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

[pyranota]

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

[eddyb]

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)

[pyranota]

Thank you for correction, @eddyb