crossgen2: Add Length Prefix to Crossgen-Generated Wasm

src/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.cs

@@ -26,8 +26,9 @@ public ObjectNode.ObjectData Encode(ISymbolDefinitionNode symbolDefinitionNode)
         {
 #if READYTORUN
             byte[] encodedThunk = new byte[FunctionBody.EncodeSize()];
+            FunctionBody.Encode(encodedThunk);
+
             Relocation[] relocs = new Relocation[FunctionBody.EncodeRelocationCount()];
-            FunctionBody.Encode(encodedThunk.AsSpan());
             FunctionBody.EncodeRelocations(relocs.AsSpan());
 
             return new ObjectNode.ObjectData(encodedThunk, relocs, 1, new ISymbolDefinitionNode[] { symbolDefinitionNode });

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

@@ -84,20 +84,6 @@ private protected ObjectWriter(NodeFactory factory, ObjectWritingOptions options
         private protected SectionWriter GetOrCreateSection(ObjectNodeSection section)
             => GetOrCreateSection(section, default, default);
 
-        private readonly SectionWriter.Params _defaultParams = new SectionWriter.Params
-        {
-            LengthEncodeFormat = LengthEncodeFormat.None
-        };
-
-        /// <summary>
-        /// Some architectures may require section-specific params for the writer. For example, on Wasm,
-        /// certain sections require length prefixes before each object entry which the section writer does support,
-        /// but this has to be indicated by a particular implementation.
-        /// </summary>
-        /// <param name="section"></param>
-        /// <returns></returns>
-        private protected virtual SectionWriter.Params WriterParams(ObjectNodeSection section) => _defaultParams;
-
         /// <summary>
         /// Get or creates an object file section.
         /// </summary>
@@ -139,8 +125,7 @@ private protected SectionWriter GetOrCreateSection(ObjectNodeSection section, Ut
             return new SectionWriter(
                 this,
                 sectionIndex,
-                sectionData,
-                WriterParams(section));
+                sectionData);
         }
 
         private protected bool ShouldShareSymbol(ObjectNode node)
@@ -482,13 +467,9 @@ public virtual void EmitObject(Stream outputFileStream, IReadOnlyCollection<Depe
 
                 if (nodeContents.Relocs is not null)
                 {
-                    // For platforms such as Wasm where we must prepend the length before writing blocks,
-                    // we need to adjust the offset of the relocation by the length prefix
-                    uint additionalOffset = sectionWriter.HasLengthPrefix ? sectionWriter.LengthPrefixSize(nodeContents.Data.Length) : 0;
-
                     blocksToRelocate.Add(new BlockToRelocate(
                         sectionWriter.SectionIndex,
-                        sectionWriter.Position + additionalOffset,
+                        sectionWriter.Position,
                         nodeContents.Data,
                         nodeContents.Relocs));
 

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

@@ -11,69 +11,26 @@
 
 namespace ILCompiler.ObjectWriter
 {
-    internal enum LengthEncodeFormat
-    {
-        ULEB128,
-        None,
-    }
-
     internal struct SectionWriter
     {
         private readonly ObjectWriter _objectWriter;
         private readonly SectionData _sectionData;
 
-        private readonly Params _params;
-
         public int SectionIndex { get; init; }
         public readonly IBufferWriter<byte> Buffer => _sectionData.BufferWriter;
 
-        public struct Params
-        {
-            public LengthEncodeFormat LengthEncodeFormat;
-        }
-
-        public bool HasLengthPrefix => _params.LengthEncodeFormat != LengthEncodeFormat.None;
-
         internal SectionWriter(
             ObjectWriter objectWriter,
             int sectionIndex,
-            SectionData sectionData,
-            Params ps)
+            SectionData sectionData)
         {
             _objectWriter = objectWriter;
             SectionIndex = sectionIndex;
             _sectionData = sectionData;
-            _params = ps;
-        }
-
-        public readonly void EmitLengthPrefix(ulong length)
-        {
-            switch (_params.LengthEncodeFormat)
-            {
-                case LengthEncodeFormat.ULEB128:
-                    WriteULEB128(length);
-                    break;
-                case LengthEncodeFormat.None:
-                    break;
-                default:
-                    throw new InvalidOperationException("Length prefix encoding not specified");
-            }
-        }
-
-        public readonly uint LengthPrefixSize(int length)
-        {
-            switch (_params.LengthEncodeFormat)
-            {
-                case LengthEncodeFormat.ULEB128:
-                    return DwarfHelper.SizeOfULEB128((ulong)length);
-                default:
-                    return 0;
-            }
         }
 
         public readonly void EmitData(ReadOnlyMemory<byte> data)
         {
-            EmitLengthPrefix((ulong)data.Length);
             _sectionData.AppendData(data);
         }
 

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

@@ -85,13 +85,17 @@ private int BodyContentSize()
 
         public int EncodeSize()
         {
-            return BodyContentSize();
+            int bodySize = BodyContentSize();
+            int sizePrefixLength = (int)DwarfHelper.SizeOfULEB128((ulong)bodySize);
+            return sizePrefixLength + bodySize;
         }
 
         public int Encode(Span<byte> buffer)
         {
-            _locals.CopyTo(buffer);
-            int pos = _locals.Length;
+            int contentSize = BodyContentSize();
+            int pos = DwarfHelper.WriteULEB128(buffer, (ulong)contentSize);
+            _locals.CopyTo(buffer.Slice(pos));
+            pos += _locals.Length;
             pos += _body.Encode(buffer.Slice(pos));
 
             return pos;
@@ -104,8 +108,10 @@ public int EncodeRelocationCount()
 
         public int EncodeRelocations(Span<Relocation> buffer)
         {
+            uint bodySize = (uint)BodyContentSize();
+            int bodySizePrefixLength = (int)DwarfHelper.SizeOfULEB128(bodySize);
             int relocsEncoded = _body.EncodeRelocations(buffer);
-            WasmExpr.OffsetRelocationsByOffset(buffer.Slice(0, relocsEncoded), _locals.Length);
+            WasmExpr.OffsetRelocationsByOffset(buffer.Slice(0, relocsEncoded), bodySizePrefixLength + _locals.Length);
             return relocsEncoded;
         }
     }

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

@@ -359,9 +359,6 @@ private void InsertWasmStub(Utf8String name, WasmFunctionBody body)
             byte[] data = new byte[codeSize];
             body.Encode(data);
 
-            // We must emit the length prefix explicitly
-            Debug.Assert(!codeWriter.HasLengthPrefix);
-            codeWriter.WriteULEB128((ulong)codeSize);
             codeWriter.EmitData(data);
             _uniqueSymbols.Add(name.ToString(), _methodCount);
             _methodCount++;
@@ -480,14 +477,6 @@ private protected override ObjectNodeSection GetEmitSection(ObjectNodeSection se
             return section;
         }
 
-        private protected override SectionWriter.Params WriterParams(ObjectNodeSection section)
-        {
-            return new SectionWriter.Params
-            {
-                LengthEncodeFormat = LengthEncodeFormat.None
-            };
-        }
-
         private protected override void CreateSection(ObjectNodeSection section, Utf8String comdatName, Utf8String symbolName, int sectionIndex, Stream sectionStream)
         {
             WasmSectionType sectionType = GetWasmSectionType(section);