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20997f6ad8
Use undef for uninitialized bytes in constants Fixes #83657 This generates good code when the const is fully uninit, e.g. ```rust #[no_mangle] pub const fn fully_uninit() -> MaybeUninit<[u8; 10]> { const M: MaybeUninit<[u8; 10]> = MaybeUninit::uninit(); M } ``` generates ```asm fully_uninit: ret ``` as you would expect. There is no improvement, however, when it's partially uninit, e.g. ```rust pub struct PartiallyUninit { x: u64, y: MaybeUninit<[u8; 10]> } #[no_mangle] pub const fn partially_uninit() -> PartiallyUninit { const X: PartiallyUninit = PartiallyUninit { x: 0xdeadbeefcafe, y: MaybeUninit::uninit() }; X } ``` generates ```asm partially_uninit: mov rax, rdi mov rcx, qword ptr [rip + .L__unnamed_1+16] mov qword ptr [rdi + 16], rcx movups xmm0, xmmword ptr [rip + .L__unnamed_1] movups xmmword ptr [rdi], xmm0 ret .L__unnamed_1: .asciz "\376\312\357\276\255\336\000" .zero 16 .size .L__unnamed_1, 24 ``` which copies a bunch of zeros in place of the undef bytes, the same as before this change. Edit: generating partially-undef constants isn't viable at the moment anyways due to #84565, so it's disabled |
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