Integrated: 8316679: C2 SuperWord: wrong result, load should not be moved before store if not comparable

[Emanuel Peter]

On Thu, 21 Sep 2023 14:19:58 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> **Context**
> In `SuperWord::output`, we replace the scalars with vectors, pack by pack.
> 
> In [JDK-8052081](https://bugs.openjdk.org/browse/JDK-8052081) ([changeset](https://github.com/openjdk/jdk/commit/7764490363a5963212e040baa2b79946cf4487bf)) we added an optimization to move the memory state of a LoadVector as early as possible. We go up the memory chain, and skip any `StoreVector` that provably can never alias with  the `LoadVector` (provably no overlap).
> 
> **Details**
> 
> For this, we must understand the following function `VPointer::cmp`:
> 
> https://github.com/openjdk/jdk/blob/96781ba33d4717c8df2b9ba4cd5d66858cf5c2d1/src/hotspot/share/opto/vectorization.hpp#L110-L120
> 
> We have 4 possible states: `NotComparable` means that we cannot determine the relative offsets between the two memory addresses. We have no guarantee about aliasing at all. `Less` means that one address lies before the other, `Greater` means that it comes after. `Equal` means that the two memory regions have an overlap or are identical. To determine overlap, we must know the position of the pointer, as well as the `memory_size` of each memory reference. `VPointer::not_equal` is true iff we get states `Less` or `Greater` (we know there cannot be overlap). `VPointer::comparable` is true iff the state is `NotComparable`, hence we have no guarantee about aliasing.
> 
> **Problem**
> 
> Currently, there are 2 bugs in this "move-load-before-store" logic:
> 1. [JDK-8316679](https://bugs.openjdk.org/browse/JDK-8316679) ([PR](https://github.com/openjdk/jdk/pull/15864)): We do not only skip the `not_equal` but also the `!comparable` case. However, when the two pointers are not comparable, that could be due to different invariants for example (see `TestMovingLoadBeforeStore.java`). The two pointers are not comparable, because we do not know the relative position, due to the invariants. But this exactly means that we cannot know at compile-time that there will not be aliasing. Thus, we cannot allow moving a load before such a store. Otherwise we may get wrong results. (*)
> 2. [JDK-8316594](https://bugs.openjdk.org/browse/JDK-8316594) ([PR](https://github.com/openjdk/jdk/pull/15866)): Currently, we only compare the first element of the load-pack with the `StoreVector`. This is not correct, we need to compare all elements of the load-pack with the `StoreVector`. Otherwise it may be that the first element lays below/before the memory region of the `StoreVector` (Less), but the last element of the load-pack for example c...

This pull request has now been integrated.

Changeset: 48f1a925
Author:    Emanuel Peter <epeter at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/48f1a925e528cc9f8cd6c727129918e0e49b3429
Stats:     110 lines in 2 files changed: 103 ins; 4 del; 3 mod

8316679: C2 SuperWord: wrong result, load should not be moved before store if not comparable

Reviewed-by: kvn, thartmann

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PR: https://git.openjdk.org/jdk/pull/15864