Fixed Android 11 incompatibility with ValueSlot ptr tagging #104
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The prior size optimization in ValueSlot used the high byte of the `_pointer` field to distinguish between an actual pointer and an inline 7-bit Value; it assumed if the upper byte is zero then it's a pointer. This isn't true anymore in Android: "Starting in Android 11, for 64- bit processes, all heap allocations have an implementation defined tag set in the top byte of the pointer on devices with kernel support for ARM Top-byte Ignore (TBI)." Switched to using the _low_ byte as a tag, with a value of 0xFF denoting inline, and anything else being a pointer. This works because any Value* we store there must have at least one zero bit in the LSB; see the comment in ValueSlot.hh for details.
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Tested on macOS, needs testing on ARM obviously, esp. with Android 11! |
bmeike
approved these changes
Aug 4, 2021
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OK, I'll update the Fleece submodule in LiteCore now. |
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The prior size optimization in ValueSlot used the high byte of the
_pointerfield to distinguish between an actual pointer and aninline 7-bit Value; it assumed if the upper byte is zero then it's a
pointer.
This isn't true anymore in Android: "Starting in Android 11, for 64-
bit processes, all heap allocations have an implementation defined tag
set in the top byte of the pointer on devices with kernel support for
ARM Top-byte Ignore (TBI)."
Switched to using the low byte as a tag, with a value of 0xFF
denoting inline, and anything else being a pointer. This works because
any Value* we store there must have at least one zero bit in the
LSB; see the comment in ValueSlot.hh for details.