RFR: 8317121: vector_masked_load instruction is moved too early after JDK-8286941

[Tobias Hartmann]

On Mon, 2 Oct 2023 19:17:40 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

> [JDK-8286941](https://bugs.openjdk.org/browse/JDK-8286941) changed how masked vector operations are created and processed. It calls `LoadNode::Ideal()` which tries to find best place in code for it.
> Unfortunately some places do not expect to have vector access to modified memory.
> In `LoadNode::find_previous_arraycopy()` the call to `ArrayCopyNode::modifies()` assumes that it access only one element and did not take into account that a vector may access wider memory.
> The same issue exists with Unsafe mismatching (bigger size) access.
> 
> The fix is to take into account size of memory access.
> 
> New regression test is added which show the issue.
> 
> Tested tier1-5. xcomp, stress. Testing tier6-7 in progress.

Looks good to me.

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Marked as reviewed by thartmann (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16015#pullrequestreview-1654429649