::TYPE_NOBITS,
+                               kCodeAlignment,
+                               desc->code_start(),
+                               0,
+                               desc->code_size(),
+                               ELFSection::FLAG_ALLOC | ELFSection::FLAG_EXEC));
+
+  CreateSymbolsTable(desc, &elf, text_section_index);
+
+  CreateDWARFSections(desc, &elf);
+
+  elf.Write(&w);
+
+  return CreateCodeEntry(w.buffer(), w.position());
+}
+
+
+static bool SameCodeObjects(void* key1, void* key2) {
+  return key1 == key2;
+}
+
+
+static HashMap entries(&SameCodeObjects);
+
+
+static uint32_t HashForCodeObject(Code* code) {
+  static const uintptr_t kGoldenRatio = 2654435761u;
+  uintptr_t hash = reinterpret_cast<uintptr_t>(code->address());
+  return static_cast<uint32_t>((hash >> kCodeAlignmentBits) * kGoldenRatio);
+}
+
+
+static const intptr_t kLineInfoTag = 0x1;
+
+
+static bool IsLineInfoTagged(void* ptr) {
+  return 0 != (reinterpret_cast<intptr_t>(ptr) & kLineInfoTag);
+}
+
+
+static void* TagLineInfo(GDBJITLineInfo* ptr) {
+  return reinterpret_cast<void*>(
+      reinterpret_cast<intptr_t>(ptr) | kLineInfoTag);
+}
+
+
+static GDBJITLineInfo* UntagLineInfo(void* ptr) {
+  return reinterpret_cast<GDBJITLineInfo*>(
+      reinterpret_cast<intptr_t>(ptr) & ~kLineInfoTag);
+}
+
+
+void GDBJITInterface::AddCode(Handle<String> name,
+                              Handle<Script> script,
+                              Handle<Code> code) {
+  if (!FLAG_gdbjit) return;
+
+  // Force initialization of line_ends array.
+  GetScriptLineNumber(script, 0);
+
+  if (!name.is_null()) {
+    SmartPointer<char> name_cstring = name->ToCString(DISALLOW_NULLS);
+    AddCode(*name_cstring, *code, *script);
+  } else {
+    AddCode("", *code, *script);
+  }
+}
+
+
+void GDBJITInterface::AddCode(const char* name,
+                              Code* code,
+                              Script* script) {
+  if (!FLAG_gdbjit) return;
+  AssertNoAllocation no_gc;
+
+  HashMap::Entry* e = entries.Lookup(code, HashForCodeObject(code), true);
+  if (e->value != NULL && !IsLineInfoTagged(e->value)) return;
+
+  GDBJITLineInfo* lineinfo = UntagLineInfo(e->value);
+  CodeDescription code_desc(name,
+                            code,
+                            script != NULL ? Handle<Script>(script)
+                                           : Handle<Script>(),
+                            lineinfo);
+
+  if (!FLAG_gdbjit_full && !code_desc.is_line_info_available()) {
+    delete lineinfo;
+    entries.Remove(code, HashForCodeObject(code));
+    return;
+  }
+
+  jit_code_entry* entry = CreateELFObject(&code_desc);
+  ASSERT(!IsLineInfoTagged(entry));
+
+  delete lineinfo;
+  e->value = entry;
+
+  RegisterCodeEntry(entry);
+}
+
+
+void GDBJITInterface::AddCode(GDBJITInterface::CodeTag tag,
+                              const char* name,
+                              Code* code) {
+  if (!FLAG_gdbjit) return;
+
+  EmbeddedVector<char, 256> buffer;
+  StringBuilder builder(buffer.start(), buffer.length());
+
+  builder.AddString(Tag2String(tag));
+  if ((name != NULL) && (*name != '\0')) {
+    builder.AddString(": ");
+    builder.AddString(name);
+  } else {
+    builder.AddFormatted(": code object %p", static_cast<void*>(code));
+  }
+
+  AddCode(builder.Finalize(), code);
+}
+
+
+void GDBJITInterface::AddCode(GDBJITInterface::CodeTag tag,
+                              String* name,
+                              Code* code) {
+  if (!FLAG_gdbjit) return;
+  AddCode(tag, name != NULL ? *name->ToCString(DISALLOW_NULLS) : NULL, code);
+}
+
+
+void GDBJITInterface::AddCode(GDBJITInterface::CodeTag tag, Code* code) {
+  if (!FLAG_gdbjit) return;
+
+  AddCode(tag, "", code);
+}
+
+
+void GDBJITInterface::RemoveCode(Code* code) {
+  if (!FLAG_gdbjit) return;
+
+  HashMap::Entry* e = entries.Lookup(code, HashForCodeObject(code), false);
+  if (e == NULL) return;
+
+  if (IsLineInfoTagged(e->value)) {
+    delete UntagLineInfo(e->value);
+  } else {
+    jit_code_entry* entry = static_cast<jit_code_entry*>(e->value);
+    UnregisterCodeEntry(entry);
+    DestroyCodeEntry(entry);
+  }
+  e->value = NULL;
+  entries.Remove(code, HashForCodeObject(code));
+}
+
+
+void GDBJITInterface::RegisterDetailedLineInfo(Code* code,
+                                               GDBJITLineInfo* line_info) {
+  ASSERT(!IsLineInfoTagged(line_info));
+  HashMap::Entry* e = entries.Lookup(code, HashForCodeObject(code), true);
+  ASSERT(e->value == NULL);
+  e->value = TagLineInfo(line_info);
+}
+
+
+} }  // namespace v8::internal
+#endif