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[cDAC][wasm] Filter unexpected register slots in non-active frames #135093

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Merged
radekdoulik merged 2 commits into from
Oct 8, 2026
Merged

[cDAC][wasm] Filter unexpected register slots in non-active frames #135093

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f7923ba
[cDAC][wasm] Filter unexpected register slots in non-active frames
radekdoulik Oct 2, 2026
1608fca
[cDAC][wasm] Match native fallback for unexpected register slots
radekdoulik Oct 4, 2026
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3 changes: 3 additions & 0 deletions docs/design/datacontracts/GCInfo.md
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Expand Up @@ -665,6 +665,9 @@ For encodings without interruptible ranges (`HAS_INTERRUPTIBLE_RANGES` is false,

**Slot filtering**: Before reporting any slot, the algorithm checks:
- **Scratch registers**: Only reported for the active/leaf frame (`ActiveStackFrame` flag).
Valid Wasm GC info never contains register slots, so the WebAssembly platform
decoder's scratch-register predicate always returns false and asserts in debug
builds if called, mirroring the native unsupported-platform fallback.
- **Scratch stack slots**: Only reported for the active/leaf frame (slots in the outgoing/scratch area).
- **FP-based-only mode** (`ReportFPBasedSlotsOnly`): Only frame-register-relative stack slots are reported; all register slots and non-frame-relative stack slots are skipped.

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12 changes: 11 additions & 1 deletion ...gnostics.DataContractReader.Contracts/Contracts/GCInfo/PlatformTraits/WasmGCInfoTraits.cs
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@@ -1,6 +1,8 @@
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Diagnostics;

namespace Microsoft.Diagnostics.DataContractReader.Contracts.GCInfoHelpers;

// Mirrors Wasm32GcInfoEncoding in src/coreclr/inc/gcinfotypes.h.
Expand Down Expand Up @@ -37,5 +39,13 @@ internal sealed class WasmGCInfoTraits : IGCInfoTraits
public static int NUM_INTERRUPTIBLE_RANGES_ENCBASE => 1;
public static bool HAS_FIXED_STACK_PARAMETER_SCRATCH_AREA => false;
public static bool HAS_INTERRUPTIBLE_RANGES => false;
public static bool IsScratchRegister(uint regNum) => false;

// Valid Wasm GC info has no register slots; RyuJIT never emits one. Mirror the
// native fallback (gcinfodecoder.cpp PORTABILITY_ASSERT): flag the unexpected
// call in debug builds and otherwise return false, like other unimplemented platforms.
public static bool IsScratchRegister(uint regNum)
{
Debug.Fail("Wasm GC info must not contain register slots.");
return false;
}
}
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