[wasm][coreclr] Fix struct layout

[radekdoulik]

Fixes #122294

The default layout is now equal to the struct layout in C. The auto layout is updated in the same way.

[Copilot]

Pull request overview

This PR fixes struct layout alignment for WebAssembly (WASM) in CoreCLR to match C language struct layout rules. The changes ensure that when a type requires 8-byte alignment on WASM, the actual field alignment requirement is consulted rather than hardcoding the alignment to 8 bytes, which aligns the behavior with C struct layout semantics.

• Updates auto layout handling to use actual field alignment requirements on WASM when types require 8-byte alignment
• Updates default layout handling with the same WASM-specific logic for consistency

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/coreclr/vm/methodtablebuilder.cpp	Adds TARGET_WASM-specific logic in auto layout to use max(8, GetFieldAlignmentRequirement()) instead of hardcoded 8-byte alignment
src/coreclr/vm/classlayoutinfo.cpp	Adds TARGET_WASM-specific logic in default layout to use max(8, GetFieldAlignmentRequirement()) instead of hardcoded 8-byte alignment

[jkotas]

It looks like we have Arm32 bug here. It does not make sense for this ifdef to exist. Also, we may want to match !RequiresAlign8 logic that ignores GetFieldAlignmentRequirement for types with GC references. So maybe something like this?

            if (pNestedType.GetMethodTable()->ContainsGCPointers())
            {
                // this field type has GC pointers in it, which need to be pointer-size aligned
                placementInfo.m_alignment = TARGET_POINTER_SIZE;
            }
            else
            {
                placementInfo.m_alignment = pNestedType.GetMethodTable()->GetFieldAlignmentRequirement();
            }
#if defined(FEATURE_64BIT_ALIGNMENT)
            if (pNestedType.RequiresAlign8())
            {
                placementInfo.m_alignment = max(placementInfo.m_alignment, 8);
            }
#endif

[jkotas]

Also, this logic is duplicates in

runtime/src/coreclr/tools/Common/TypeSystem/Common/MetadataFieldLayoutAlgorithm.cs

Line 733 in 80d86b9

// This does not account for types that are marked IsAlign8Candidate due to 8-byte fields

for the AOT compiler. We should do the same change there as well.

[jkoritzinsky]

I would agree with @jkotas. This is a bug for types with higher alignment requirements than 8 bytes. This shouldn't be wasm specific.

Also, when updating the layout algorithm, we generally have to bump the major R2R version and min-compatible major R2R version.

[jkotas]

cc @jkoritzinsky @davidwrighton - field layout algorithm change

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