Add GPU backend probe with automatic fallback for older GPUs

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Older GPUs without usable DirectX 12 Feature Level 12_0 or working
Vulkan support (e.g. legacy Intel iGPUs and other old parts), and old
GLES drivers whose shader compiler rejects iced's own shaders, crash
at startup or on the first drawing viewport: StagingBelt aborts in
Buffer::get_mapped_range (panic=abort, silent exit) on the DX12 path,
and shader compilation fails inside iced's gradient pipeline on the
GLES path. Selecting a working backend via WGPU_BACKEND before iced
boots avoids both classes.

- Probe each candidate backend (Windows: dx12,vulkan,gl; Linux:
  vulkan,gl) in an isolated --gpu-probe child: device request,
  staging-style mapped upload, render pipeline creation (its fragment
  shader reads a private vec4 array, catching old-Mesa compile
  failures), offscreen render and readback. First fully-passing
  backend wins; explicit --backend / WGPU_BACKEND / --safe-mode
  always win, and Windows keeps preferring DX12/Vulkan (#55).
- Software adapters (WARP/llvmpipe/SwiftShader) are the last resort
  before a best-effort GL attempt: slow but alive.
- The crash sentinel names the attempted backend so only implicated
  backends are skipped next launch.
- Winning probes report adapter limits; GPUs without shader storage
  buffers get the packed compatibility renderer automatically, with
  a startup command-line notice so the switch is never silent.

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This commit is contained in:

Karim Jerbi committed 2026-09-18 11:26:25 +01:00

1 parent 472bb9883d

commit 2b38f798c0

7 files changed

+1105 -15

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src/gpu_backend.rs

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