Reduce the size of native debug info generated by JIT.

Remove some ELF file overheads:
 - Do not produce program headers.
 - Do not page align strtab.
 - Do not write oat_patches sections.

This more than halves the size of JIT native debug info.
Since we generate many small entries, the overheads added up.

Change-Id: I27d95548c61e2e38c3683d6f5eb870a2db6e812d

4 files changed, 94 insertions, 42 deletions

diff --git a/compiler/elf_builder.h b/compiler/elf_builder.h
index 3d24d19919..bc7c83ed7d 100644
--- a/compiler/elf_builder.h
+++ b/compiler/elf_builder.h
@@ -110,18 +110,27 @@ class ElfBuilder FINAL {
       CHECK(sections.empty() || sections.back()->finished_);
       // The first ELF section index is 1. Index 0 is reserved for NULL.
       section_index_ = sections.size() + 1;
-      // Push this section on the list of written sections.
-      sections.push_back(this);
+      // Page-align if we switch between allocated and non-allocated sections,
+      // or if we change the type of allocation (e.g. executable vs non-executable).
+      if (!sections.empty()) {
+        if (header_.sh_flags != sections.back()->header_.sh_flags) {
+          header_.sh_addralign = kPageSize;
+        }
+      }
       // Align file position.
       if (header_.sh_type != SHT_NOBITS) {
-        header_.sh_offset = RoundUp(owner_->stream_.Seek(0, kSeekCurrent), header_.sh_addralign);
-        owner_->stream_.Seek(header_.sh_offset, kSeekSet);
+        header_.sh_offset = owner_->AlignFileOffset(header_.sh_addralign);
+      } else {
+        header_.sh_offset = 0;
       }
       // Align virtual memory address.
       if ((header_.sh_flags & SHF_ALLOC) != 0) {
-        header_.sh_addr = RoundUp(owner_->virtual_address_, header_.sh_addralign);
-        owner_->virtual_address_ = header_.sh_addr;
+        header_.sh_addr = owner_->AlignVirtualAddress(header_.sh_addralign);
+      } else {
+        header_.sh_addr = 0;
       }
+      // Push this section on the list of written sections.
+      sections.push_back(this);
     }
 
     // Finish writing of this section.
@@ -170,8 +179,8 @@ class ElfBuilder FINAL {
     // and it will be zero-initialized when the ELF file is loaded in the running program.
     void WriteNoBitsSection(Elf_Word size) {
       DCHECK_NE(header_.sh_flags & SHF_ALLOC, 0u);
-      Start();
       header_.sh_type = SHT_NOBITS;
+      Start();
       header_.sh_size = size;
       End();
     }
@@ -293,12 +302,13 @@ class ElfBuilder FINAL {
         dynamic_(this, ".dynamic", SHT_DYNAMIC, SHF_ALLOC, &dynstr_, 0, kPageSize, sizeof(Elf_Dyn)),
         eh_frame_(this, ".eh_frame", SHT_PROGBITS, SHF_ALLOC, nullptr, 0, kPageSize, 0),
         eh_frame_hdr_(this, ".eh_frame_hdr", SHT_PROGBITS, SHF_ALLOC, nullptr, 0, 4, 0),
-        strtab_(this, ".strtab", 0, kPageSize),
+        strtab_(this, ".strtab", 0, 1),
         symtab_(this, ".symtab", SHT_SYMTAB, 0, &strtab_),
         debug_frame_(this, ".debug_frame", SHT_PROGBITS, 0, nullptr, 0, sizeof(Elf_Addr), 0),
         debug_info_(this, ".debug_info", SHT_PROGBITS, 0, nullptr, 0, 1, 0),
         debug_line_(this, ".debug_line", SHT_PROGBITS, 0, nullptr, 0, 1, 0),
         shstrtab_(this, ".shstrtab", 0, 1),
+        started_(false),
         virtual_address_(0) {
     text_.phdr_flags_ = PF_R | PF_X;
     bss_.phdr_flags_ = PF_R | PF_W;
@@ -357,16 +367,25 @@ class ElfBuilder FINAL {
     virtual_address_ = address;
   }
 
