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Fix input/output scalar merging in compile #4658

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Merged
zcbenz merged 2 commits into from
Oct 9, 2026
Merged

Fix input/output scalar merging in compile #4658

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99b1116
Fix input/output scalar merging in compile
michalk8 Oct 8, 2026
ca0401f
nit
zcbenz Oct 9, 2026
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22 changes: 9 additions & 13 deletions mlx/compile.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -650,11 +650,16 @@ void compile_simplify(
return pa.is_equivalent(pb);
};

// Merge scalars
std::unordered_map<uintptr_t, array> output_map;
for (auto& o : outputs) {
output_map.insert({o.id(), o});
}

// Merge scalars, except outputs
std::vector<array> new_tape;
for (auto& arr : tape) {
// Check if we can merge scalars
if (is_scalar(arr)) {
if (is_scalar(arr) && output_map.find(arr.id()) == output_map.end()) {
auto scalar = scalars.find(get_scalar_rep(arr));
if (scalar->second.id() != arr.id()) {
merge(scalar->second, arr, parents_map);
Expand All @@ -668,10 +673,6 @@ void compile_simplify(

// Remove no-ops
{
std::unordered_map<uintptr_t, array> output_map;
for (auto& o : outputs) {
output_map.insert({o.id(), o});
}
for (auto& arr : tape) {
if (!arr.has_primitive() || !is_noop(arr.primitive())) {
new_tape.push_back(std::move(arr));
Expand All @@ -693,11 +694,6 @@ void compile_simplify(
tape_order.insert({tape[i].id(), i});
}

std::unordered_set<uintptr_t> output_set;
for (auto& o : outputs) {
output_set.insert(o.id());
}

// Multi-pass merge only keeping non-orphaned arrays in the tape
for (int pass = 0; pass < passes; ++pass) {
for (auto& arr : tape) {
Expand All @@ -717,7 +713,7 @@ void compile_simplify(
auto& src = parents->second[src_idx].first;
auto& dst = parents->second[dst_idx].first;
if (src.id() != dst.id() && array_equivalent(src, dst) &&
output_set.find(src.id()) == output_set.end()) {
output_map.find(src.id()) == output_map.end()) {
merge(dst, src, parents_map);
mask[src_idx] = true;
}
Expand Down Expand Up @@ -783,7 +779,7 @@ void compile_simplify(
}
return false;
} else {
return output_set.find(a.id()) == output_set.end();
return output_map.find(a.id()) == output_map.end();
}
};
bool discard = maybe_merge_parents(arr);
Expand Down
18 changes: 18 additions & 0 deletions python/tests/test_compile.py
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Original file line number Diff line number Diff line change
Expand Up @@ -765,6 +765,24 @@ def fn(x):
self.assertEqual(cfn(x4).shape, fn(x4).shape)
self.assertEqual(cfn(x8).shape, fn(x8).shape)

def test_compile_scalar_constant_output(self):
for shapeless in [False, True]:
fun = mx.compile(lambda x: (x + 1.0, mx.array(1.0)), shapeless=shapeless)
out, c = fun(mx.array(3.0))
self.assertEqual(out.item(), 4.0)
self.assertEqual(c.item(), 1.0)

s = mx.array(7.0)
out, c = mx.compile(lambda x: (x + 7.0, s))(mx.array([1.0, 2.0]))
self.assertEqual(out.tolist(), [8.0, 9.0])
self.assertEqual(c.item(), 7.0)

# The gradient is the seed constant of the backward pass
fun = mx.compile(mx.value_and_grad(lambda x: x + 1.0))
value, grad = fun(mx.array(2.0))
self.assertEqual(value.item(), 3.0)
self.assertEqual(grad.item(), 1.0)

def test_compile_with_constant(self):
# Test float
@partial(mx.compile)
Expand Down
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