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Convert common sub-functions as common sub-expressions #2788

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127 changes: 125 additions & 2 deletions src/Nonlinear/parse.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -36,6 +36,77 @@ function parse_expression(::Model, ::Expression, x::Any, ::Int)
)
end

function _extract_subexpression!(expr::Expression, root::Int)
n = length(expr.nodes)
# The whole subexpression is continuous in the tape
first_out = first_value = last_value = nothing
for i in root:n
node = expr.nodes[i]
if i != root && node.parent < root
first_out = i
break
end
index = node.index
if node.type == NODE_VALUE
if isnothing(first_value)
first_value = node.index
last_value = first_value
else
last_value = node.index
end
index -= first_value - 1
end
expr.nodes[i] =
Node(node.type, index, i == root ? -1 : node.parent - root + 1)
end
if isnothing(first_out)
I = root:n
else
I = root:(first_out-1)
end
if isnothing(first_value)
V = nothing
else
V = first_value:last_value
end
if !isnothing(first_out)
for i in (last(I)+1):n
node = expr.nodes[i]
index = node.index
if node.type == NODE_VALUE && !isnothing(V)
@assert index >= last(V)
index -= length(V)
end
parent = node.parent
if parent > root
@assert parent > last(I)
parent -= length(I) - 1
end
expr.nodes[i] = Node(node.type, index, parent)
end
end
return I, V
end

function _extract_subexpression!(data::Model, expr::Expression, root::Int)
parent = expr.nodes[root].parent
I, V = _extract_subexpression!(expr, root)
subexpr =
Expression(expr.nodes[I], isnothing(V) ? Float64[] : expr.values[V])
push!(data.expressions, subexpr)
index = ExpressionIndex(length(data.expressions))
expr.nodes[root] = Node(NODE_SUBEXPRESSION, index.value, parent)
if length(I) > 1
deleteat!(expr.nodes, I[2:end])
if !isnothing(V)
deleteat!(expr.values, V)
end
else
@assert isnothing(V)
end
return index, I
end

function parse_expression(
data::Model,
expr::Expression,
Expand All @@ -46,7 +117,59 @@ function parse_expression(
while !isempty(stack)
parent_node, arg = pop!(stack)
if arg isa MOI.ScalarNonlinearFunction
_parse_without_recursion_inner(stack, data, expr, arg, parent_node)
if haskey(data.cache, arg)
subexpr = data.cache[arg]
if subexpr isa Tuple{Expression,Int}
_expr, _node = subexpr
subexpr, I = _extract_subexpression!(data, _expr, _node)
if expr === _expr
if parent_node > first(I)
@assert parent_node > last(I)
parent_node -= length(I) - 1
end
for i in eachindex(stack)
_parent_node = stack[i][1]
if _parent_node > first(I)
@assert _parent_node > last(I)
stack[i] =
(_parent_node - length(I) + 1, stack[i][2])
end
end
end
for (key, val) in data.cache
if val isa Tuple{Expression,Int}
__expr, __node = val
if _expr === __expr && __node > first(I)
if __node <= last(I)
data.cache[key] = (
data.expressions[subexpr.value],
__node - first(I) + 1,
)
else
data.cache[key] =
(__expr, __node - length(I) + 1)
end
end
end
end
data.cache[arg] = subexpr
end
parse_expression(
data,
expr,
subexpr::ExpressionIndex,
parent_node,
)
else
_parse_without_recursion_inner(
stack,
data,
expr,
arg,
parent_node,
)
data.cache[arg] = (expr, length(expr.nodes))
end
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So I think we can tidy up all of this, but I'm not opposed to this part necessarily. We could be cleverer in how we do this.

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Yes, this version should be working and tested so now it could be refactored :)

else
# We can use recursion here, because ScalarNonlinearFunction only
# occur in other ScalarNonlinearFunction.
Expand Down Expand Up @@ -82,7 +205,7 @@ function _parse_without_recursion_inner(stack, data, expr, x, parent)
parent = length(expr.nodes)
# Args need to be pushed onto the stack in reverse because the stack is a
# first-in last-out datastructure.
for arg in reverse(x.args)
for arg in Iterators.Reverse(x.args)
push!(stack, (parent, arg))
end
return
Expand Down
12 changes: 11 additions & 1 deletion src/Nonlinear/types.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -76,9 +76,10 @@ tree.
struct Expression
nodes::Vector{Node}
values::Vector{Float64}
Expression() = new(Node[], Float64[])
end

