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[Wasm RyuJIT] Generate element section and populate function pointer table #125531

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Merged
kg merged 5 commits into from
Mar 17, 2026
Merged

[Wasm RyuJIT] Generate element section and populate function pointer table #125531

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5ea3a63
Checkpoint: Rename r2r_start
kg Mar 13, 2026
ff02e62
Checkpoint: Generate element containing sequential function pointers …
kg Mar 13, 2026
cf12138
Fix initial size of our ftn pointer table being 0
kg Mar 13, 2026
d435d70
Improvement from Adam
kg Mar 13, 2026
b981ec9
Address feedback
kg Mar 17, 2026
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74 changes: 66 additions & 8 deletions src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,7 @@ internal static class WasmObjectNodeSection
public static readonly ObjectNodeSection CombinedDataSection = new ObjectNodeSection("wasm.alldata", SectionType.Writeable, needsAlign: false);
public static readonly ObjectNodeSection FunctionSection = new ObjectNodeSection("wasm.function", SectionType.ReadOnly, needsAlign: false);
public static readonly ObjectNodeSection ExportSection = new ObjectNodeSection("wasm.export", SectionType.ReadOnly, needsAlign: false);
public static readonly ObjectNodeSection ElementSection = new ObjectNodeSection("wasm.element", SectionType.ReadOnly, needsAlign: false);
public static readonly ObjectNodeSection MemorySection = new ObjectNodeSection("wasm.memory", SectionType.ReadOnly, needsAlign: false);
public static readonly ObjectNodeSection TableSection = new ObjectNodeSection("wasm.table", SectionType.ReadOnly, needsAlign: false);
public static readonly ObjectNodeSection ImportSection = new ObjectNodeSection("wasm.import", SectionType.ReadOnly, needsAlign: false);
Expand Down Expand Up @@ -143,6 +144,28 @@ private void WriteMemoryExport(string name, int memoryIndex) =>
private void WriteGlobalExport(string name, int globalIndex) =>
WriteExport(name, WasmExportKind.Global, globalIndex);

private int _numElements;
private void WriteFunctionElement(WasmInstructionGroup e0, ReadOnlySpan<int> functionIndices)
{
SectionWriter writer = GetOrCreateSection(WasmObjectNodeSection.ElementSection);
// e0:expr y*:list(funcidx)
// elem (ref func) (ref.func y)* (active 0 e0)
writer.WriteULEB128(0);

// FIXME: Add a way to encode directly into the writer without a scratch buffer
int encodeSize = e0.EncodeSize();
int bytesWritten = e0.Encode(writer.Buffer.GetSpan(encodeSize));
Debug.Assert(bytesWritten == encodeSize);
writer.Buffer.Advance((int)bytesWritten);

writer.WriteULEB128((ulong)functionIndices.Length);

foreach (int index in functionIndices)
writer.WriteULEB128((ulong)index);

_numElements++;
}

private List<WasmSection> _sections = new();
private Dictionary<string, int> _sectionNameToIndex = new();
private Dictionary<ObjectNodeSection, WasmSectionType> _sectionToType = new()
Expand All @@ -151,6 +174,7 @@ private void WriteGlobalExport(string name, int globalIndex) =>
{ WasmObjectNodeSection.FunctionSection, WasmSectionType.Function },
{ WasmObjectNodeSection.TableSection, WasmSectionType.Table },
{ WasmObjectNodeSection.ExportSection, WasmSectionType.Export },
{ WasmObjectNodeSection.ElementSection, WasmSectionType.Element },
{ WasmObjectNodeSection.ImportSection, WasmSectionType.Import },
{ ObjectNodeSection.WasmTypeSection, WasmSectionType.Type },
{ ObjectNodeSection.WasmCodeSection, WasmSectionType.Code }
Expand All @@ -171,12 +195,21 @@ protected internal override void UpdateSectionAlignment(int sectionIndex, int al
// This is a no-op for now under Wasm
}

WasmInstructionGroup GetImageFunctionPointerBaseOffset(int offset)
{
return new WasmInstructionGroup([
Global.Get(ImageFunctionPointerBaseGlobalIndex),
I32.Const(offset),
I32.Add,
]);
}

private WasmDataSection CreateCombinedDataSection()
{
WasmInstructionGroup GetR2RStartOffset(int offset)
WasmInstructionGroup GetImageBaseOffset(int offset)
{
return new WasmInstructionGroup([
Global.Get(R2RStartGlobalIndex),
Global.Get(ImageBaseGlobalIndex),
I32.Const(offset),
I32.Add,
]);
Expand All @@ -189,7 +222,7 @@ WasmInstructionGroup GetR2RStartOffset(int offset)
{
Debug.Assert(wasmSection.Type == WasmSectionType.Data);
WasmDataSegment segment = new WasmDataSegment(wasmSection.Stream, wasmSection.Name, WasmDataSectionType.Active,
GetR2RStartOffset(offset));
GetImageBaseOffset(offset));
segments.Add(segment);
offset += segment.ContentSize;
}
Expand Down Expand Up @@ -267,8 +300,8 @@ private void WriteTableSection()
writer.WriteByte(0x01); // number of tables
writer.WriteByte(0x70); // element type: funcref
writer.WriteByte(0x01); // table limits: flags (1 = has maximum)
writer.WriteULEB128((ulong)0);
writer.WriteULEB128((ulong)_methodCount); // table limits: initial size in number of entries
writer.WriteULEB128((ulong)_methodCount); // minimum
writer.WriteULEB128((ulong)_methodCount); // maximum
}

