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Fix inconsistent optimization with scaled objectives (#8998)

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When the LP optimizer returns the same blocker expression in successive
iterations of geometric_lex (e.g., due to nonlinear constraints like
mod/to_int preventing the LP from exploring the full feasible region),
the loop now falls back to using the model-based lower bound to push
harder instead of breaking immediately.

This fixes the case where minimize(3*a) incorrectly returned -162
while minimize(a) correctly returned -infinity with the same constraints.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
This commit is contained in:
Lev Nachmanson 2026-03-15 07:02:13 -10:00 • committed by Lev Nachmanson
parent fbeb4b22eb
commit 5df80705aa
1 changed files with 8 additions and 1 deletions
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9
src/opt/optsmt.cpp
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@ -253,7 +253,14 @@ namespace opt {
}
last_objective = obj;
if (bound == last_bound) {
break;
// LP didn't produce a new blocker. If the model-based lower bound
// is strictly better than what the LP found, use it to push the LP
// further. This handles cases where nonlinear constraints, mod,
// to_int, prevent the LP from seeing the full feasible region.
if (m_lower[obj_index].is_finite() && m_lower[obj_index] > obj)
bound = m_s->mk_ge(obj_index, m_lower[obj_index]);
if (bound == last_bound)
break;
}
m_s->assert_expr(bound);
last_bound = bound;