RFR: 8287244: Add bound check in indexed memory access var handle [v2]

[Paul Sandoz]

On Tue, 24 May 2022 17:53:38 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Indices start at zero. The ceilDiv operation is needed so that the operation returns the first index outisde the range (it's a bit subtle, sorry, but I don't know how else to express).
>
> Here's a concrete example:
> 
> Consider a sequence layout with 6 elements. Then:
> 
> element count = 6
> valid indices 0, 1, 2, 3, 4, 5
> 
> Now consider a var handle that is obtained by calling the path element method, passing the following parameters
> 
> start = 1
> step = 2
> 
> This sets up the following mapping between logical an physical indices:
> 
> 0 -> 1
> 1 -> 3
> 2 -> 5
> 
> Where on the LHS we have the logical index (the one passed to the VH) and on the RHS we have the actual index it is translated to.
> 
> Note that the index map has three elements. So the upper bound (exclusive) of the index map is 3 - that is, we can pass indices 0, 1, 2.
> 
> According to the formula shown in the javadoc:
> 
> B = ceilDiv((elementCount - start) / step);
> 
> so:
> 
> B = ceilDiv((6 - 1) / 2)
>     = ceilDiv(5 / 2)
> 
> Note how, w/o ceilDiv we'd get 2 (the last valid index), and not 3 (the first invalid index).

The terms `x_1, x_2, ... x__n` are defined, but `x_0` is not. 

I think you can refer to the first index out of bounds as the exclusive upper bound of the range?

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