-  void Start() {
-    // Reserve space for ELF header and program headers.
-    // We do not know the number of headers until later, so
-    // it is easiest to just reserve a fixed amount of space.
-    int size = sizeof(Elf_Ehdr) + sizeof(Elf_Phdr) * kMaxProgramHeaders;
+  // Reserve space for ELF header and program headers.
+  // We do not know the number of headers until later, so
+  // it is easiest to just reserve a fixed amount of space.
+  // Program headers are required for loading by the linker.
+  // It is possible to omit them for ELF files used for debugging.
+  void Start(bool write_program_headers = true) {
+    int size = sizeof(Elf_Ehdr);
+    if (write_program_headers) {
+      size += sizeof(Elf_Phdr) * kMaxProgramHeaders;
+    }
     stream_.Seek(size, kSeekSet);
+    started_ = true;
     virtual_address_ += size;
+    write_program_headers_ = write_program_headers;
   }
 
   void End() {
+    DCHECK(started_);
+
     // Write section names and finish the section headers.
     shstrtab_.Start();
     shstrtab_.Write("");
@@ -386,8 +405,7 @@ class ElfBuilder FINAL {
       shdrs.push_back(section->header_);
     }
     Elf_Off section_headers_offset;
-    section_headers_offset = RoundUp(stream_.Seek(0, kSeekCurrent), sizeof(Elf_Off));
-    stream_.Seek(section_headers_offset, kSeekSet);
+    section_headers_offset = AlignFileOffset(sizeof(Elf_Off));
     stream_.WriteFully(shdrs.data(), shdrs.size() * sizeof(shdrs[0]));
 
     // Flush everything else before writing the program headers. This should prevent
@@ -395,14 +413,21 @@ class ElfBuilder FINAL {
     // and partially written data if we suddenly lose power, for example.
     stream_.Flush();
 
-    // Write the initial file headers.
-    std::vector<Elf_Phdr> phdrs = MakeProgramHeaders();
+    // The main ELF header.
     Elf_Ehdr elf_header = MakeElfHeader(isa_);
-    elf_header.e_phoff = sizeof(Elf_Ehdr);
     elf_header.e_shoff = section_headers_offset;
-    elf_header.e_phnum = phdrs.size();
     elf_header.e_shnum = shdrs.size();
     elf_header.e_shstrndx = shstrtab_.GetSectionIndex();
+
+    // Program headers (i.e. mmap instructions).
+    std::vector<Elf_Phdr> phdrs;
+    if (write_program_headers_) {
+      phdrs = MakeProgramHeaders();
+      CHECK_LE(phdrs.size(), kMaxProgramHeaders);
+      elf_header.e_phoff = sizeof(Elf_Ehdr);
+      elf_header.e_phnum = phdrs.size();
+    }
+
     stream_.Seek(0, kSeekSet);
     stream_.WriteFully(&elf_header, sizeof(elf_header));
     stream_.WriteFully(phdrs.data(), phdrs.size() * sizeof(phdrs[0]));
@@ -492,6 +517,14 @@ class ElfBuilder FINAL {
     return &stream_;
   }
 
+  off_t AlignFileOffset(size_t alignment) {
+     return stream_.Seek(RoundUp(stream_.Seek(0, kSeekCurrent), alignment), kSeekSet);
+  }
+
+  Elf_Addr AlignVirtualAddress(size_t alignment) {
+     return virtual_address_ = RoundUp(virtual_address_, alignment);
+  }
+
  private:
   static Elf_Ehdr MakeElfHeader(InstructionSet isa) {
     Elf_Ehdr elf_header = Elf_Ehdr();
@@ -666,9 +699,13 @@ class ElfBuilder FINAL {
   // List of used section in the order in which they were written.
   std::vector<Section*> sections_;
 
+  bool started_;
+
   // Used for allocation of virtual address space.
   Elf_Addr virtual_address_;
 
