JDK-8314994 : C2: simplify the array cloning intrinsic implementation
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: compiler
  • Affected Version: 22
  • Priority: P4
  • Status: Open
  • Resolution: Unresolved
  • Submitted: 2023-08-25
  • Updated: 2023-08-28
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tbdUnresolved
Related Reports
Relates :  
JDK-8312749 - Generational ZGC: Tests crash with assert(index == 0 || is_power_of_2(index))
Relates :  
JDK-8315082 - [REDO] Generational ZGC: Tests crash with assert(index == 0 || is_power_of_2(index))
Description
Currently, the 'length' input of ArrayCopy nodes created from array cloning is expressed in double-words [1], instead of number of array elements as it could be intuitively expected. Furthermore, depending on the VM configuration (e.g. value of UseCompressedClassPointers), this double-word length might cover parts of the array header such as the array length field in order to align the start of the array copy to double words [2]. Arguably, these are array copy implementation details that should not be dealt with at ArrayCopy creation time.

A potentially simpler alternative, suggested by [~ayang], would be to (in his own words) "pass the actual array length (#slots) as length and move the merge-bytes-to-words-copying optimization to a lower level, e.g. inside conjoint_jbytes. Ofc, BarrierSetC2::clone_at_expansion and its derived siblings need to be adjusted accordingly, e.g. to use the actual elem-type." (see discussion in JDK-8312749 [3] for context).

[1] https://github.com/openjdk/jdk/blob/d02405917406a355a11741bb278ea58c3a4642fb/src/hotspot/share/gc/shared/c2/barrierSetC2.cpp#L678
[2] https://github.com/openjdk/jdk/blob/d02405917406a355a11741bb278ea58c3a4642fb/src/hotspot/share/gc/shared/c2/barrierSetC2.cpp#L650-L671
[3] https://github.com/openjdk/jdk/pull/15288