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lp: avoid heap allocation when relocating coefficients in static_matrix::remove_element

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remove_element uses swap-remove but deep-copied the relocated tail coefficient.
In namespace lp the coefficient type is rational (copyable), so this allocates
on the heap for large coefficients. Since the tail element is popped right after,
relocate large coefficients by swapping instead, avoiding the allocation; small
coefficients still use a plain copy (cheaper than swapping mpz internals).

Reported in Z3Prover/bench#3143 (remove_element was the top hotspot). Verified
correctness-neutral (identical results, 92/92 unit tests); ~1.5% faster on the
certora large-coefficient set, neutral on small-coefficient inputs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: fd5693e4-3b1d-4dac-b3be-2942ae6f31f8
This commit is contained in:
Lev Nachmanson 2026-07-13 09:39:07 -07:00
parent 98e1f5ca2d
commit 4cb997c128
1 changed files with 12 additions and 2 deletions
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14
src/math/lp/static_matrix_def.h
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@ -462,12 +462,22 @@ namespace lp {
column_cell& cs = m_columns[row_el_iv.var()][column_offset];
unsigned row_offset = cs.offset();
if (column_offset != column_vals.size() - 1) {
auto & cc = column_vals[column_offset] = column_vals.back(); // copy from the tail
auto & cc = column_vals[column_offset] = column_vals.back(); // column cells are tiny, a plain copy is optimal
m_rows[cc.var()][cc.offset()].offset() = column_offset;
}
if (row_offset != row_vals.size() - 1) {
auto & rc = row_vals[row_offset] = row_vals.back(); // copy from the tail
row_cell<T> & rc = row_vals[row_offset];
row_cell<T> & tail = row_vals.back();
rc.var() = tail.var();
rc.offset() = tail.offset();
// Relocating the tail coefficient: swap to steal an already allocated (big) value and
// avoid a heap allocation; for small coefficients a direct copy is cheaper than swapping
// the mpz internals. See Z3Prover/bench#3143.
if (rc.coeff().is_big() || tail.coeff().is_big())
rc.coeff().swap(tail.coeff());
else
rc.coeff() = tail.coeff();
m_columns[rc.var()][rc.offset()].offset() = row_offset;
}