RFR: 8257624: C2: PhaseMacroExpand::eliminate_macro_nodes() crashes on out-of-bounds access into macro node array

[Nils Eliasson]

On Wed, 2 Dec 2020 19:13:31 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> Elimination of a single macro node may trigger removal of some other macro nodes (e.g., see `PhaseMacroExpand::process_users_of_allocation()`  which can eliminate `AllocateCopy` if it is a user of `Allocate` node being scalar replaced). But `PhaseMacroExpand::eliminate_macro_nodes()` doesn't take it into account: it iterates over the array backwards one by one and if it is unfortunate to eliminate multiple elements when it is at the very end of the array, it crashes on an out-of-bounds access.
> 
> The fix is to adjust current position on every iteration. If there are multiple elements removed, current position can point at an element which is already processed, but that's benign.
> 
> Testing:
> - [x] failing tests
> - [x] hs-tier-1-6 w/ -XX:+AlwaysIncrementalInline

Looks good!

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Marked as reviewed by neliasso (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1572