[Interpreter] localloc does not guarantee platform stack alignment

[radekdoulik]

FrameDataAllocator::Alloc() in src/coreclr/vm/interpframeallocator.cpp aligns localloc buffer sizes to sizeof(void*) (4 bytes on WASM, 8 bytes on 64-bit platforms):

size = ALIGN_UP(size, sizeof(void*));

The JIT guarantees localloc returns pointers aligned to STACK_ALIGN (16 bytes on most platforms including WASM — see src/coreclr/jit/targetwasm.h), but the interpreter does not honor this. The fragment backing memory comes from raw malloc() with no explicit alignment, and subsequent bump allocations within a fragment are only pointer-size aligned.

This causes the ShowLocallocAlignment regression test to fail on WASM (and would fail on any platform using the interpreter) because stackalloc returns 8-byte aligned addresses instead of the expected 16-byte alignment.

Repro

Run JIT/Regression/Dev11/External/dev11_239804/ShowLocallocAlignment on a platform using the CoreCLR interpreter (e.g. WASM).

Expected

Localloc returns a 16-byte aligned pointer (matching STACK_ALIGN).

Actual

Localloc returns a pointer aligned only to sizeof(void*).

Suggested fix

Change the alignment in FrameDataAllocator::Alloc() to use INTERP_STACK_ALIGNMENT (16) instead of sizeof(void*), and ensure the returned pointer (not just the size) is aligned.

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[radekdoulik]

@janvorli @BrzVlad should we align FrameDataAllocator::Alloc() to INTERP_STACK_ALIGNMENT?

[BrzVlad]

Sounds good. We could use AlignedAllocate here as well.