[release/9.0] Ensure the layout of StackAllocatedBox<T> matches boxed layouts
#107075
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The boxed layout of a struct always has its data at +8, as evidenced by `Object::UnBox`. This means that `StackAllocatedBox<T>` should have `Pack = 1`, otherwise this may not be the case. In the test failure we had a `StackAllocatedBox<Int128>` which had its `_value` field at offset 16. After object stack allocation this meant that we were saving data in padding of the structure, which promotion does not guarantee to preserve. Fix #106947
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jkotas
approved these changes
Aug 28, 2024
jeffschwMSFT
approved these changes
Aug 28, 2024
AndyAyersMS
approved these changes
Aug 28, 2024
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This was referenced Aug 29, 2024
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Backport of #107050 to release/9.0
/cc @jakobbotsch
Customer Impact
The JIT may produce silent bad codegen when stack allocating a box of a type with alignment larger than 8 (such as
Int128orVector128). In those cases it is possible for some of the data of the struct value to be silently discarded.Regression
Rare edge case not found immediately with the stack allocation work.
Testing
Internal tests failed under a JIT stress mode (#106947). The fix was manually verified on those tests with the jit stress mode applied.
Also added assert to make other potential bugs of this flavor much more easy to discover.
Risk
Low.