BBQueue backing storage is put in .data instead of .bss

[jonas-schievink]

See jonas-schievink/rubble#100

This is a regression from 0.3.0 to 0.4.0.

[jonas-schievink]

Hmm, https://github.com/rust-embedded/cortex-m-rt/issues/116 claims that MaybeUninit data will end up in .bss, but there was also rtic-rs/rtic#52 and rust-lang/rust#41315, which has apparently now been fixed, but maybe there is another rustc bug behind this?

[jamesmunns]

For reference, here is where I am explicitly zeroing the backing structure, and likely the source of why this is showing up in .data:

https://docs.rs/bbqueue/0.4.0/src/bbqueue/atomic.rs.html#127-128

[jamesmunns]

It's a little disappointing that the compiler isn't picking up that this is just zeroed in static (as bss) (or the optimizer decided that a memcpy is better than a memset)

[jamesmunns]

Also also for reference, I did try manually swapping the write_bytes call to a loop with a loop + ptr::write, and the generated code seemed the same, though I might need something more like https://github.com/japaric/r0/blob/master/src/lib.rs#L167-L176 to deter the optimizer, at the cost of a slower initialization.