Malformed SPIRV after assignment to mutable reference

[FractalFir]

Issue originally found by rustlantis.

This reduced example:

#![no_std]
use spirv_std::spirv;
#[spirv(compute(threads(1)))]
pub fn fn1(#[spirv(storage_buffer, descriptor_set = 0, binding = 0)] mut addr: &mut f32) {
    addr = addr;
    (*addr) = 0.0;
}

Results in invalid SPIR-V:

error: malformed SPIR-V (in OpStore: undefined ID %33)
  |
  = note: while lowering SPIR-V module to SPIR-T (spirt::spv::lower)
  = note: input SPIR-V module was saved to /home/michal/rust-gpu/target/spirv-builder/spirv-unknown-vulkan1.1/release/deps/sky_shader.spirt-lower-from-spv-input.spv

[eddyb]

This was my minimal repro:

#[spirv(fragment)]
pub fn self_assignment(mut x: f32, y: &mut f32) {
    x = x;
    *y = x;
}

Crucially, the type of the self-assignment doesn't actually matter, the legacy mem2reg pass (which replaces variables with their contents, just like the LLVM mem2reg pass) is just buggy for this.

(My qptr replacement doesn't have this bug so I might be able to find what is wrong with the legacy mem2reg)

Also, I could've sworn I've seen this bug before, but maybe in the context of loops? (which made me assume it was a much more complicated issue)

---

Yupp this is the bug report I was thinking of:

• Malformed SPIR-V when compiling a simple loop EmbarkStudios/rust-gpu#1076

Can't believe none of us tried x = x; to see if it worked...