+  size_t write_program_headers_;
+
   DISALLOW_COPY_AND_ASSIGN(ElfBuilder);
 };
 
diff --git a/compiler/elf_writer_debug.cc b/compiler/elf_writer_debug.cc
index d1f50073a0..ca8cd68b33 100644
--- a/compiler/elf_writer_debug.cc
+++ b/compiler/elf_writer_debug.cc
@@ -1148,13 +1148,19 @@ class DebugInfoWriter {
     writer.Write(types);
   }
 
-  void End() {
+  void End(bool write_oat_patches) {
     builder_->GetDebugInfo()->End();
-    builder_->WritePatches(".debug_info.oat_patches",
-                           ArrayRef<const uintptr_t>(debug_info_patches_));
+    if (write_oat_patches) {
+      builder_->WritePatches(".debug_info.oat_patches",
+                             ArrayRef<const uintptr_t>(debug_info_patches_));
+    }
     builder_->WriteSection(".debug_abbrev", &debug_abbrev_buffer_);
-    builder_->WriteSection(".debug_loc", &debug_loc_);
-    builder_->WriteSection(".debug_ranges", &debug_ranges_);
+    if (!debug_loc_.empty()) {
+      builder_->WriteSection(".debug_loc", &debug_loc_);
+    }
+    if (!debug_ranges_.empty()) {
+      builder_->WriteSection(".debug_ranges", &debug_ranges_);
+    }
   }
 
  private:
@@ -1357,10 +1363,12 @@ class DebugLineWriter {
     return buffer.size();
   }
 
-  void End() {
+  void End(bool write_oat_patches) {
     builder_->GetDebugLine()->End();
-    builder_->WritePatches(".debug_line.oat_patches",
-                           ArrayRef<const uintptr_t>(debug_line_patches));
+    if (write_oat_patches) {
+      builder_->WritePatches(".debug_line.oat_patches",
+                             ArrayRef<const uintptr_t>(debug_line_patches));
+    }
   }
 
  private:
@@ -1370,7 +1378,8 @@ class DebugLineWriter {
 
 template<typename ElfTypes>
 static void WriteDebugSections(ElfBuilder<ElfTypes>* builder,
-                               const ArrayRef<const MethodDebugInfo>& method_infos) {
+                               const ArrayRef<const MethodDebugInfo>& method_infos,
+                               bool write_oat_patches) {
   // Group the methods into compilation units based on source file.
   std::vector<CompilationUnit> compilation_units;
   const char* last_source_file = nullptr;
@@ -1394,7 +1403,7 @@ static void WriteDebugSections(ElfBuilder<ElfTypes>* builder,
     for (auto& compilation_unit : compilation_units) {
       line_writer.WriteCompilationUnit(compilation_unit);
     }
-    line_writer.End();
+    line_writer.End(write_oat_patches);
   }
 
   // Write .debug_info section.
@@ -1404,7 +1413,7 @@ static void WriteDebugSections(ElfBuilder<ElfTypes>* builder,
     for (const auto& compilation_unit : compilation_units) {
       info_writer.WriteCompilationUnit(compilation_unit);
     }
-    info_writer.End();
+    info_writer.End(write_oat_patches);
   }
 }
 
@@ -1484,13 +1493,14 @@ static void WriteDebugSymbols(ElfBuilder<ElfTypes>* builder,
 template <typename ElfTypes>
 void WriteDebugInfo(ElfBuilder<ElfTypes>* builder,
                     const ArrayRef<const MethodDebugInfo>& method_infos,
-                    CFIFormat cfi_format) {
+                    CFIFormat cfi_format,
+                    bool write_oat_patches) {
   // Add methods to .symtab.
   WriteDebugSymbols(builder, method_infos, true /* with_signature */);
   // Generate CFI (stack unwinding information).
-  WriteCFISection(builder, method_infos, cfi_format, true /* write_oat_patches */);
+  WriteCFISection(builder, method_infos, cfi_format, write_oat_patches);
   // Write DWARF .debug_* sections.
-  WriteDebugSections(builder, method_infos);
+  WriteDebugSections(builder, method_infos, write_oat_patches);
 }
 
