RFR: 8338731: MemoryLayout::offsetHandle can return a negative offset

[Chen Liang]

On Wed, 21 Aug 2024 13:26:58 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> When working on startup improvements, I noticed that the method handle returned by `MemoryLayout::offsetHandle` can overflow if the client calls the handle with a base offset that is too big.
> 
> In other similar situations, the layout API always fails with `ArithmeticException` (see `MemoryLayout::scale`), so we should do the same here.
> 
> The fix is to use a `Math::addExact(long, long)` for the outermost add operation in the computation of the offset method handle. That outermost computation in fact is the only one that can overflow: it is an addition between a user-provided base offset `B` and a layout offset `L`. `L` is guaranteed not to overflow, by construction (as `L` is derived from a layout path). But `B` + `L` might overflow, so the new logic checks for that.

They should have the same performance characteristics. They have different permissions for access checks, and since `Long` is exported public API and `sum` is public both will pass.

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PR Comment: https://git.openjdk.org/jdk/pull/20662#issuecomment-2307175829