Reduce the number of sections in R2R PE files

docs/design/coreclr/botr/readytorun-platform-native-envelope.md

@@ -13,9 +13,11 @@ The tentative high-level design is outlined below. As we implement this support,
 ## crossgen2: producing Mach-O object files
 
 Mach‑O support will only be supported for composite ReadyToRun when the target OS is macOS. It will be opt-in via a new `crossgen2` flag:
+
 - `--obj-format macho`
 
 `crossgen2` will:
+
 - Produce a Mach-O object file as the composite R2R image with the `RTR_HEADER` export for the `READYTORUN_HEADER`.
 - Mark each input IL assembly as a component R2R assembly: `READYTORUN_FLAG_COMPONENT`.
 - Mark each input IL assembly with a new flag indicating that the associated composite image is in the platform-native format: `READYTORUN_FLAG_PLATFORM_NATIVE_IMAGE`
@@ -28,20 +30,14 @@ There's a few cases in the R2R format that are not natively represented in the M
 
 #### Sections
 
-Sections folded into `__TEXT,__text` that is in other sections in the PE envelope:
-
-- CLR metadata: In the PE format, put in .cormeta, corresponds to the PE Header's "COR Header directory"
-- Win32 Resources: In the PE format, put in .rsrc, corresponds to the PE Header's "Win32 Resources Header directory"
-- Managed Unwind Info: In the Mach-O format, this section is expected to be in the Mach-O unwind format. The unwind info used by the runtime must be in the Windows unwind format.
-- GC Info: Entries correspond to the unwind info.
 
 Data moved out of `__TEXT,__text`:
 
 - Precompiled managed code has been moved into `__TEXT,__managedcode`. `__TEXT,__text` gets special treatment by the linker and `__TEXT,__managedcode` matches NativeAOT.
+- Read-only data such as jump tables, CLR metadata, Win32 Resources, managed unwind info, gc info, and the R2R headers are moved to `__TEXT,__const`
 
 Data that stays in the corresponding locations as the PE envelope:
 
-- Read-only data such as jump tables and the R2R headers: `__TEXT,__const`
 - Read-write data, such as fixup tables: `__DATA,__data`
 - Import thunks: `__TEXT,__text`
 

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

@@ -32,7 +32,6 @@ public ObjectNodeSection(string name, SectionType type, string comdatName)
         public ObjectNodeSection(string name, SectionType type) : this(name, type, null)
         { }
 
-        public static readonly ObjectNodeSection XDataSection = new ObjectNodeSection("xdata", SectionType.ReadOnly);
         public static readonly ObjectNodeSection DataSection = new ObjectNodeSection("data", SectionType.Writeable);
         public static readonly ObjectNodeSection ReadOnlyDataSection = new ObjectNodeSection("rdata", SectionType.ReadOnly);
         public static readonly ObjectNodeSection FoldableReadOnlyDataSection = new ObjectNodeSection("rdata", SectionType.ReadOnly);
@@ -51,10 +50,5 @@ public ObjectNodeSection(string name, SectionType type) : this(name, type, null)
 
         public static readonly ObjectNodeSection ModulesWindowsContentSection = new ObjectNodeSection(".modules$I", SectionType.ReadOnly);
         public static readonly ObjectNodeSection ModulesUnixContentSection = new ObjectNodeSection("__modules", SectionType.Writeable);
-
-        public static readonly ObjectNodeSection DebugDirectorySection = new ObjectNodeSection("debug", SectionType.Debug);
-        public static readonly ObjectNodeSection CorMetaSection = new ObjectNodeSection("cormeta", SectionType.ReadOnly);
-        public static readonly ObjectNodeSection Win32ResourcesSection = new ObjectNodeSection("rsrc", SectionType.ReadOnly);
-        public static readonly ObjectNodeSection PDataSection = new ObjectNodeSection("pdata", SectionType.ReadOnly);
     }
 }

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

@@ -38,7 +38,7 @@ public virtual int CompareToImpl(ISortableNode other, CompilerComparer comparer)
             throw new NotImplementedException("Multiple nodes of this type are not supported");
         }
 
-        protected enum ObjectNodePhase
+        protected internal enum ObjectNodePhase
         {
             /// <summary>
             /// Nodes should only be placed in this phase if they have strict output ordering requirements that
@@ -49,7 +49,7 @@ protected enum ObjectNodePhase
             Late,
         }
 
