SPIR-T panics with assertion failed: list_eq_key(children) ==\n list_eq_key(self.func_def_body.at(unstructured_region).def().children).

[Schell Carl Scivally]

While debugging an issue with nested loops I simplified my shader to one loop as a sanity check and then hit this compiler panic. It seems to be caused by looping indefinitely (in my shader I forgot to increment a value used in the break condition).

Expected Behaviour

Compiler should error instead of panic.

Actual Behavior

Compiler panics with:

thread 'rustc' panicked at 'assertion failed: list_eq_key(children) ==\n    list_eq_key(self.func_def_body.at(unstructured_region).def().children)',
  /Users/schell/.cargo/registry/src/index.crates.io-6f17d22bba15001f/spirt-0.2.0/src/cfg.rs:1214:13

Example & Steps To Reproduce

Compile this shader:

use glam::{Vec3, Vec4, Vec4Swizzles, Vec3Swizzles};
use spirv_std::{image::Cubemap, Sampler, spirv};

#[cfg(target_arch = "spirv")]
use spirv_std::num_traits::Float;

#[spirv(fragment)]
pub fn fragment_convolve_diffuse_irradiance(
    environment_texture: &Cubemap,
    sampler: &Sampler,
    local_pos: Vec3,
    frag_color: &mut Vec4,
) {
    let pi: f32 = 3.1415927;

    let normal = local_pos.normalize_or_zero();
    let mut irradiance = Vec3::new(0.0, 0.0, 0.0);
    let right = Vec3::new(0.0, 1.0, 0.0).cross(normal).normalize_or_zero();
    let up = normal.cross(right).normalize_or_zero();

    let nr_samples = 0.0;
    irradiance += environment_texture.sample(*sampler, up).xyz();
    loop {
        if nr_samples > 2.0 {
            break;
        }

        irradiance += environment_texture.sample(*sampler, up).xyz();
    }
    let color = irradiance * (pi / nr_samples);
    *frag_color = color.xyz().extend(1.0);
}

System Info

note: rustc 1.70.0-nightly (84dd17b56 2023-04-14) running on aarch64-apple-darwin

note: compiler flags: --crate-type dylib -C opt-level=3 -C embed-bitcode=no -C codegen-units=1 -Z unstable-options -Z codegen-backend=/Users/schell/code/renderling/shaders/target/release/librustc_codegen_spirv.dylib -Z binary-dep-depinfo -C symbol-mangling-version=v0 -Z crate-attr=feature(register_tool) -Z crate-attr=register_tool(rust_gpu) -C llvm-args=--module-output=multiple

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
note: rust-gpu version 0.8.0

• Rust: [e.g. 1.49.0-nightly (1eaadebb3 2020-10-21)]
• OS: [e.g. macOS 10.15.7]
• GPU: [e.g. Intel(R) UHD Graphics 630]
• SPIR-V: [e.g. v2020.3 unknown hash, 2020-06-12T01:06:18]

