Prune commentary and stale Rust port notes
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@@ -1,33 +1,3 @@
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//! Why a GPU test segfaults *after* it has passed, and what stops it.
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//!
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//! Measured here 2026-09-08, on this VM's Venus adapter. Destroying the
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//! last `VkInstance` makes the Vulkan loader `dlclose` the ICD; Mesa's
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//! ICD (`/usr/lib/libvulkan_virtio.so`) registers a `pthread_key_create`
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//! destructor pointing into its own text and is not linked `-z nodelete`,
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//! so glibc calls that destructor through unmapped memory when the thread
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//! that used Vulkan exits. libtest runs every `#[test]` on a spawned
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//! thread, which is why it looked like "wgpu crashes on drop": the drop
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//! itself completes, and the crash lands as the thread unwinds.
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//!
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//! The four modes are the experiment, and each is one variable:
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//!
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//! | mode | what it does | 2026-09-08 |
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//! |---|---|---|
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//! | `main` | wgpu instance + device on the main thread, dropped | exits 0 |
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//! | `thread` | the same on a spawned thread | **SIGSEGV** |
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//! | `keep` | the same, but the instance is never dropped | exits 0 |
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//! | `raw` | raw Vulkan (`ash`), instance + device, spawned thread | **SIGSEGV** |
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//!
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//! `raw` is the one that says whose bug it is: no wgpu is involved, so
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//! there is nothing for wgpu or a caller to fix in its drop order. `keep`
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//! is the fix -- hold one `wgpu::Instance` for the process, which is what
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//! wgpu asks for anyway. `iris/tests/mask_sdf.rs` does exactly that.
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//!
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//! `VK_LOADER_DISABLE_DYNAMIC_LIBRARY_UNLOADING=1` also makes every mode
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//! exit cleanly, which is the confirmation that the unload is the
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//! mechanism -- but it is an environment variable every caller would have
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//! to remember, so it belongs in this comment rather than in a script.
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use ash::vk;
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use pollster::block_on;
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use wgpu::*;
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@@ -42,12 +12,9 @@ fn main() {
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other => panic!("unknown mode {other:?}: main | thread | keep | raw"),
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};
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std::thread::spawn(body).join().expect("the spawned thread");
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// Not reached when the thread's exit takes the process with it.
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eprintln!("thread joined");
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}
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/// A wgpu instance and device, opened and closed. `keep_instance` is the
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/// fix under test: everything else still drops normally.
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fn wgpu_open_and_close(keep_instance: bool) {
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let instance = Instance::default();
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let adapter =
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@@ -71,8 +38,6 @@ fn wgpu_open_and_close(keep_instance: bool) {
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eprintln!("wgpu closed");
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}
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/// The same shape with no wgpu in it at all, which is what makes this a
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/// loader/driver bug rather than a wgpu one.
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fn raw_vulkan_open_and_close() {
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unsafe {
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let entry = ash::Entry::load().expect("vulkan loader");
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@@ -1,55 +1,13 @@
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//! Ask a device whether it can give iris the GPU it asks for.
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//!
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//! Until 2026-09-04 iris's renderer bound every texture it had drawn as one
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//! binding array and indexed it non-uniformly from the shader, which needed
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//! descriptor indexing and a very large per-stage binding-array limit
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//! (101,000 elements: 100,000 textures and 1,000 samplers,
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//! `UiLimits::default`). That was ordinary on a desktop and, per
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//! TEXTURES.md's "iris's binding array does not survive real Android
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//! hardware", not available on a real share of Android GPUs -- and it failed
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//! outright on this emulator's software Vulkan, which is what this rig
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//! caught first. iris now asks for nothing beyond wgpu's own defaults (see
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//! `iris/src/default/render.rs`): the glyph atlas is one `texture_2d_array`
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//! and a standalone image is its own ordinary bind group, and neither needs
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//! descriptor indexing. This rig still asks `request_device` for exactly
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//! what iris asks for, so it keeps being the answer to "does iris's actual
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//! device request succeed here" rather than a guess from reading the code.
