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Upstream ILGPU Issues Analysis

Analysis of open issues in the original ILGPU repo to determine which bugs are inherited and fixable in SpawnDev.ILGPU v3.3.0.

Actionable Bugs (Testable & Potentially Fixable)

1. ✅ #1361 — MathF.CopySign argument order swapped on GPU — FIXED in v3.3.0
Severity High — silent wrong results
Affected All GPU backends (CUDA, OpenCL, WebGPU, WebGL)
Reproducible? Yes — simple kernel: CopySign(x, -1) should return -x
Root cause XMath.CopySign intrinsic passed (sign, magnitude) instead of (magnitude, sign) to the backend copysign instruction
Fix complexity Low — swapped the two arguments in the intrinsic mapping
Testable ✅ CopySignTest passes on all backends

2. ✅ #1309 — uint to float cast goes through double — FIXED in v3.3.0
Severity Medium — crashes on devices without fp64 support
Affected OpenCL devices without double precision (Intel integrated GPUs)
Reproducible? Yes — any (float)someUint cast in a kernel
Root cause IL conv.r.un + conv.r4 treated as uint→double→float instead of direct uint→float, emitting fp64 ops on devices that don't support them
Fix complexity Low — added direct uint→float conversion path in the IL-to-IR converter
Testable ✅ UintToFloatCastTest passes on all backends

3. ✅ #1479 — Infinite compilation with large local arrays — FIXED in v3.3.0
Severity High — 10+ minute compile, 10+ GB RAM for new int[1_000_000]
Affected All backends
Reproducible? Yes — any kernel with new int[N] where N is large
Root cause LowerArrays.Lower unrolled zero-initialization into N individual store IR nodes regardless of array size
Fix complexity Medium — added threshold (32 elements); small arrays keep unrolled stores, large arrays emit a proper IR loop
Testable ✅ All 366 existing tests pass across CUDA/OpenCL/CPU

4. ✅ #1538 — Internal Compiler Error with nested struct properties — FIXED in v3.3.0
Severity Medium — prevents kernel compilation
Affected All backends
Reproducible? Yes — deeply nested record struct parameters + static struct member access
Root cause StructureType.Slice used SliceRecursive/DirectFields to extract sub-spans, but type unification could merge types with different field orderings (e.g., {float, Vec3} vs {Vec3, float}), causing wrong slices
Fix complexity Medium — changed Slice to use flat Fields directly instead of SliceRecursive
Testable ✅ NestedStructICETest passes on all 8 backends (CPU, CUDA, OpenCL, WebGPU, WebGL, Wasm)

5. ✅ #1540 — H100/H200 not working — Already fixed in our fork
Severity High — H100/H200 GPUs crash immediately
Affected CUDA (Hopper architecture SM_90)
Reproducible? Only on H100/H200 hardware (we don't have this)
Root cause Original ILGPU 1.5.x didn't include SM_90 in architecture tables
Fix complexity Already fixed — our fork's CudaArchitecture.Generated.cs includes SM_90, SM_100, SM_101, SM_120
Testable Would need H100/H200 to verify, but code is clearly present

6. ✅ #1539 — OpenCL produces wrong results for complex kernels — FIXED in v3.3.0
Severity High — silent wrong results
Affected All OpenCL backends (not just AMD — reproduces on NVIDIA too)
Reproducible? Yes — BVH ray traversal kernel with while-loop stack-based traversal
Root cause intermediatePhiVariables dictionary in CLCodeGenerator.GenerateCodeInternal() persisted across blocks, causing stale intermediate phi variables from one block's phi swap to be incorrectly used as source values in a different block's phi bindings
Fix complexity Low (one-line fix) — added intermediatePhiVariables.Clear() at the start of each block's phi binding processing
Testable ✅ BVHRayTraversalTest passes on all backends (CPU, CUDA, OpenCL, WebGPU, WASM)

Not Actionable (For Us)

Issue Why Not
#1535 — .NET Standard 2.1 We target .NET 10, not relevant
#1359 — DebugInformationManager PDB loading exception — very environment-specific
#1263 — Radix sort too many resources Algorithm-specific, configuration-dependent. Likely Debug vs Release IL verbosity.
#1542, #1476, #1508 Questions, not bugs

Fix Summary

  1. #1361 (CopySign) ✅ Fixed — swapped argument order in PTX/OpenCL intrinsic
  2. #1309 (uint→float) ✅ Fixed — direct uint→float conversion without double intermediate
  3. #1479 (local array unrolling) ✅ Fixed — threshold-based loop for large arrays
  4. #1538 (struct ICE) ✅ Fixed — StructureType.Slice uses flat fields instead of SliceRecursive
  5. #1540 (H100/H200) ✅ Already fixed — SM_90+ architecture tables present in fork
  6. #1539 (OpenCL wrong results) ✅ Fixed — intermediatePhiVariables.Clear() per-block in OpenCL code generator