[wasm] Encode struct alignment in thunk signatures

[davidwrighton]

Supports structs with 16 byte alignment. Alternative PR to #132244

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

[azure-pipelines]

Azure Pipelines:
Successfully started running 4 pipeline(s).
12 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[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.