Integrated: 8310190: C2 SuperWord: AlignVector is broken, generates misaligned packs

[Emanuel Peter]

On Thu, 6 Jul 2023 14:13:01 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> I want to push this in JDK23.
> After this fix here, I'm doing [this refactoring](https://github.com/openjdk/jdk/pull/16620).
> 
> To calm your nerves: most of the changes are in auto-generated tests, and tests in general.
> 
> **Context**
> 
> `-XX:+AlignVector` ensures that SuperWord only creates LoadVector and StoreVector that can be memory aligned. This is achieved by iterating in the pre-loop until we reach the alignment boundary, then we can start the main loop properly aligned. However, this is not possible in all cases, sometimes some memory accesses cannot be guaranteed to be aligned, and we need to reject vectorization (at least partially, for some of the packs).
> 
> Alignment is split into two tasks:
>  - Alignment Correctness Checks: only relevant if `-XX:+AlignVector`. Need to reject vectorization if alignment is not possible. We must check if the address of the vector load/store is aligned with (divisible by) `ObjectAlignmentInBytes`.
>  - Alignment by adjusting pre-loop limit: alignment is desirable even if `-XX:-AlignVector`. We would like to align the vectors with their vector width.
> 
> **Problem**
> 
> I have recently found a bug with our AlignVector [JDK-8310190](https://bugs.openjdk.org/browse/JDK-8310190).
> In that bug, we perform a misaligned memory vector access, which results in a `SIGBUS` on an ARM32 machine.
> Thanks @fg1417 for confirming this!
> Hence, we need to fix the alignment correctness checks.
> 
> While working on this task, I also found some bugs in the "alignment by adjusting pre-loop limit": there were cases where it did not align the vectors correctly.
> 
> **Problem Details**
> 
> Reproducer:
> 
> 
>     static void test(short[] a, short[] b, short mask) {
>         for (int i = 0; i < RANGE; i+=8) {
>             // Problematic for AlignVector
>             b[i+0] = (short)(a[i+0] & mask); // best_memref, align 0
> 
>             b[i+3] = (short)(a[i+3] & mask); // pack at offset 6 bytes
>             b[i+4] = (short)(a[i+4] & mask);
>             b[i+5] = (short)(a[i+5] & mask);
>             b[i+6] = (short)(a[i+6] & mask);
>         }
>     }
> 
> 
> During `SuperWord::find_adjacent_refs` we used to check if the references are expected to be aligned. For that, we look at each "group" of references (eg all `LoadS`) and take the reference with the lowest offset. For that chosen reference, we check if it is alignable. If yes, we accept all references of that group, if no we reject all.
> 
> This is problematic as shown in this example. We have references at index offset `0, 3, 4, 5, 6`, and by...

This pull request has now been integrated.

Changeset: 827c71da
Author:    Emanuel Peter <epeter at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/827c71dac9a5732f70bc7341743bce314cad302f
Stats:     8892 lines in 23 files changed: 7569 ins; 362 del; 961 mod

8310190: C2 SuperWord: AlignVector is broken, generates misaligned packs

Co-authored-by: Christian Hagedorn <chagedorn at openjdk.org>
Reviewed-by: kvn, chagedorn

-------------

PR: https://git.openjdk.org/jdk/pull/14785