XPRESS Changelog#

20241227#

Fix acceptance options for nonlinear formulas (still disabled by default.)
Option pre:solve_nlp
Updated to Xpress 9.5 (44.01.01):
- Perfromance improvements in the Global optimization engine and in the MIP solver
- New column-based presolve eliminations to reduce model size when free variables are present in the model instance.
- In models with big-M coefficients, additional tightening of coefficients is performed.
- Knapsack cover inequalities are now strengthened by using superadditive lifting.

20240823#

Updated to Xpress 43.01.03
Added hybrid gradient algorithm (set bar:alg to 4)
Option bar:cpuplatform defaults to -2. To replicate the previous versions behaviour, set to -1.

20240724#

Option acc:_all
- Useful to disable all reformulations (acc:_all=2), or force linearization (acc:_all=0).
Option cvt:prod
- Controls reformulation of binary products into logical constraints.
Faster input of quadratic expressions.

20240617#

Multi-objective emulator
- obj:multi=2 forces emulation, even if MO natively supported.
- Fixed a bug in the objective degradation suffixes .objasbtol, .objreltol.

20240606#

Fix mip:basis (post-solving as fixed MIP), although fix + resolve from AMPL can be faster.
Fix reporting infeasibility with obj:multi=1.

20240604#

Presolve division by constant, resulting in fewer constraints
Fix no-solution case in multi-objective emulator

20240529#

Multi-objective emulator
- All flat MP solvers support multi-objective mode (obj:multi=1), either natively, or via emulation.
- Suffixes .objpriority, .objweight, .objabstol, .objreltol.
- [BREAKING] Default intuitive handling of .objweight, see option obj:multi:weight, even when natively supported.

20240429#

[BREAKING] Merged report_times and timing; they are now aliases, set the value to 1 to have basic info, to 2 to have more detailed info.

20240320#

SOS constraints.
- Fixed handling of SOS2 constraints created by AMPL as reformulations of PL expressions (option pl_linearize 1, default; set to 0 to use the solver’s native PL functions or MP linearization.)
- Disallow repeated weights for SOS constraints (suffixes .sosno/.ref.)
Native handling of POW(x, INT).
- Power expressions with positive integer exponent are passed natively to the solvers accepting them, vs previously quadratic or linear reformulation.
Option report_times.
Unused acc: options.
- The constraint acceptance options acc:... for non-handled constraints are ignored (previously triggered error.)

20240312#

Updated Xpress to 42.01.5
Xpress Global Solver
- Native implementation of most supported non-linear functions. Not enabled by default.

20240306#

Xpress Global Solver
- Apply global (MI)NLP solver for non-convex non-linear models.

20240115#

Solve result codes
- List codes by running (solver) -!
- [BREAKING] Standardized codes. Major changes:
  - 100-199 (solved?) means solution candidate provided, but can be suboptimal/infeasible
  - 300-349 means unbounded problem but feasible solution returned
  - 400-449 means limit/interrupt but feasible
- [BREAKING] sol:chk:fail returns code 150 (solved?)
Improved translation of SOCP constraints.
- Options cvt:socp, cvt:socp2qc.
Compact solution check warnings
Fixed presolve of the power function #226.

20231117#

MP update: fixed graceful exit on Ctrl-C from AMPL in Linux and fixed issue with reading text-format NL files

20231103#

Improved translation of logical constraints: inlining of nested disjunctions and conjunctions; fewer auxiliary binary variables.

20231017#

Fixed a bug in NL reader on Windows.

20230919#

mp_options. Receive mp_options from AMPL (for all MP solvers). They are parsed before (solvername)_options.
Solution checking: relative tolerance sol:chk:feastolrel; options sol:chk:round, sol:chk:prec.

20230831#

Solution checking, options sol:chk:* (experimental).
Preprocess And/Or constraints.

20230817#

Alternative solutions: solve status equal to that of the final solution.
Fixed a bug causing repeated names for auxiliary variables and constraints.
Option values can be assigned without ‘=’.
Fixed a bug where equivalent conditional comparisons were not unified.

20230808#

Updates for Xpress 9.2 (42.01.01), which include many performance improvements in the MIP solver.
Non-convex quadratic problems are now solved to global optimality by default.
Added options ‘mip:heurshiftprop’, ‘tech:backgroundthreads’ and ‘tech:backgroundselect’.
The range of the control ‘mip:feasibilityjump’ was extended.

20230728#

Option ‘tech:writesolution’.

20230727#

Fixed a memory leak #217
Fixed quadratic constraints

20230726#

MP: fixed inequalities of integer expressions with non-integer constants, see test_int_non_int.mod.

20230724#

Option [solver_]auxfiles rc; transfers names of variables and linear constraints into the model; (solver)_options ‘cvt:names=0-3’ controls names.

20230714#

Options barrier/primal/dual/network like in ASL.

20230621#

Fix quadratic objective with repeated subexpressions.
Fix reformulation of ==> / else.

20230616#

Changes in MP.
Eliminated spurious warnings
Fixed passing of quadratic objective

20230607#

Amended detection and display of licensing errors
Licensing allows now usage of an XPRESS license from an AMPL-based deployment

20230603#

Added option ‘tech:logfile’ to enable output to a log file

20230424#

Changes in the MP library: added variable names support and removed spurious starting solution

20230227#

Eliminated warning message when a non feasible solution is added as a starting point for the MIP search

20230207#

Changes in the MP library

20221228#

Changes in MP

20221222#

Bug fixes in MP

20221211#

Changes in MP: added the ==> else operator Implemented implication with ‘else’: constr1 ==> constr2 [else constr3]
Changes in MP: PLApproxRelTol, PLApproxDomain Parameters to control piecewise-linear approximation. cvt:plapprox:reltol default value changed from 1e-5 to 0.01.

20221208#

First release of the MP-based driver for Xpress (solver version 9.0)