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Guard scatter_axis against 64-bit outputs on the GPU #3695

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Guard scatter_axis against 64-bit outputs on the GPU #3695

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6 changes: 6 additions & 0 deletions mlx/backend/metal/indexing.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -519,6 +519,12 @@ void GatherAxis::eval_gpu(const std::vector<array>& inputs, array& out) {
}

void ScatterAxis::eval_gpu(const std::vector<array>& inputs, array& out) {
if (size_of(out.dtype()) == 8) {
std::ostringstream msg;
msg << "[ScatterAxis::eval_gpu] Does not support " << out.dtype();
throw std::invalid_argument(msg.str());
}

auto& src = inputs[0];
auto& idx = inputs[1];
auto& upd = inputs[2];
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8 changes: 8 additions & 0 deletions mlx/ops.cpp
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Expand Up @@ -3529,6 +3529,14 @@ array scatter_axis(
return a;
}

// TODO, remove when scatter_axis supports 64-bit outputs
if (to_stream(s).device == Device::gpu && size_of(a.dtype()) == 8) {

@zcbenz zcbenz Jun 17, 2026

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The check in metal/indexing.cpp along is enough, and it actually works in the cuda backend, the test should also be updated for metal only

std::ostringstream msg;
msg << prefix << " GPU scatter does not yet support " << a.dtype()
<< " for the input or updates.";
throw std::invalid_argument(msg.str());
}

auto upd = astype(values, a.dtype(), s);

// Squeeze leading singletons out of update
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14 changes: 14 additions & 0 deletions python/tests/test_ops.py
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Expand Up @@ -1353,6 +1353,20 @@ def test_put_along_axis(self):
self.assertEqual(b.size, 0)
self.assertEqual(b.shape, a.shape)

# 64-bit outputs are not supported on the GPU and should raise a clean
# error rather than failing the Metal JIT build.
idx = mx.array([[0], [1], [2], [3]])
for dt in (mx.int64, mx.uint64):
x = mx.zeros((4, 8), dtype=dt)
upd = mx.ones((4, 1), dtype=dt)
if mx.default_device() == mx.gpu:
with self.assertRaises(ValueError):
mx.eval(mx.put_along_axis(x, idx, upd, axis=1))
else:
out = mx.put_along_axis(x, idx, upd, axis=1)
self.assertEqual(out.dtype, dt)
mx.eval(out)

def test_split(self):
a = mx.array([1, 2, 3])
splits = mx.split(a, 3)
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