Creating a nested MaybeUninit array unnecessarily calls memset

[abgros]

This function calls memset:

fn create_uninit_array() -> [[MaybeUninit<u8>; 4]; 200] {
    [[MaybeUninit::<u8>::uninit(); 4]; 200]
}

and this one doesn't:

fn create_uninit_array_fast() -> [MaybeUninit<u8>; 800] {
    [MaybeUninit::<u8>::uninit(); 800]
}

I would have expected the same assembly to be generated for both cases.

See on Godbolt

[saethlin]

This is pretty subtle. We only get the deoptimization if the inner array length is 2, 4, or 8. Because the deoptimization happens in SROA + EarlyCSE. The lowering we do here involves creating uninit alloca then storing from them repeatedly into a much larger allocation. SROA confuses the story then EarlyCSE for some reason replaces a store of an undef SSA value with a store of 0 instead of a const undef.