Gpu::new built the 27 compute pipelines one after another. On DX12 each one is compiled by FXC
(issue #471), single-threaded, inside create_compute_pipeline, so every process waited ~2.2 s for
them on an Intel Core Ultra 7 165U iGPU (app.gpu 2.4 s median) while the other cores idled.
build_kernels creates the shader modules and layouts on the calling thread and shares the pipelines
out to worker threads (std::thread::scope, an atomic job counter; the calling thread works too, so
every kernel is built even when no thread can be started). wgpu's error scopes are per thread, so
each worker pushes its own validation and internal scopes: a refused kernel or a panicked worker
still means no GPU, never a half-built one. app.gpu: 2.4 s -> 1.2 s median on that machine, exports
pixel-identical. The gpu: info line now reports the kernel count and build time.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018QWU3BMJMENFqFqxDHzV3b