private void PrependCount(WasmSection section, int count)
Expand All @@ -289,6 +322,7 @@ private WasmSection SectionByName(string name)
WasmObjectNodeSection.FunctionSection.Name,
WasmObjectNodeSection.TableSection.Name,
WasmObjectNodeSection.ExportSection.Name,
WasmObjectNodeSection.ElementSection.Name,
ObjectNodeSection.WasmCodeSection.Name,
WasmObjectNodeSection.CombinedDataSection.Name,
];
Expand Down Expand Up @@ -394,7 +428,7 @@ private unsafe void ResolveRelocations(MemoryStream sectionStream, List<Symbolic

Span<byte> ReadRelocToDataSpan(SymbolicRelocation reloc, byte[] buffer)
{
Span<byte> relocContents = buffer.AsSpan(0, Relocation.GetSize(reloc.Type));
Span<byte> relocContents = buffer.AsSpan(0, Relocation.GetSize(reloc.Type));
sectionStream.Position = reloc.Offset;
sectionStream.ReadExactly(relocContents);
return relocContents;
Expand All @@ -408,13 +442,15 @@ void WriteRelocFromDataSpan(SymbolicRelocation reloc, byte *pData)
}

const int StackPointerGlobalIndex = 0;
const int R2RStartGlobalIndex = 1;
const int ImageBaseGlobalIndex = 1;
const int ImageFunctionPointerBaseGlobalIndex = 2;

private WasmImport[] _defaultImports = new[]
{
null, // placeholder for memory, which is set up dynamically in WriteImports()
new WasmImport("env", "__stack_pointer", import: new WasmGlobalImportType(WasmValueType.I32, WasmMutabilityType.Mut), index: StackPointerGlobalIndex),
new WasmImport("env", "__r2r_start", import: new WasmGlobalImportType(WasmValueType.I32, WasmMutabilityType.Const), index: R2RStartGlobalIndex),
new WasmImport("env", "__image_base", import: new WasmGlobalImportType(WasmValueType.I32, WasmMutabilityType.Const), index: ImageBaseGlobalIndex),
new WasmImport("env", "__image_function_pointer_base", import: new WasmGlobalImportType(WasmValueType.I32, WasmMutabilityType.Const), index: ImageFunctionPointerBaseGlobalIndex),
};

private void WriteImports()
Expand Down Expand Up @@ -454,11 +490,28 @@ private void WriteExports()
}
}

private void WriteElements()
{
// Generate the function pointer table element that contains function pointers for all of our functions
int[] functionIndices = new int[_uniqueSymbols.Count];
// NOTE: This relies on items in _uniqueSymbols being assigned sequentially and that iteration over Values is order-preserving.
// BCL Dictionary preserves insertion order so as long as we keep using it, we would get the function indices in the order they were added.
_uniqueSymbols.Values.CopyTo(functionIndices, 0);
// Enforce that the function pointers are sequential so that (image_function_pointer_base + 0) == ftn index 0
#if DEBUG
for (int i = 0; i < _uniqueSymbols.Count; i++) {
Debug.Assert(functionIndices[i] == i);
}
#endif
WriteFunctionElement(GetImageFunctionPointerBaseOffset(0), functionIndices);
}

// For now, this function just prepares the function, exports, and type sections for emission by prepending the counts.
private protected override void EmitSymbolTable(IDictionary<Utf8String, SymbolDefinition> definedSymbols, SortedSet<Utf8String> undefinedSymbols)
{
WriteImports();
WriteExports();
WriteElements();

int funcIdx = _sectionNameToIndex[WasmObjectNodeSection.FunctionSection.Name];
PrependCount(_sections[funcIdx], _methodCount);
Expand All @@ -469,6 +522,11 @@ private protected override void EmitSymbolTable(IDictionary<Utf8String, SymbolDe
int exportIdx = _sectionNameToIndex[WasmObjectNodeSection.ExportSection.Name];
PrependCount(_sections[exportIdx], _numExports);

if (_sectionNameToIndex.TryGetValue(WasmObjectNodeSection.ElementSection.Name, out int elementIdx))
{
PrependCount(_sections[elementIdx], _numElements);
}

PrependCount(SectionByName(WasmObjectNodeSection.ImportSection.Name), _numImports);
}
}
Expand Down
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