Primitive boolean array packing

Roman Kennke rkennke at
Sun Oct 7 18:29:33 UTC 2018

IIRC, some CPUs even have instructions for directly getting/setting
bits, without explicit mask/shift. AArch64 can do that I think. Not sure
if it's more efficient though.


> On 10/07/2018 04:13 PM, Aleksey Shipilev wrote:
>> On 10/07/2018 02:38 PM, B. Blaser wrote:
>>> Thanks Aleksey, you're absolutely right but a programmer can still
>>> disable this feature and use regular boolean arrays if necessary.
>> I fully expect the JVM feature/flag what optionally breaks the Java
>> specification will be flat out rejected.
> It doesn't break the Java specification:
>     Notes
>     The baload instruction is used to load values from both byte and
>     boolean arrays. In Oracle's Java Virtual Machine implementation,
>     boolean arrays - that is, arrays of type T_BOOLEAN (§2.2,
>     §newarray) - are implemented as arrays of 8-bit values. Other
>     implementations may implement packed boolean arrays; the baload
>     instruction of such implementations must be used to access those
>     arrays.
>> My educated guess from the subword atomic operations over
>> VarHandles: the local latency of uncontended CAS would still be too
>> significant to ignore. Over years, we hoped CAS overhead would
>> become negligible, but it still isn't
> Probably so. Numbers would be nice.

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