[Migrated] Large array variable initialization (`let arr = [0; 32 * 1024];`) compiles very slowly.

[rust-gpu-bot]

Issue automatically imported from old repo: EmbarkStudios/rust-gpu#1132
Old labels: t: bug
Originally creatd by pyranota on 2024-02-11T13:28:58Z

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EDIT(@eddyb): while there are reasons to avoid such arrays, the slowdown is the immediate Rust-GPU bug - I've left unchanged the original text, but used strikethrough for parts that don't apply, IMO.

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User should not be able to create large sized arrays in rust-gpu shaders.
Stack is limited, and large sized data-structures should be moved in buffers.
Plus it makes compilation incredibly slow (I assume this syntax [0; SIZE] translates to spirv's [0, 0, 0, 0, 0, ... SIZE])

Expected Behaviour

spirv-builder should throw an error, when user creates large array in stack

Example & Steps To Reproduce

1. Setup basic rust-gpu shader
2. let arr = [0; 32 * 1024];
3. cargo build

System Info

• Rust: rustc 1.75.0 (82e1608df 2023-12-21)
• OS: Pop!_OS
• GPU: nvidia 1060
• SPIR-V: SPIRV-Tools v2022.2-dev unknown hash, 2022-02-16T16:37:15

[rust-gpu-bot]

Comment from eddyb (CONTRIBUTOR) on 2024-02-20T12:14:25Z

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I suspect let mut array = [0; 32 * 1024]; is worse than something like:

let mut array = MaybeUninit::<[_; 32 * 1024]>::zeroed().assume_init();
// or `core::mem::zeroed`?

I think we have an optimization for e.g. memset(0), but not [expr; N].
(haven't checked the code though, but it could be as simple as using the better approach in more places)

[rust-gpu-bot]

Comment from pyranota on 2024-02-20T12:57:44Z

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Thank you for correction, @eddyb