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Fix unsigned overflow in parallel solver conflict budget escalation (#10076)

Browse source 
Z3 4.16.0 introduced a cube-and-conquer parallel solver that regressed
easy QF_LIRA problems from <1s to hanging indefinitely. Workers start
with a 1000-conflict budget and multiply by 1.5× on each timeout, but
after ~38 escalations the `unsigned` cast overflows, causing the budget
to oscillate chaotically (e.g. 3.27B → 618M → 927M → … never reaching a
stable large value). For sub-cubes that require more conflicts than any
value in the oscillation window, the worker loops forever.

## Changes

- **`src/smt/smt_parallel.h`** – `update_max_thread_conflicts()`:
replace raw `(unsigned)(mul * val)` cast with saturating arithmetic that
caps at `UINT_MAX`, eliminating the UB and the oscillation:
  ```cpp
// Before – UB when product > UINT_MAX, budget oscillates after ~38
escalations
  m_config.m_threads_max_conflicts =
(unsigned)(m_config.m_max_conflict_mul *
m_config.m_threads_max_conflicts);

  // After – saturates at UINT_MAX
double next = m_config.m_max_conflict_mul *
m_config.m_threads_max_conflicts;
m_config.m_threads_max_conflicts = (next >= (double)UINT_MAX) ? UINT_MAX
: (unsigned)next;
  ```

- **`src/solver/parallel_tactical.cpp`** – Identical
saturating-arithmetic fix in the `parallel_tactical2` worker's
`update_max_thread_conflicts()`, which is the code path actually
exercised for QF_LIRA problems.

- **`src/smt/smt_parallel.cpp`** – Worker's initial per-cube conflict
budget is now sourced from `m_smt_params.m_threads_max_conflicts` (the
user-visible `smt.threads.max_conflicts` parameter) instead of being
hardcoded to 1000, so users who leave the parameter at its default get
an unlimited initial budget matching Z3 4.12.x portfolio behaviour.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Nikolaj Bjorner <nbjorner@microsoft.com>
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Copilot 2026-07-10 15:25:57 -07:00 • committed by GitHub
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src/solver/parallel_tactical.cpp
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@ -1025,8 +1025,9 @@ class parallel_solver {
unsigned m_num_initial_atoms = 0;
void update_max_thread_conflicts() {
m_config.m_threads_max_conflicts = static_cast<unsigned>(
m_config.m_max_conflict_mul * m_config.m_threads_max_conflicts);
// Use saturating arithmetic to avoid unsigned overflow / undefined behaviour.
double next = m_config.m_max_conflict_mul * m_config.m_threads_max_conflicts;
m_config.m_threads_max_conflicts = (next >= (double)UINT_MAX) ? UINT_MAX : static_cast<unsigned>(next);
}
lbool check_cube(expr_ref_vector const& cube) {