 static void XzCompress(const std::vector<uint8_t>* src, std::vector<uint8_t>* dst) {
@@ -1571,10 +1581,12 @@ static ArrayRef<const uint8_t> WriteDebugElfFileForMethodInternal(
   buffer.reserve(KB);
   VectorOutputStream out("Debug ELF file", &buffer);
   std::unique_ptr<ElfBuilder<ElfTypes>> builder(new ElfBuilder<ElfTypes>(isa, &out));
-  builder->Start();
+  // No program headers since the ELF file is not linked and has no allocated sections.
+  builder->Start(false /* write_program_headers */);
   WriteDebugInfo(builder.get(),
                  ArrayRef<const MethodDebugInfo>(&method_info, 1),
-                 DW_DEBUG_FRAME_FORMAT);
+                 DW_DEBUG_FRAME_FORMAT,
+                 false /* write_oat_patches */);
   builder->End();
   CHECK(builder->Good());
   // Make a copy of the buffer.  We want to shrink it anyway.
@@ -1601,12 +1613,12 @@ static ArrayRef<const uint8_t> WriteDebugElfFileForClassesInternal(
   buffer.reserve(KB);
   VectorOutputStream out("Debug ELF file", &buffer);
   std::unique_ptr<ElfBuilder<ElfTypes>> builder(new ElfBuilder<ElfTypes>(isa, &out));
-  builder->Start();
-
+  // No program headers since the ELF file is not linked and has no allocated sections.
+  builder->Start(false /* write_program_headers */);
   DebugInfoWriter<ElfTypes> info_writer(builder.get());
   info_writer.Start();
   info_writer.WriteTypes(types);
-  info_writer.End();
+  info_writer.End(false /* write_oat_patches */);
 
   builder->End();
   CHECK(builder->Good());
@@ -1630,11 +1642,13 @@ ArrayRef<const uint8_t> WriteDebugElfFileForClasses(const InstructionSet isa,
 template void WriteDebugInfo<ElfTypes32>(
     ElfBuilder<ElfTypes32>* builder,
     const ArrayRef<const MethodDebugInfo>& method_infos,
-    CFIFormat cfi_format);
+    CFIFormat cfi_format,
+    bool write_oat_patches);
 template void WriteDebugInfo<ElfTypes64>(
     ElfBuilder<ElfTypes64>* builder,
     const ArrayRef<const MethodDebugInfo>& method_infos,
-    CFIFormat cfi_format);
+    CFIFormat cfi_format,
+    bool write_oat_patches);
 template void WriteMiniDebugInfo<ElfTypes32>(
     ElfBuilder<ElfTypes32>* builder,
     const ArrayRef<const MethodDebugInfo>& method_infos);
diff --git a/compiler/elf_writer_debug.h b/compiler/elf_writer_debug.h
index e19da088da..8e8472f678 100644
--- a/compiler/elf_writer_debug.h
+++ b/compiler/elf_writer_debug.h
@@ -33,7 +33,8 @@ struct MethodDebugInfo;
 template <typename ElfTypes>
 void WriteDebugInfo(ElfBuilder<ElfTypes>* builder,
                     const ArrayRef<const MethodDebugInfo>& method_infos,
-                    CFIFormat cfi_format);
+                    CFIFormat cfi_format,
+                    bool write_oat_patches);
 
 template <typename ElfTypes>
 void WriteMiniDebugInfo(ElfBuilder<ElfTypes>* builder,
diff --git a/compiler/elf_writer_quick.cc b/compiler/elf_writer_quick.cc
index 6bf080a083..f2a95f2396 100644
--- a/compiler/elf_writer_quick.cc
+++ b/compiler/elf_writer_quick.cc
@@ -151,7 +151,7 @@ void ElfWriterQuick<ElfTypes>::WriteDebugInfo(
     const ArrayRef<const dwarf::MethodDebugInfo>& method_infos) {
   if (compiler_options_->GetGenerateDebugInfo()) {
     // Generate all the debug information we can.
-    dwarf::WriteDebugInfo(builder_.get(), method_infos, kCFIFormat);
+    dwarf::WriteDebugInfo(builder_.get(), method_infos, kCFIFormat, true /* write_oat_patches */);
   }
   if (compiler_options_->GetGenerateMiniDebugInfo()) {
     // Generate only some information and compress it.