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Fix Metal row reductions on negative-stride views #4267

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zcbenz merged 2 commits into from
Aug 16, 2026
+12 −2
Merged

Fix Metal row reductions on negative-stride views #4267

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f06ca82
Fix Metal row reductions on negative-stride views
FU-max-boop Aug 15, 2026
2bf38e3
Run negative-stride reduction test on all backends
FU-max-boop Aug 16, 2026
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3 changes: 2 additions & 1 deletion mlx/backend/metal/kernels/reduction/reduce_row.h
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Expand Up @@ -337,7 +337,8 @@ template <

// lid.x * N_READS breaks the per_thread_row_reduce interface a bit. Maybe it
// needs a small refactor.
in += elem_to_loc<IdxT>(out_idx, shape, strides, ndim) + lid.x * N_READS;
in +=
elem_to_loc<IdxT>(out_idx, shape, strides, ndim) + IdxT(lid.x) * N_READS;

LoopedElemToLoc<NDIMS, IdxT, (NDIMS > 2)> loop(reduce_ndim);
const device T* row;
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11 changes: 11 additions & 0 deletions python/tests/test_reduce.py
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Expand Up @@ -47,6 +47,17 @@ def test_expand_sums(self):
np.allclose(z_npy, np.array(z_mlx), atol=1e-4)
)

@unittest.skipIf(not mx.metal.is_available(), "Metal is not available")
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def test_row_reduce_negative_stride(self):
x_npy = np.arange(1, 131).reshape(2, 65)[::-1]
x_mlx = mx.arange(1, 131).reshape(2, 65)[::-1]

for op in ["sum", "max", "min", "mean", "var"]:
with self.subTest(op=op):
expected = getattr(np, op)(x_npy, axis=-1)
actual = getattr(mx, op)(x_mlx, axis=-1, stream=mx.gpu)
self.assertTrue(np.allclose(expected, actual))

def test_dtypes(self):
int_dtypes = [
"int8",
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