Hook up emitting the Mach-O format with the expectation of getting put into a dylib.

Author: jkoritzinsky

src/coreclr/tools/Common/Compiler/ObjectWriter/MachObjectWriter.cs

@@ -72,8 +72,8 @@ internal sealed partial class MachObjectWriter : UnixObjectWriter
         /// </summary>
         private readonly string _baseSymbolName;
 
-        public MachObjectWriter(NodeFactory factory, ObjectWritingOptions options)
-            : base(factory, options)
+        public MachObjectWriter(NodeFactory factory, ObjectWritingOptions options, OutputInfoBuilder outputInfoBuilder = null)
+            : base(factory, options, outputInfoBuilder)
         {
             switch (factory.Target.Architecture)
             {
@@ -98,8 +98,8 @@ public MachObjectWriter(NodeFactory factory, ObjectWritingOptions options)
             _targetOS = factory.Target.OperatingSystem;
         }
 
-        public MachObjectWriter(NodeFactory factory, ObjectWritingOptions options, string baseSymbolName)
-            : this(factory, options)
+        public MachObjectWriter(NodeFactory factory, ObjectWritingOptions options, OutputInfoBuilder outputInfoBuilder, string baseSymbolName)
+            : this(factory, options, outputInfoBuilder)
         {
             _baseSymbolName = baseSymbolName;
         }

src/coreclr/tools/Common/Compiler/ObjectWriter/UnixObjectWriter.cs

@@ -25,8 +25,8 @@ private sealed record UnixSectionDefinition(string SymbolName, Stream SectionStr
         private static readonly ObjectNodeSection LsdaSection = new ObjectNodeSection(".dotnet_eh_table", SectionType.ReadOnly);
         private static readonly ObjectNodeSection EhFrameSection = new ObjectNodeSection(".eh_frame", SectionType.UnwindData);
 
-        protected UnixObjectWriter(NodeFactory factory, ObjectWritingOptions options)
-            : base(factory, options)
+        protected UnixObjectWriter(NodeFactory factory, ObjectWritingOptions options, OutputInfoBuilder outputInfoBuilder = null)
+            : base(factory, options, outputInfoBuilder)
         {
         }
 

src/coreclr/tools/aot/ILCompiler.ReadyToRun/CodeGen/ReadyToRunContainerFormat.cs

@@ -9,6 +9,7 @@ namespace ILCompiler.DependencyAnalysis
 {
     public enum ReadyToRunContainerFormat
     {
-        PE
+        PE,
+        MachO,
     }
 }

src/coreclr/tools/aot/ILCompiler.ReadyToRun/CodeGen/ReadyToRunObjectWriter.cs

@@ -193,11 +193,28 @@ public void EmitReadyToRunObjects(ReadyToRunContainerFormat format, Logger logge
                     timeDateStamp = inputPeReader.PEHeaders.CoffHeader.TimeDateStamp;
                 }
 
-                PEObjectWriter objectWriter = new(_nodeFactory, ObjectWritingOptions.None, _outputInfoBuilder, _objectFilePath, _customPESectionAlignment, timeDateStamp);
+                ObjectWriter.ObjectWriter objectWriter;
 
-                if (_nodeFactory.CompilationModuleGroup.IsCompositeBuildMode && _componentModule == null)
+                if (format == ReadyToRunContainerFormat.PE)
+                {
+                    PEObjectWriter peWriter = new(_nodeFactory, ObjectWritingOptions.None, _outputInfoBuilder, _objectFilePath, _customPESectionAlignment, timeDateStamp);
+
+                    if (_nodeFactory.CompilationModuleGroup.IsCompositeBuildMode && _componentModule == null)
+                    {
+                        peWriter.AddExportedSymbol("RTR_HEADER");
+                    }
+                    objectWriter = peWriter;
+                }
+                else if (format == ReadyToRunContainerFormat.MachO)
+                {
+                    // Use the base symbol name emitted into .dylib files,
+                    // with the expectation that the R2R object will be linked into
+                    // a dylib.
+                    objectWriter = new MachObjectWriter(_nodeFactory, ObjectWritingOptions.None, _outputInfoBuilder, baseSymbolName: "__mh_dylib_header");
+                }
+                else
                 {
-                    objectWriter.AddExportedSymbol("RTR_HEADER");
+                    throw new UnreachableException();
                 }
 
                 using FileStream stream = new FileStream(_objectFilePath, FileMode.Create);

src/coreclr/tools/aot/crossgen2/Crossgen2RootCommand.cs

@@ -415,6 +415,7 @@ private static ReadyToRunContainerFormat MakeOutputFormat(ArgumentResult result)
             return result.Tokens[0].Value.ToLowerInvariant() switch
             {
                 "pe" => ReadyToRunContainerFormat.PE,
+                "macho" => ReadyToRunContainerFormat.MachO,
                 _ => throw new CommandLineException(SR.InvalidOutputFormat)
             };
         }