Backtrace

Backtrace

stack backtrace:
   0:        0x102eb5894 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hec12b20601f2bdf0
   1:        0x102f089a0 - core::fmt::write::h84e52a09c8a95be7
   2:        0x102eab344 - std::io::Write::write_fmt::h694b0891aa3b64d5
   3:        0x102eb56a8 - std::sys_common::backtrace::print::hdefc03f01b0d83f1
   4:        0x102eb80bc - std::panicking::default_hook::{{closure}}::hc4c0bdc7bd459aa9
   5:        0x102eb7e7c - std::panicking::default_hook::h6dc5d3ac0202ebe6
   6:        0x10470d6f4 - rustc_codegen_spirv::__rustc_codegen_backend::{{closure}}::h6edd25957ff152e7
   7:        0x102eb877c - std::panicking::rust_panic_with_hook::h04718b2e9bd107f5
   8:        0x102eb8534 - std::panicking::begin_panic_handler::{{closure}}::h1c251c8e7c7e5969
   9:        0x102eb5cb4 - std::sys_common::backtrace::__rust_end_short_backtrace::hc2f6697e294cde53
  10:        0x102eb8308 - _rust_begin_unwind
  11:        0x102f34420 - core::panicking::panic_fmt::hcabee496664f2172
  12:        0x102f34490 - core::panicking::panic::hc56ef5c96c7e745b
  13:        0x1049027e8 - spirt::cfg::Structurizer::structurize_func::h8ccbca14134c4bed
  14:        0x10488ac74 - spirt::passes::legalize::structurize_func_cfgs::h5a9bb2f8c14010b3
  15:        0x1046b6974 - rustc_codegen_spirv::linker::link::h666c87acc0f36829
  16:        0x10465bae0 - rustc_codegen_spirv::link::link::h87c14272e01e3168
  17:        0x10470bf54 - <rustc_codegen_spirv::SpirvCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::link::ha3f63258b51407e5
  18:        0x10b47f7f8 - <rustc_interface[b2f1c94be2f1d7cc]::queries::Linker>::link
  19:        0x10b3b3da4 - rustc_span[dc18a910ba5122f1]::set_source_map::<core[723a1ab69fb7b501]::result::Result<(), rustc_span[dc18a910ba5122f1]::ErrorGuaranteed>, rustc_interface[b2f1c94be2f1d7cc]::interface::run_compiler<core[723a1ab69fb7b501]::result::Result<(), rustc_span[dc18a910ba5122f1]::ErrorGuaranteed>, rustc_driver_impl[27bd541b75fbd8b5]::run_compiler::{closure#1}>::{closure#0}::{closure#0}>
  20:        0x10b3c7184 - std[b04369226ecd3a5b]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[b2f1c94be2f1d7cc]::util::run_in_thread_pool_with_globals<rustc_interface[b2f1c94be2f1d7cc]::interface::run_compiler<core[723a1ab69fb7b501]::result::Result<(), rustc_span[dc18a910ba5122f1]::ErrorGuaranteed>, rustc_driver_impl[27bd541b75fbd8b5]::run_compiler::{closure#1}>::{closure#0}, core[723a1ab69fb7b501]::result::Result<(), rustc_span[dc18a910ba5122f1]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[723a1ab69fb7b501]::result::Result<(), rustc_span[dc18a910ba5122f1]::ErrorGuaranteed>>
  21:        0x10b3b8364 - <<std[b04369226ecd3a5b]::thread::Builder>::spawn_unchecked_<rustc_interface[b2f1c94be2f1d7cc]::util::run_in_thread_pool_with_globals<rustc_interface[b2f1c94be2f1d7cc]::interface::run_compiler<core[723a1ab69fb7b501]::result::Result<(), rustc_span[dc18a910ba5122f1]::ErrorGuaranteed>, rustc_driver_impl[27bd541b75fbd8b5]::run_compiler::{closure#1}>::{closure#0}, core[723a1ab69fb7b501]::result::Result<(), rustc_span[dc18a910ba5122f1]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[723a1ab69fb7b501]::result::Result<(), rustc_span[dc18a910ba5122f1]::ErrorGuaranteed>>::{closure#1} as core[723a1ab69fb7b501]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  22:        0x102ec0e24 - std::sys::unix::thread::Thread::new::thread_start::h5343322b5f9311e3
  23:        0x18df97fa8 - __pthread_joiner_wake

[Eduard-Mihai Burtescu]

I've edited the description and title to explicitly mention the message.

This is a "known" panic I have yet to investigate because so far I've only hit it with RUSTGPU_CODEGEN_ARGS=--no-early-report-zombies, which can let some malformed SPIR-V rustc_codegen_spirv generates through, so I just assumed that was the cause here.

If it can happen without RUSTGPU_CODEGEN_ARGS, then this deserves a lot more scrutiny. I'll attempt to minimize the testcase provided (is it complete though? there's no #[spirv(...)], so that's just dead code? EDIT: was just a missing #[spirv(fragment)], added that to the issue description, can confirm it repros).

[Eduard-Mihai Burtescu]

This reduces somewhat bizarrely:

use spirv_std::spirv;

#[inline(never)]
fn bug(cond: bool) {
    if (if cond { true } else { false }) {}
    loop {}
}

#[spirv(fragment)]
pub fn main() {
    bug(false);
}

I'll have to move --dump-spirt-passes into a helper type that implements Drop so panics don't stop it from showing useful information (in general it seems sensible to do that sort of thing, just because removing 100% of possible panics is hard, esp. before we have a SPIR-T validator).

[Eduard-Mihai Burtescu]

Starting to suspect it's in part caused by this structurizer special-casing, which has a FIXME:

            // Structured return, the function is fully structurized.
            //
            // FIXME(eddyb) also support structured return when the whole body
            // is divergent, by generating undef constants (needs access to the
            // whole `FuncDecl`, not just `FuncDefBody`, to get the right types).

I forgot about this since, because it apparently never(?) caused any issues in practice, but e.g. "always reach an infinite loop" Rust-GPU shaders wouldn't end up 100% structured - my best guess for why spirv-val doesn't complain is that the actual body is structured, there's just a single unstructured branch at the very start, which SPIR-V doesn't care about ("structured SPIR-V" only puts requirements on merging selects/loops).