ObjWriter in C# (#95876)

This is reimplementation of NativeAOT ObjWriter in pure C# instead of depending on LLVM. It implements Mach-O, ELF, COFF object file emitter with DWARF and CodeView debugging information. Only x64 and arm64 targets are implemented to cover officially supported platforms (win-x86 code is present and lightly tested; linux-x86 code is present and incomplete, only serves as a test bed for emitting 32-bit ELF files if we ever need that).

Original object writer code is still present and can be used by setting the `DOTNET_USE_LLVM_OBJWRITER=1` environment variable.

**Thanks to @am11 for helping with testing and debugging this, @xoofx for making LibObjectFile which helped kickstart this project, @PaulusParssinen for tips about branchless U/SLEB128 size calculation, and all the people on the .NET team who helped push this forward!**

Fixes #77178

src/coreclr/tools/Common/Compiler/DependencyAnalysis/ObjectNodeSection.cs

@@ -9,6 +9,7 @@ public enum SectionType
  Writeable,
  Executable,
  Uninitialized,
+ Debug,
  }
 
  /// <summary>

src/coreclr/tools/Common/Compiler/DependencyAnalysis/Relocation.cs

@@ -24,6 +24,7 @@ public enum RelocType
  // This is a special NGEN-specific relocation type
  // for relative pointer (used to make NGen relocation
  // section smaller)
+ IMAGE_REL_SECTION = 0x79, // 16 bit section index containing target
 
  IMAGE_REL_BASED_ARM64_PAGEBASE_REL21 = 0x81, // ADRP
  IMAGE_REL_BASED_ARM64_PAGEOFFSET_12A = 0x82, // ADD/ADDS (immediate) with zero shift, for page offset
@@ -465,9 +466,13 @@ public static unsafe void WriteValue(RelocType relocType, void* location, long v
  switch (relocType)
  {
  case RelocType.IMAGE_REL_BASED_ABSOLUTE:
+ case RelocType.IMAGE_REL_BASED_ADDR32NB:
  case RelocType.IMAGE_REL_BASED_HIGHLOW:
  case RelocType.IMAGE_REL_BASED_REL32:
- case RelocType.IMAGE_REL_BASED_ADDR32NB:
+ case RelocType.IMAGE_REL_BASED_RELPTR32:
+ case RelocType.IMAGE_REL_SECREL:
+ case RelocType.IMAGE_REL_TLSGD:
+ case RelocType.IMAGE_REL_TPOFF:
  case RelocType.IMAGE_REL_SYMBOL_SIZE:
  case RelocType.IMAGE_REL_FILE_ABSOLUTE:
  *(int*)location = (int)value;

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/ObjectWriter/CodeView/CodeViewFileTableBuilder.cs

@@ -0,0 +1,70 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Buffers;
+using System.Buffers.Binary;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.IO;
+using System.Text;
+
+using static ILCompiler.ObjectWriter.CodeViewNative;
+
+namespace ILCompiler.ObjectWriter
+{
+ internal sealed class CodeViewFileTableBuilder
+ {
+ private readonly MemoryStream _stringTableWriter = new();
+ private readonly MemoryStream _fileTableWriter = new();
+ private readonly Dictionary<string, uint> _fileNameToIndex = new();
+
+ public CodeViewFileTableBuilder()
+ {
+ // Insert empty entry at the beginning of string table
+ _stringTableWriter.Write(stackalloc byte[2] { 0, 0 });
+ }
+
+ public uint GetFileIndex(string fileName)
+ {
+ if (fileName == "")
+ {
+ // Match the placeholder value from LLVM. We need to use a non-empty
+ // string to ensure that the null terminator of the UTF-8 representation
+ // is not treated as the terminator of the whole file name table.
+ fileName = "<stdin>";
+ }
+
+ if (_fileNameToIndex.TryGetValue(fileName, out uint fileIndex))
+ {
+ return fileIndex;
+ }
+ else
+ {
+ uint stringTableIndex = (uint)_stringTableWriter.Position;
+ _stringTableWriter.Write(Encoding.UTF8.GetBytes(fileName));
+ _stringTableWriter.WriteByte(0);
+
+ uint fileTableIndex = (uint)_fileTableWriter.Position;
+ Span<byte> fileTableEntry = stackalloc byte[sizeof(uint) + sizeof(uint)];
+ BinaryPrimitives.WriteUInt32LittleEndian(fileTableEntry, stringTableIndex);
+ BinaryPrimitives.WriteUInt32LittleEndian(fileTableEntry.Slice(4), 0);
+ _fileTableWriter.Write(fileTableEntry);
+
+ _fileNameToIndex.Add(fileName, fileTableIndex);
+
+ return fileTableIndex;
+ }
+ }
+
+ public void Write(SectionWriter sectionWriter)
+ {
+ sectionWriter.WriteLittleEndian<uint>((uint)DebugSymbolsSubsectionType.FileChecksums);
+ sectionWriter.WriteLittleEndian<uint>((uint)_fileTableWriter.Length);
+ sectionWriter.EmitData(_fileTableWriter.GetBuffer().AsMemory(0, (int)_fileTableWriter.Length));
+ sectionWriter.WriteLittleEndian<uint>((uint)DebugSymbolsSubsectionType.StringTable);
+ sectionWriter.WriteLittleEndian<uint>((uint)_stringTableWriter.Length);
+ sectionWriter.EmitData(_stringTableWriter.GetBuffer().AsMemory(0, (int)_stringTableWriter.Length));
+ }
+ }
+}