In attempting to read a [u8; 8] from an MMIO register (to re-compose into a native u64 from little-endian bytes), the read_volatile seems to always return zero. However, when attempting to read a u64 it works fine. To my mind, reading 8 bytes as an array and reading 8 bytes as an integer should be equivalent.

What could be causing this?

EDIT: It appears that in release mode, the behaviour of read_volatile is much more clear here. It effectively translates to 8 1-byte reads, which are then re-composed into a single register. This is very weird to me.

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