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Skip ComplexF16 sparse tests on the LTS stack #760

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11 changes: 10 additions & 1 deletion docs/src/lts.md
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Original file line number Diff line number Diff line change
Expand Up @@ -155,11 +155,20 @@ This workaround is independent of `ONEAPI_LTS` and can be enabled on its own.
`cumsum` are unaffected. It is not reproducible on Xe-LP hardware with the same NEO 25.18 /
IGC 2.11 user-mode driver, so it looks specific to IGC's PVC code generation. There is no
workaround in oneAPI.jl yet.
- **`ComplexF16` sparse reductions.** With Julia 1.13, `sum(A; dims)` and other reductions
along a dimension of a `ComplexF16` sparse matrix return wrong real parts; the imaginary
parts are correct. IGC's load/store combining packs the complex accumulator into a layout
struct, and copying that struct emits a stray move that overwrites the real field with
another lane's data. This is not specific to PVC: the LTS IGC 2.11.7 produces the same
wrong results on DG2. It was fixed upstream in IGC 2.26.0 (and 2.25.10) by
[intel/intel-graphics-compiler@15614f2](https://github.com/intel/intel-graphics-compiler/commit/15614f288101c71739ff41d4e21cef6e4dad3e8e),
which has not been backported to the 2.11 LTS line (checked up to 2.11.54). Building the
module with `-igc_opts 'EnableLdStCombine=0'` avoids it.

## Running the test suite

The test suite reads the same switch and skips the cases the LTS stack cannot support (the
`BFloat16` element type, the BFloat16 example, `ComplexF32` in the accumulate tests). On a multi-tile node, pinning each test
`BFloat16` element type, the BFloat16 example, `ComplexF32` in the accumulate tests, `ComplexF16` in the sparse tests). On a multi-tile node, pinning each test
worker to its own tile avoids the oversubscription that provokes the dropped-tail
corruption:

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6 changes: 6 additions & 0 deletions test/runtests.jl
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Expand Up @@ -144,6 +144,12 @@ init_worker_code = quote
if oneL0.LTS[] && test === TestSuite.test_accumulate
filter!(!=(ComplexF32), typs)
end
# The LTS IGC (2.11) miscompiles the ComplexF16 accumulator of GPUArrays' sparse
# reductions with Julia 1.13, corrupting the real parts. Fixed upstream in IGC 2.26
# (intel/intel-graphics-compiler@15614f2), not in the LTS line; see docs/src/lts.md.
if oneL0.LTS[] && test === TestSuite.test_sparse
filter!(!=(ComplexF16), typs)
end
return typs
end

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