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Update find_concat_same_inputs to handle dimension > 1 #5142

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27bfbcf
Update find_concat_same_inputs to handle dimension > 1
pfultz2 Aug 16, 2026
bc737c6
Simplify
pfultz2 Aug 16, 2026
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35 changes: 20 additions & 15 deletions src/simplify_algebra.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1095,11 +1095,9 @@ struct find_concat_op
}
};

// Collapse `concat(x, x, ..., x)` (N copies of the same instruction) into a
// single `multibroadcast` when the concat axis has length 1 in the source
// tensor. This is the common shape that shows up in MoE / KV-cache / RoPE
// expansion code where a tensor is replicated N times along an axis. The
// rewrite turns an O(output_size) memcpy into a strided view.
// Rewrite `concat(x, x, ..., x)` (N copies of the same instruction) as a
// broadcast-based tiling of x. This pattern shows up in MoE / KV-cache /
// RoPE expansion code where a tensor is replicated N times along an axis.
struct find_concat_same_input
{
auto matcher() const { return match::name("concat")(match::same_inputs()); }
Expand All @@ -1119,18 +1117,25 @@ struct find_concat_same_input
if(axis < 0 or axis >= lens.size())
return;

// Safe (no data movement) case: the concat axis is size 1 in the
// source, so it can be broadcast to N. The general lens[axis] > 1
// case requires unsqueeze + multGibroadcast + reshape and is left
// to a follow-up matcher.
if(lens[axis] != 1)
return;
const auto& out_lens = ins->get_shape().lens();

auto out_lens = lens;
out_lens[axis] = inputs.size();
assert(out_lens == ins->get_shape().lens());
// The concat axis is size 1 in the source, so replicating it is a
// strided view with no data movement.
if(lens[axis] == 1)
{
m.replace_instruction(ins, make_op("multibroadcast", {{"out_lens", out_lens}}), x);
return;
}

m.replace_instruction(ins, make_op("multibroadcast", {{"out_lens", out_lens}}), x);
// General case: tile the axis by unsqueezing a unit dim before it,
// broadcasting that dim to N, then folding it back into the axis.
auto unsqueezed = m.insert_instruction(ins, make_op("unsqueeze", {{"axes", {axis}}}), x);
auto bcast_lens = unsqueezed->get_shape().lens();
assert(bcast_lens[axis] == 1);
bcast_lens[axis] = inputs.size();
auto bcast = m.insert_instruction(
ins, make_op("multibroadcast", {{"out_lens", bcast_lens}}), unsqueezed);
m.replace_instruction(ins, make_op("reshape", {{"dims", out_lens}}), bcast);
}
};

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71 changes: 63 additions & 8 deletions test/simplify_algebra_test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3130,11 +3130,15 @@ TEST_CASE(simplify_dot_horiz_same_constant)

migraphx::module m2;
{
auto input = m2.add_parameter("input", s);
auto a = m2.add_literal(migraphx::generate_literal(s, 0));
auto concat = m2.add_instruction(migraphx::make_op("concat", {{"axis", 2}}), a, a);
auto dot = m2.add_instruction(migraphx::make_op("dot"), input, concat);
auto x = m2.add_instruction(
auto input = m2.add_parameter("input", s);
auto a = m2.add_literal(migraphx::generate_literal(s, 0));
auto unsqueeze = m2.add_instruction(migraphx::make_op("unsqueeze", {{"axes", {2}}}), a);
auto bcast = m2.add_instruction(
migraphx::make_op("multibroadcast", {{"out_lens", {3, 2, 2, 2}}}), unsqueeze);
auto reshape =
m2.add_instruction(migraphx::make_op("reshape", {{"dims", {3, 2, 4}}}), bcast);
auto dot = m2.add_instruction(migraphx::make_op("dot"), input, reshape);
auto x = m2.add_instruction(
migraphx::make_op("slice", {{"axes", {2}}, {"starts", {0}}, {"ends", {2}}}), dot);
auto y = m2.add_instruction(
migraphx::make_op("slice", {{"axes", {2}}, {"starts", {2}}, {"ends", {4}}}), dot);
Expand Down Expand Up @@ -5743,9 +5747,60 @@ TEST_CASE(simplify_concat_same_input_axis_not_one)

migraphx::module m2;
{
auto x = m2.add_parameter("x", s);
auto concat = m2.add_instruction(migraphx::make_op("concat", {{"axis", 1}}), x, x);
m2.add_return({concat});
auto x = m2.add_parameter("x", s);
auto unsqueeze = m2.add_instruction(migraphx::make_op("unsqueeze", {{"axes", {1}}}), x);
auto bcast = m2.add_instruction(
migraphx::make_op("multibroadcast", {{"out_lens", {2, 2, 4, 4}}}), unsqueeze);
auto reshape =
m2.add_instruction(migraphx::make_op("reshape", {{"dims", {2, 8, 4}}}), bcast);
m2.add_return({reshape});
}
EXPECT(m1 == m2);
}

TEST_CASE(simplify_concat_same_input_tile_axis_one_not_last)
{
auto s = migraphx::shape{migraphx::shape::float_type, {2, 3, 5}};
migraphx::module m1;
{
auto x = m1.add_parameter("x", s);
auto concat = m1.add_instruction(migraphx::make_op("concat", {{"axis", 1}}), x, x, x);
m1.add_return({concat});
}
run_pass(m1);

migraphx::module m2;
{
auto x = m2.add_parameter("x", s);
auto unsqueeze = m2.add_instruction(migraphx::make_op("unsqueeze", {{"axes", {1}}}), x);
auto bcast = m2.add_instruction(
migraphx::make_op("multibroadcast", {{"out_lens", {2, 3, 3, 5}}}), unsqueeze);
auto reshape =
m2.add_instruction(migraphx::make_op("reshape", {{"dims", {2, 9, 5}}}), bcast);
m2.add_return({reshape});
}
EXPECT(m1 == m2);
}

TEST_CASE(simplify_concat_same_input_tile_axis_zero)
{
auto s = migraphx::shape{migraphx::shape::float_type, {3, 2}};
migraphx::module m1;
{
auto x = m1.add_parameter("x", s);
auto concat = m1.add_instruction(migraphx::make_op("concat", {{"axis", 0}}), x, x, x);
m1.add_return({concat});
}
run_pass(m1);

migraphx::module m2;
{
auto x = m2.add_parameter("x", s);
auto unsqueeze = m2.add_instruction(migraphx::make_op("unsqueeze", {{"axes", {0}}}), x);
auto bcast = m2.add_instruction(
migraphx::make_op("multibroadcast", {{"out_lens", {3, 3, 2}}}), unsqueeze);
auto reshape = m2.add_instruction(migraphx::make_op("reshape", {{"dims", {9, 2}}}), bcast);
m2.add_return({reshape});
}
EXPECT(m1 == m2);
}
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