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fix: properly handle aliasing elements of unscalarized array variables #4533

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Merged
AayushSabharwal merged 1 commit into from
May 14, 2026
+30 −4
Merged

fix: properly handle aliasing elements of unscalarized array variables #4533

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24 changes: 20 additions & 4 deletions src/systems/alias_elimination.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -145,6 +145,7 @@ function find_perfect_aliases!(
c1 == c2 && push!(eqs_to_rm, ieq)
end

eqs_to_substitute = Int[]
for (root, group_vars) in alias_sets
target = group_target[root]
state.always_present[target] = true
Expand All @@ -165,8 +166,7 @@ function find_perfect_aliases!(
dnbors = copy(𝑑neighbors(graph, v))
for e in dnbors
snbors = 𝑠neighbors(graph, e)
eqs[e] = substitute(eqs[e], subs)
original_eqs[e] = substitute(original_eqs[e], subs)
push!(eqs_to_substitute, e)
Graphs.rem_edge!(graph, e, v)
Graphs.add_edge!(graph, e, target)
end
Expand All @@ -185,8 +185,7 @@ function find_perfect_aliases!(
dnbors = copy(𝑑neighbors(graph, dv))
for e in dnbors
snbors = 𝑠neighbors(graph, e)
eqs[e] = substitute(eqs[e], subs)
original_eqs[e] = substitute(original_eqs[e], subs)
push!(eqs_to_substitute, e)
Graphs.rem_edge!(graph, e, dv)
Graphs.add_edge!(graph, e, dtarget)
end
Expand All @@ -197,6 +196,23 @@ function find_perfect_aliases!(
end
end

# We need to handle unscalarized array variables
for k in keys(subs)
k, isarr = split_indexed_var(k)
isarr || continue
haskey(subs, k) && continue
subs[k] = Symbolics.SConst(collect(k))
end

ir = get_irstructure(sys)
subber = SU.IRSubstituter{false}(ir, subs)
for e in eqs_to_substitute
# Double substitute to handle unscalarized array variables. First one
# substitutes `x` to `[x[1], x[2]]`. The second substitutes `x[1]` and `x[2]`.
eqs[e] = subber(subber(eqs[e]))
original_eqs[e] = subber(subber(original_eqs[e]))
end

# After substitution, alias equations that connected a sticky non-target
# variable to a zero-priority variable become structurally identical to the
# direct alias between the sticky variable and the target (since the
Expand Down
10 changes: 10 additions & 0 deletions test/reduction.jl
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Original file line number Diff line number Diff line change
@@ -1,4 +1,5 @@
using ModelingToolkit, OrdinaryDiffEq, Test, NonlinearSolve, LinearAlgebra
using Symbolics
using OrdinaryDiffEqRosenbrock
using ModelingToolkit: topsort_equations, t_nounits as t, D_nounits as D, unwrap

Expand Down Expand Up @@ -374,3 +375,12 @@ end

@test Set(unknowns(sys)) == Set([e, c, d])
end

@testset "`eliminate_perfect_aliases!` correctly handles unscalarized arrays" begin
@variables x(t)[1:2] y(t)[1:2]
@mtkcompile sys = System([D(x) ~ x, y[1] ~ y[2], dot(x, y) ~ 1], t)
@test any(equations(sys)) do eq
isequal(eq, 0 ~ 1 - dot(x, Symbolics.SConst([y[2], y[2]]))) ||
isequal(eq, 0 ~ 1 - dot(x, Symbolics.SConst([y[1], y[1]])))
end
end
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