RFR: 8287244: Add bound check in indexed memory access var handle

[Maurizio Cimadamore]

On Tue, 24 May 2022 14:40:56 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> Constructing indexed var handles using the `MemoryLayout` API produces `VarHandle` which do not check the input indices for out-of-bounds conditions.
> While this can never result in a VM crash (after all the memory segment will protect against "true" OOB access), it is still possible for an access expression to refer to parts of a segment that are logically unrelated.
> 
> This patch adds a "logical" bound check to all indexed var handles generated using the layout API.
> Benchmarks are not affected by the check. Users are still able to create custom "unchecked" var handles, using the combinator API in `MethodHandles`.

src/java.base/share/classes/java/lang/foreign/MemoryLayout.java line 537:

> 535:          *
> 536:          * <ul>
> 537:          *    <li>if {@code F > 0}, then {@code B = ceilDiv(C - S, F)}</li>

These formulas come from the formula for computing the accessed index A:

`A = S + I * F`

And then deriving the value for I, by equating `A = C` (for F > 0) and `A = -1` (for F < 0) - that is equating the accessed index to the "first" out of bound index. `ceilDiv` ensures there is "some room" between the max/min index and the selected one.

src/java.base/share/classes/jdk/internal/foreign/LayoutPath.java line 109:

> 107:         SequenceLayout seq = (SequenceLayout)layout;
> 108:         checkSequenceBounds(seq, index);
> 109:         long elemSize = seq.elementLayout().bitSize();

I've simplified the code here, as it still had traces of attempts to avoid the call to `bitSize` (this method used to be partial).

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PR: https://git.openjdk.java.net/jdk/pull/8868