Integrated: 8347006: LoadRangeNode floats above array guard in arraycopy intrinsic

[Tobias Hartmann]

On Wed, 8 Jan 2025 12:07:16 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> C2's arraycopy intrinsic adds guards that check that the source and destination objects are arrays:
> https://github.com/openjdk/jdk/blob/afe543414f58a04832d4f07dea88881d64954a0b/src/hotspot/share/opto/library_call.cpp#L5917-L5919
> 
> If these guards pass, the array length is loaded:
> https://github.com/openjdk/jdk/blob/afe543414f58a04832d4f07dea88881d64954a0b/src/hotspot/share/opto/library_call.cpp#L5930-L5933
> 
> But since the `LoadRangeNode` is not pinned, it might float above the array guard:
> https://github.com/openjdk/jdk/blob/afe543414f58a04832d4f07dea88881d64954a0b/src/hotspot/share/opto/graphKit.cpp#L1214
> 
> If the object is not an array, we will read garbage. That's usually fine because the result will not be used (the array guard will trigger) but with `-XX:+UseCompactObjectHeaders` it can happen that the memory right after the header is not mapped and we crash.
> 
> The fix is to add a `CheckCastPPNode` to propagate the information that the operand is an array and prevent the load from floating. 
> 
> Thanks to @shipilev for identifying the root cause!
> 
> I was able to reliably reproduce the issue with `compiler/arraycopy/TestArrayCopyNoInit.java` and `-XX:-UseTLAB -XX:+UnlockExperimentalVMOptions -XX:+UseCompactObjectHeaders` on Linux AArch64 and verified that the fix solves the problem.
> 
> Best regards,
> Tobias

This pull request has now been integrated.

Changeset: 82e2a791
Author:    Tobias Hartmann <thartmann at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/82e2a791225a289ba32360bf415274c4b48b9e00
Stats:     58 lines in 5 files changed: 14 ins; 0 del; 44 mod

8347006: LoadRangeNode floats above array guard in arraycopy intrinsic

Reviewed-by: roland, qamai, kvn

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PR: https://git.openjdk.org/jdk/pull/22967