-        protected enum ObjectNodeOrder
+        protected internal enum ObjectNodeOrder
         {
             //
             // The ordering of this sequence of nodes is deliberate and currently required for
@@ -66,7 +66,9 @@ protected enum ObjectNodeOrder
             ImportSectionsTableNode,
             ImportSectionNode,
             MethodEntrypointTableNode,
-
+            DebugDirectoryNode,
+            RuntimeFunctionsGCInfoNode,
+            RuntimeFunctionsTableNode,
 
             //
             // NativeAOT Nodes

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

@@ -89,6 +89,8 @@ private protected override void CreateSection(ObjectNodeSection section, Utf8Str
                 {
                     SectionType.ReadOnly =>
                         SectionCharacteristics.MemRead | SectionCharacteristics.ContainsInitializedData,
+                    SectionType.UnwindData =>
+                        SectionCharacteristics.MemRead | SectionCharacteristics.ContainsInitializedData,
                     SectionType.Writeable =>
                         SectionCharacteristics.MemRead | SectionCharacteristics.MemWrite |
                         SectionCharacteristics.ContainsInitializedData,
@@ -102,7 +104,7 @@ private protected override void CreateSection(ObjectNodeSection section, Utf8Str
                 }
             };
 
-            if (section == DebugTypesSection || section == ObjectNodeSection.DebugDirectorySection)
+            if (section == DebugTypesSection)
             {
                 sectionHeader.SectionCharacteristics =
                     SectionCharacteristics.MemRead | SectionCharacteristics.ContainsInitializedData |

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

@@ -65,6 +65,18 @@ private protected ObjectWriter(NodeFactory factory, ObjectWritingOptions options
 
         protected internal abstract void UpdateSectionAlignment(int sectionIndex, int alignment);
 
+        /// <summary>
+        /// Get the section in the image where nodes in the passed in section should actually be emitted.
+        /// </summary>
+        /// <param name="section">A node's requested section.</param>
+        /// <returns>The section to actually emit the node into.</returns>
+        /// <remarks>
+        /// Sections in an image can be very expensive, and unlike linkable formats,
+        /// sections cannot be merged after the fact.
+        /// This method allows formats that want to merge sections during emit to do so.
+        /// </remarks>
+        private protected virtual ObjectNodeSection GetEmitSection(ObjectNodeSection section) => section;
+
         private protected SectionWriter GetOrCreateSection(ObjectNodeSection section)
             => GetOrCreateSection(section, default, default);
 
@@ -87,6 +99,8 @@ private protected SectionWriter GetOrCreateSection(ObjectNodeSection section, Ut
             int sectionIndex;
             SectionData sectionData;
 
+            section = GetEmitSection(section);
+
             if (!comdatName.IsNull || !_sectionNameToSectionIndex.TryGetValue(section.Name, out sectionIndex))
             {
                 sectionData = new SectionData(section.Type == SectionType.Executable ? _insPaddingByte : (byte)0);
@@ -426,6 +440,11 @@ public virtual void EmitObject(Stream outputFileStream, IReadOnlyCollection<Depe
 
                         _outputInfoBuilder?.AddSymbol(new OutputSymbol(sectionWriter.SectionIndex, (ulong)(sectionWriter.Position + n.Offset), alternateCName));
                     }
+
+                    if (node.Phase == (int)SortableDependencyNode.ObjectNodePhase.Ordered)
+                    {
+                        RecordWellKnownSymbol(currentSymbolName, (SortableDependencyNode.ObjectNodeOrder)node.ClassCode);
+                    }
                 }
 
                 if (nodeContents.Relocs is not null)
@@ -590,6 +609,10 @@ public virtual void EmitObject(Stream outputFileStream, IReadOnlyCollection<Depe
             }
         }
 
+        private protected virtual void RecordWellKnownSymbol(Utf8String currentSymbolName, SortableDependencyNode.ObjectNodeOrder classCode)
+        {
+        }
+
         private protected virtual void EmitSymbolRangeDefinition(Utf8String rangeNodeName, Utf8String startNodeName, Utf8String endNodeName, SymbolDefinition endSymbol)
         {
             if (!_definedSymbols.TryGetValue(startNodeName, out var startSymbol))