Fix issue #114504 by not emitting empty gcrefmaps in R2R (#118594)

* Add important comment so it's possible to find the source of AuxiliaryDataRva
* Skip emitting an RVA for the GCRefMap if it contains no methods

Author: kg

src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRun/GCRefMapNode.cs

@@ -34,6 +34,8 @@ public GCRefMapNode(ImportSectionNode importSection)
 
         public int Offset => 0;
 
+        public bool IsEmpty => _methods.Count == 0;
+
         public void AddImport(Import import)
         {
             lock (_methods)
@@ -53,9 +55,9 @@ public override ObjectData GetData(NodeFactory factory, bool relocsOnly = false)
             if (_methods.Count == 0 || relocsOnly)
             {
                 return new ObjectData(
-                    data: Array.Empty<byte>(), 
-                    relocs: Array.Empty<Relocation>(), 
-                    alignment: 1, 
+                    data: Array.Empty<byte>(),
+                    relocs: Array.Empty<Relocation>(),
+                    alignment: 1,
                     definedSymbols: new ISymbolDefinitionNode[] { this });
             }
 

src/coreclr/tools/aot/ILCompiler.ReadyToRun/Compiler/DependencyAnalysis/ReadyToRun/ImportSectionNode.cs

@@ -41,7 +41,6 @@ protected override int GetAlignmentRequirement(NodeFactory factory)
         private readonly byte _entrySize;
         private readonly string _name;
         private readonly bool _emitPrecode;
-        private readonly bool _emitGCRefMap;
 
         private bool _materializedSignature;
 
@@ -52,12 +51,11 @@ public ImportSectionNode(string name, ReadyToRunImportSectionType importType, Re
             _flags = flags;
             _entrySize = entrySize;
             _emitPrecode = emitPrecode;
-            _emitGCRefMap = emitGCRefMap;
 
             _imports = new ImportTable(_name + "_ImportBegin", entrySize);
             _signatures = new ArrayOfEmbeddedPointersNode<Signature>(_name + "_SigBegin", new EmbeddedObjectNodeComparer(CompilerComparer.Instance));
             _signatureList = new List<Signature>();
-            _gcRefMap = _emitGCRefMap ? new GCRefMapNode(this) : null;
+            _gcRefMap = emitGCRefMap ? new GCRefMapNode(this) : null;
         }
 
         public void MaterializeSignature(NodeFactory r2rFactory)
@@ -89,10 +87,7 @@ public void AddImport(NodeFactory factory, Import import)
                 _signatureList.Add(import.ImportSignature.Target);
             }
 
-            if (_emitGCRefMap)
-            {
-                _gcRefMap.AddImport(import);
-            }
+            _gcRefMap?.AddImport(import);
         }
 
         public string Name => _name;
@@ -135,8 +130,10 @@ public override void EncodeData(ref ObjectDataBuilder dataBuilder, NodeFactory f
                 dataBuilder.EmitUInt(0);
             }
 
-            if (_emitGCRefMap)
+            // If we emit an Rva for an empty gcrefmap, it will have no size header and be impossible for r2rdump to read correctly
+            if (_gcRefMap?.IsEmpty == false)
             {
+                // This indirectly generates the AuxiliaryDataRva by emitting a placeholder 0 which will be replaced later
                 dataBuilder.EmitReloc(_gcRefMap, RelocType.IMAGE_REL_BASED_ADDR32NB, 0);
             }
             else
@@ -149,7 +146,7 @@ public override IEnumerable<DependencyListEntry> GetStaticDependencies(NodeFacto
         {
             yield return new DependencyListEntry(_imports, "Import section fixup data");
             yield return new DependencyListEntry(_signatures, "Import section signatures");
-            if (_emitGCRefMap)
+            if (_gcRefMap != null)
             {
                 yield return new DependencyListEntry(_gcRefMap, "GC ref map");
             }

src/tests/Common/CLRTest.CrossGen.targets

@@ -110,8 +110,7 @@ if [ ! -z ${RunCrossGen2+x} ]%3B then
         done
 
         echo -o:"$__OutputFile" >> "$__ResponseFile"
-        ## Enable once #114504 is fixed
-        ## echo -O >> "$__ResponseFile"
+        echo -O >> "$__ResponseFile"
         echo --targetarch:$(TargetArchitecture) >> "$__ResponseFile"
         echo --targetos:$(TargetOS) >> "$__ResponseFile"
         echo --verify-type-and-field-layout >> "$__ResponseFile"