Missed optimization when using const-blocks for MaybeUninit arrays

[kmdreko]

Please see the assembly from these functions (Rust 1.85.0 --release):

use std::mem::MaybeUninit;

pub struct SmallVec<T> {
    pub len: usize,
    pub arr: [MaybeUninit<T>; 24],
}

#[unsafe(no_mangle)]
pub fn uninit_arr_via_const() -> SmallVec<String> {
    SmallVec { len: 0, arr: [const { MaybeUninit::uninit() }; 24] }
}

#[unsafe(no_mangle)]
pub fn uninit_arr_via_assume_init() -> SmallVec<String> {
    SmallVec { len: 0, arr: unsafe { MaybeUninit::uninit().assume_init() } }
}

uninit_arr_via_const:
	pushq	%rbx
	movq	%rdi, %rbx
	movl	$584, %edx
	xorl	%esi, %esi
	callq	*memset@GOTPCREL(%rip)
	movq	%rbx, %rax
	popq	%rbx
	retq

uninit_arr_via_assume_init:
	movq	%rdi, %rax
	movq	$0, 576(%rdi)
	retq

The const {} variant compiles with code that seems to zero-out the arr field. This should not be necessary and the other version (correctly) only initializes the len field.

This is particularly troublesome because the documentation in multiple places expresses that const-blocks are the preferred way to create a [MaybeUnint; _].

[saethlin]

As with most such examples, you can get the codegen you want with more const:

#[unsafe(no_mangle)]
pub fn uninit_arr_via_const() -> SmallVec<String> {
    SmallVec { len: 0, arr: const { [const { MaybeUninit::uninit() }; 24] } }
}

So it's nice that works, but I'll try to fix this properly.

[saethlin]

I'm not tagging this as LLVM because the bug here is our bug. We are giving the backend a loop for the assignment of [const { MaybeUninit::uninit() }; 24].

[kmdreko]

@saethlin I'm truly impressed by your swiftness and precision. The fix works like a charm! Just wanted to share my appreciation, keep it up! 😊