https://github.com/user-attachments/assets/c23471cb-4ae8-423b-a1cb-7ea0003b4811

Software information

EVE Online (CCP, Carbon engine), DX12 mode. The issue triggers when opening certain new-UI-engine (Photon-UI) windows (e.g., character sheet, starmap, etc.). In-game shader quality LOW works around the issue; medium or higher reproduces it. Reverting vkd3d-proton to v2.11 also eliminates the problem.

System information

• GPU: GeForce RTX 5060Ti
• Driver: 595.80
• Wine version: Proton Experimental 11.0 (wine-proton-exp-11.0-amd64) / also tested with vanilla Wine builds
• VKD3D-Proton version: v2.12 and later (v2.11 is GOOD)

Log files

• warn2.11-1.log (v2.11, GOOD — shows shader rejection)
• warn2.14-1.log (v2.14, BAD — shader accepted silently)
• warn-debug-1.log (v2.14 + ICB diagnostic patch, shows the large ICB shader)
• Problematic shader binaries: 5d529c6772b51d66.dxbc + 5d529c6772b51d66.spv: 5d529c6772b51d66.tar.gz

Root cause (bisected with AI assistance)

I bisected the 175 commits between v2.11 and v2.12 using binary search, with builds compiled by my AI assistant (Hermes Agent). The trigger commit is:

6fe77929 — "vkd3d-shader: Fix maximum ICB size" (Philip Rebohle, 2024-01-29)

This commit corrected MAX_IMMEDIATE_CONSTANT_BUFFER_SIZE from 4096 dwords to 16384 dwords (the correct D3D12 spec limit: 4096 vec4 × 4).

Before the fix (v2.11):
EVE loads a compute shader (SM 5.1) with an ICB of 10096 dwords (2524 vec4). The old limit rejects it:

Unexpected immediate constant buffer size 10096.
Encountered unrecognized or invalid instruction.
Failed to compile shader, vkd3d result -3.

EVE silently falls back to an alternative shader path → normal performance.

After the fix (v2.12+):
The same shader's 10096-dword ICB is now accepted, but the generated SPIR-V is catastrophically inefficient. The ICB is placed in Private storage class as a 2524-element OpConstantComposite initializer:

%icb = OpVariable %_ptr_Private__arr_v4float_uint_2524 Private %11820

Each thread gets its own copy. With 64 threads/workgroup (the shader uses LocalSize 64 1 1), that's ~2.58 MB of private memory per workgroup, all spilling to VRAM. This saturates GPU memory bandwidth and causes the observed FPS collapse (100+ → ~20 FPS, GPU 100%).

Why this matters beyond EVE

The 6fe77929 fix is correct for D3D12 spec compliance. The bug is that vkd3d-shader's codegen for large ICBs is pathologically inefficient — it should place ICB data in a Uniform Buffer or StorageBuffer rather than per-thread Private memory when the ICB exceeds a reasonable threshold (e.g., >256 vec4). Other games with large ICB compute shaders are likely affected but may not trigger the shader frequently enough for users to notice.