[wasm] Encode struct alignment in thunk signatures

[davidwrighton]

Supports structs passed by value with 16 byte alignment on Wasm

• Consolidate on using the alignment calculation in the jit interface to determine the alignment requirement for parameters in function calls
• Detect the difference between structures aligned to 8 byte boundaries and 16 byte boundaries in transition thunks
• Add a new encoding for pass by value structures with 16 byte alignment (A instead of S)
• Add tests to validate this. Not just on the crossgen side but also a targeted Wasm R2R test.

Co-authored-by: Copilot App 223556219+Copilot@users.noreply.github.com

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[Copilot]

Pull request overview

Updates the Wasm thunk signature-string encoding to preserve by-reference struct alignment requirements (notably 16-byte alignment) so that signature raising and transition-block layout remain correct when two same-sized structs differ in alignment.

Changes:

• Extends struct tokens from S<N> to optionally include alignment for parameters: S!<N>:<A> (while keeping struct returns as S<N>).
• Threads alignment through signature generation/parsing paths (runtime, WasmAppBuilder thunk generator, and crossgen2 tooling) and updates caching to disambiguate same-sized structs by (size, alignment) when needed.
• Adds ReadyToRun tests and updates design documentation for the extended encoding.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/tasks/WasmAppBuilder/coreclr/SignatureMapper.cs	Adds parsing/formatting support for alignment-qualified struct tokens and exposes helpers to extract size/alignment.
src/tasks/WasmAppBuilder/coreclr/InterpToNativeGenerator.cs	Aligns interpreter-stack slot offsets for indirect struct args when token alignment exceeds 8, and normalizes struct-return typedef naming.
src/coreclr/vm/wasm/helpers.cpp	Encodes parameter struct alignment into thunk signature keys while forcing return structs to remain size-only.
src/coreclr/vm/jitinterface.cpp	Adjusts Wasm alignment requirement computation to account for explicit layout and elevated field alignment in auto-layout structs.
src/coreclr/tools/Common/JitInterface/WasmLowering.cs	Implements parsing/emission for S!<N>:<A>, raises signatures using cached structs keyed by layout, and emits alignment only for parameters.
src/coreclr/tools/Common/JitInterface/CorInfoImpl.cs	Mirrors Wasm alignment requirement logic in the tool-side JIT interface implementation.
src/coreclr/tools/Common/Compiler/CompilerTypeSystemContext.Wasm.cs	Changes struct caching to key by (size, alignment) when alignment is part of the signature encoding.
src/coreclr/tools/aot/ILCompiler.ReadyToRun.Tests/WasmArgumentLayoutTests.cs	Adds/updates tests ensuring signatures and computed offsets remain correct for same-sized structs with different alignment.
docs/design/coreclr/botr/readytorun-format.md	Documents S!<N>:<A> encoding rules and clarifies slot sizing for struct tokens.

[Copilot]

Pull request overview

Copilot reviewed 10 out of 10 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/coreclr/vm/jitinterface.cpp:1964

• The comment mentions "beyond the pointer size", but the logic only adjusts the alignment when fieldAlignment > 8. On Wasm32 the pointer size is 4, so this is misleading and makes it harder to reason about why the threshold is 8.

        // Auto-layout structs can contain fields aligned beyond the pointer size on Wasm.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/tasks/WasmAppBuilder/coreclr/InterpToNativeGenerator.cs:73

• The generated C++ header comment still states that each argument is aligned to INTERP_STACK_SLOT_SIZE, but with the new 'A' struct token you now intentionally align some arguments to a 16-byte boundary (see the new slot rounding in ArgsWithSlotOffsets). Please update the emitted comment so it matches the new behavior and avoids misleading future maintainers.

        {
            var toks = SignatureMapper.ParseSignatureTokens(sig);
            if (toks[0][0] == 'S' && toks[0].Length > 1)
                structReturnSizes.Add(SignatureMapper.GetStructSize(toks[0]));
        }

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[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated no new comments.

Suppressed comments (2)

src/tests/JIT/Regression/JitBlue/WasmR2RStructAlignment/WasmR2RStructAlignment.cs:17

• This test redefines System.Runtime.BypassReadyToRunAttribute locally, but unlike other tests (e.g., Methodical) it doesn’t explain why this duplicate definition is needed. Adding a short rationale comment helps future maintainers understand that the attribute is matched by name and is intentionally duplicated in the test assembly.

namespace System.Runtime
{
    [AttributeUsage(AttributeTargets.Method)]
    internal sealed class BypassReadyToRunAttribute : Attribute
    {

src/coreclr/tools/Common/JitInterface/WasmLowering.cs:735

• The new 'A' encoding is introduced here, but the updated test suite in this PR doesn’t appear to validate that any signature actually produces an 'A' token (e.g., WasmArgumentLayoutTests has no expected signatures containing 'A'). Without a regression test that round-trips an 'A' struct (and verifies the 16-byte transition-block padding/offsets), this change can silently break without detection.

                        Debug.Assert(paramType is DefType);
                        int paramAlignment = CorInfoImpl.GetClassAlignmentRequirementStatic((DefType)paramType);
                        bool requiresAlignedSlot = paramAlignment > 8;
                        sigBuilder.Append(requiresAlignedSlot ? 'A' : 'S');
                        sigBuilder.Append(paramSize);
                        ((CompilerTypeSystemContext)paramType.Context).CacheStruct(paramType, requiresAlignedSlot);
                        result.Add(pointerType);

[davidwrighton]

/ba-g unrelated failures