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//!
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//! It runs as a plain executable with no window and no APK, because
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//! `request_adapter` needs no surface -- so it can be pushed to a device with
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//! `adb push` and run from `/data/local/tmp`, which is far cheaper than an
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//! app. What it therefore cannot answer is anything about presenting to a
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//! surface; that is the Android backend's own problem.
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mod vk;
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use wgpu::*;
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/// What `iris/src/default/render.rs` asks `request_device` for, now that the
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/// binding array is gone: nothing beyond wgpu's own default feature set.
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fn iris_features() -> Features {
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Features::empty()
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}
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/// The one non-default limit iris asks for -- unrelated to the binding array,
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/// kept for the big storage buffers behind rects/glyphs.
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const IRIS_MAX_BUFFER_SIZE: u64 = 1 << 30;
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/// Mirrors `iris_core::device_limits()` (`iris/core/src/render/mod.rs`) --
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/// cannot call it directly, since this rig is deliberately its own crate,
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/// not a workspace member (this file's own Cargo.toml comment). Keep the
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/// two in sync by hand when one changes; this rig's whole purpose is "does
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/// the device iris actually builds come back," so a stale copy here would
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/// silently stop answering that question. Zeroed rather than left at
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/// `Limits::default()`'s desktop-tier values because nothing in iris
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/// creates a `ComputePipeline` or a `@compute` shader stage -- found by
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/// grepping the whole `iris`/`iris-core` tree before this rig's comment was
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/// written -- and the unconditional default request is what crashed
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/// `request_device` on the Android emulator's software GL path
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/// (`EMU_GPU=software`, `force-gles`: SwiftShader's GL reports itself as
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/// OpenGL ES 3.0, which has no compute shaders at all, so the adapter's
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/// real limit is 0). The same would happen on a real GLES-3.0-only Android
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/// device.
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fn iris_limits() -> Limits {
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Limits {
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max_buffer_size: IRIS_MAX_BUFFER_SIZE,
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@@ -79,10 +37,6 @@ fn main() {
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" {:?} {} ({:?})",
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info.backend, info.name, info.device_type
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);
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// Compute is a *downlevel* capability, not a feature: Vulkan
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// grants it to any 1.0 device, and GLES only from ES 3.1. So
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// "can iris use a compute pass here" is this flag on every
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// adapter iris might fall back to, not just the preferred one.
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let down = adapter.get_downlevel_capabilities();
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let limits = adapter.limits();
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println!(
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@@ -143,11 +97,6 @@ fn main() {
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}
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);
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// The question that actually matters: does the device iris builds come
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// back, or does wgpu refuse it? With no features and no binding-array
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// limits requested, this is expected to succeed everywhere -- this rig
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// is what turned that from an assumption into a measurement, first on
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// this emulator's software Vulkan.
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let wanted = iris_limits();
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match pollster::block_on(adapter.request_device(&DeviceDescriptor {
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required_features: iris_features(),
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@@ -1,11 +1,3 @@
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//! The raw Vulkan half of the probe.
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//!
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//! wgpu reports a feature only after a chain of its own decisions -- which
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//! physical device, which API version, which extension list -- so "wgpu says
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//! no" and "the driver says no" are different claims. This asks
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//! `vkGetPhysicalDeviceFeatures2` itself and prints the inputs to that chain,
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//! so a disagreement can be attributed rather than guessed at.
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use ash::{Entry, vk};
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use std::ffi::CStr;
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@@ -29,9 +21,6 @@ pub fn report() {
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println!("\nraw vulkan:");
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println!(" loader instance version: {}", ver(instance_version));
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// Ask for the highest instance version the loader admits to: wgpu clamps
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// the device version by the instance's, so an instance created at 1.0
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// makes a 1.3 device look like 1.0.
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let app_info = vk::ApplicationInfo::default().api_version(instance_version);
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let create = vk::InstanceCreateInfo::default().application_info(&app_info);
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let instance = match unsafe { entry.create_instance(&create, None) } {
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