Expression() = Expression(Node[], Float64[])

function Base.:(==)(x::Expression, y::Expression)
return x.nodes == y.nodes && x.values == y.values
end
Expand Down Expand Up @@ -165,6 +166,11 @@ mutable struct Model
operators::OperatorRegistry
# This is a private field, used only to increment the ConstraintIndex.
last_constraint_index::Int64
# This is a private field, used to detect common subexpressions.
cache::Dict{
MOI.ScalarNonlinearFunction,
Union{ExpressionIndex,Tuple{Expression,Int}},
}
function Model()
return new(
nothing,
Expand All @@ -173,6 +179,10 @@ mutable struct Model
Float64[],
OperatorRegistry(),
0,
Dict{
MOI.ScalarNonlinearFunction,
Union{ExpressionIndex,Tuple{Expression,Int}},
}(),
)
end
end
Expand Down
88 changes: 88 additions & 0 deletions test/Nonlinear/Nonlinear.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -1446,6 +1446,94 @@ function test_intercept_ForwardDiff_MethodError()
return
end

function test_extract_subexpression()
model = Nonlinear.Model()
x = MOI.VariableIndex(1)
sub = MOI.ScalarNonlinearFunction(:^, Any[x, 3])
f = MOI.ScalarNonlinearFunction(:+, Any[sub, sub])
expr = Nonlinear.parse_expression(model, f)
display(expr.nodes)
@test expr == Nonlinear.Expression(
[
Nonlinear.Node(Nonlinear.NODE_CALL_MULTIVARIATE, 1, -1),
Nonlinear.Node(Nonlinear.NODE_SUBEXPRESSION, 1, 1),
Nonlinear.Node(Nonlinear.NODE_SUBEXPRESSION, 1, 1),
],
Float64[],
)
expected_sub = Nonlinear.Expression(
[
Nonlinear.Node(Nonlinear.NODE_CALL_MULTIVARIATE, 4, -1)
Nonlinear.Node(Nonlinear.NODE_MOI_VARIABLE, 1, 1)
Nonlinear.Node(Nonlinear.NODE_VALUE, 1, 1)
],
[3.0],
)
@test model.expressions == [expected_sub]
@test model.cache[sub] == Nonlinear.ExpressionIndex(1)

h = MOI.ScalarNonlinearFunction(:*, Any[2, sub, 1])
g = MOI.ScalarNonlinearFunction(:+, Any[sub, h])
expr = MOI.Nonlinear.parse_expression(model, g)
expected_g = Nonlinear.Expression(
[
Nonlinear.Node(Nonlinear.NODE_CALL_MULTIVARIATE, 1, -1)
Nonlinear.Node(Nonlinear.NODE_SUBEXPRESSION, 1, 1)
Nonlinear.Node(Nonlinear.NODE_CALL_MULTIVARIATE, 3, 1)
Nonlinear.Node(Nonlinear.NODE_VALUE, 1, 3)
Nonlinear.Node(Nonlinear.NODE_SUBEXPRESSION, 1, 3)
Nonlinear.Node(Nonlinear.NODE_VALUE, 2, 3)
],
[2.0, 1.0],
)
@test expr == expected_g
# It should have detected the sub-expressions that was the same as `f`
@test model.expressions == [expected_sub]
# This means that it didn't get to extract from `g`, let's also test
# with extraction by starting with an empty model

model = Nonlinear.Model()
MOI.Nonlinear.set_objective(model, g)
@test model.objective == expected_g
@test model.expressions == [expected_sub]
# Test that the objective function gets rewritten as we reuse `h`
# Also test that we don't change the parents in the stack of `h`
# by creating a long stack
prod = MOI.ScalarNonlinearFunction(:*, [h, x])
sum = MOI.ScalarNonlinearFunction(:*, [x, x, x, x, prod])
expr = Nonlinear.parse_expression(model, sum)
@test isempty(model.objective.values)
@test model.objective.nodes == [
Nonlinear.Node(Nonlinear.NODE_CALL_MULTIVARIATE, 1, -1),
Nonlinear.Node(Nonlinear.NODE_SUBEXPRESSION, 1, 1),
Nonlinear.Node(Nonlinear.NODE_SUBEXPRESSION, 2, 1),
]
@test model.expressions == [
expected_sub,
Nonlinear.Expression(
[
Nonlinear.Node(Nonlinear.NODE_CALL_MULTIVARIATE, 3, -1),
Nonlinear.Node(Nonlinear.NODE_VALUE, 1, 1),
Nonlinear.Node(Nonlinear.NODE_SUBEXPRESSION, 1, 1),
Nonlinear.Node(Nonlinear.NODE_VALUE, 2, 1),
],
[2.0, 1.0],
),
]
@test isempty(expr.values)
@test expr.nodes == [
Nonlinear.Node(Nonlinear.NODE_CALL_MULTIVARIATE, 3, -1),
Nonlinear.Node(Nonlinear.NODE_MOI_VARIABLE, 1, 1),
Nonlinear.Node(Nonlinear.NODE_MOI_VARIABLE, 1, 1),
Nonlinear.Node(Nonlinear.NODE_MOI_VARIABLE, 1, 1),
Nonlinear.Node(Nonlinear.NODE_MOI_VARIABLE, 1, 1),
Nonlinear.Node(Nonlinear.NODE_CALL_MULTIVARIATE, 3, 1),
Nonlinear.Node(Nonlinear.NODE_SUBEXPRESSION, 2, 6),
Nonlinear.Node(Nonlinear.NODE_MOI_VARIABLE, 1, 6),
]
return
end

end # TestNonlinear

TestNonlinear.runtests()
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