Simplifiers - Z3 Theorem Prover

Overview

Z3 simplifiers are lightweight formula transformations that reduce formula complexity before solving. Unlike tactics (which are heavy preprocessing pipelines), simplifiers are fast, composable transformations that can significantly improve solver performance.

Understanding Simplifiers

Simplifiers vs. Tactics

Simplifiers:

Lightweight transformations
Fast and incremental
Compose naturally with and-then
Applied before solving
Don’t change the solver strategy

Tactics:

Heavy preprocessing pipelines
Complete solving strategies
Transform goals into subgoals
May apply multiple techniques

When to Use Simplifiers

Use simplifiers when you want to:

Preprocess formulas before solving
Eliminate redundant constraints
Propagate constants and bounds
Simplify arithmetic expressions
Improve solver performance without changing strategy

Creating and Using Simplifiers

Basic Simplifier Usage

// Create a simplifier
Z3_simplifier simp = Z3_mk_simplifier(ctx, "simplify");
Z3_simplifier_inc_ref(ctx, simp);

// Create solver with simplifier
Z3_solver solver = Z3_mk_solver(ctx);
Z3_solver_inc_ref(ctx, solver);

Z3_solver new_solver = Z3_solver_add_simplifier(ctx, solver, simp);
Z3_solver_inc_ref(ctx, new_solver);

// Use the new solver
Z3_solver_assert(ctx, new_solver, formula);
Z3_lbool result = Z3_solver_check(ctx, new_solver);

// Cleanup
Z3_solver_dec_ref(ctx, new_solver);
Z3_solver_dec_ref(ctx, solver);
Z3_simplifier_dec_ref(ctx, simp);

Listing Available Simplifiers

unsigned num_simplifiers = Z3_get_num_simplifiers(ctx);
printf("Available simplifiers: %u\n", num_simplifiers);

for (unsigned i = 0; i < num_simplifiers; i++) {
    Z3_string name = Z3_get_simplifier_name(ctx, i);
    Z3_string descr = Z3_simplifier_get_descr(ctx, name);
    printf("  [%u] %s: %s\n", i, name, descr);
}

Common Simplifiers

Core Simplifiers

`simplify`

General-purpose simplification:

Z3_simplifier simp = Z3_mk_simplifier(ctx, "simplify");
Z3_simplifier_inc_ref(ctx, simp);

// Simplifies:
// - (and true p) => p
// - (+ 0 x) => x
// - (not (not p)) => p
// - Constant folding and propagation

Z3_solver solver = Z3_mk_solver(ctx);
Z3_solver new_solver = Z3_solver_add_simplifier(ctx, solver, simp);

Z3_simplifier_dec_ref(ctx, simp);

`propagate-values`

Propagates constants and simplifies based on known values:

Z3_simplifier simp = Z3_mk_simplifier(ctx, "propagate-values");
Z3_simplifier_inc_ref(ctx, simp);

// Propagates:
// x = 5, y = x + 3 => y = 8
// b = true, (and b c) => c

Z3_solver new_solver = Z3_solver_add_simplifier(ctx, solver, simp);

Z3_simplifier_dec_ref(ctx, simp);

`solve-eqs`

Solves for variables in equations and substitutes:

Z3_simplifier simp = Z3_mk_simplifier(ctx, "solve-eqs");
Z3_simplifier_inc_ref(ctx, simp);

// Transforms:
// (and (= x (+ y 1)) (> x 10)) => (> (+ y 1) 10)
// Eliminates variables by solving equations

Z3_solver new_solver = Z3_solver_add_simplifier(ctx, solver, simp);

Z3_simplifier_dec_ref(ctx, simp);

`elim-uncnstr`

Eliminates unconstrained variables:

Z3_simplifier simp = Z3_mk_simplifier(ctx, "elim-uncnstr");
Z3_simplifier_inc_ref(ctx, simp);

// Removes variables that don't affect satisfiability
// Example: (and (> x 0) (> y 0)) where x is unconstrained
// Can eliminate x if it appears only in that constraint

Z3_solver new_solver = Z3_solver_add_simplifier(ctx, solver, simp);

Z3_simplifier_dec_ref(ctx, simp);

Arithmetic Simplifiers

`normalize-bounds`

Normalizes arithmetic bounds:

Z3_simplifier simp = Z3_mk_simplifier(ctx, "normalize-bounds");
Z3_simplifier_inc_ref(ctx, simp);

// Normalizes:
// (>= x 5) and (>= x 3) => (>= x 5)
// Combines and simplifies bound constraints

Z3_solver new_solver = Z3_solver_add_simplifier(ctx, solver, simp);

Z3_simplifier_dec_ref(ctx, simp);

`lia2pb`

Converts linear integer arithmetic to pseudo-boolean:

Z3_simplifier simp = Z3_mk_simplifier(ctx, "lia2pb");
Z3_simplifier_inc_ref(ctx, simp);

// Converts linear arithmetic to bit-vector representation
// Useful for bounded integer problems

Z3_solver new_solver = Z3_solver_add_simplifier(ctx, solver, simp);

Z3_simplifier_dec_ref(ctx, simp);

Boolean Simplifiers

`propagate-ineqs`

Propagates inequalities:

Z3_simplifier simp = Z3_mk_simplifier(ctx, "propagate-ineqs");
Z3_simplifier_inc_ref(ctx, simp);

// Propagates:
// x < y, y < z => x < z
// Derives implied inequalities

Z3_solver new_solver = Z3_solver_add_simplifier(ctx, solver, simp);

Z3_simplifier_dec_ref(ctx, simp);

`ctx-solver-simplify`

Context-aware simplification using solver state:

Z3_simplifier simp = Z3_mk_simplifier(ctx, "ctx-solver-simplify");
Z3_simplifier_inc_ref(ctx, simp);

// Uses solver context to simplify formulas
// More powerful but slower than basic simplify

Z3_solver new_solver = Z3_solver_add_simplifier(ctx, solver, simp);

Z3_simplifier_dec_ref(ctx, simp);

Composing Simplifiers

Sequential Composition

Chain simplifiers using and-then:

// Create individual simplifiers
Z3_simplifier s1 = Z3_mk_simplifier(ctx, "propagate-values");
Z3_simplifier_inc_ref(ctx, s1);

Z3_simplifier s2 = Z3_mk_simplifier(ctx, "solve-eqs");
Z3_simplifier_inc_ref(ctx, s2);

Z3_simplifier s3 = Z3_mk_simplifier(ctx, "simplify");
Z3_simplifier_inc_ref(ctx, s3);

// Compose: first propagate values, then solve equations, then simplify
Z3_simplifier composed1 = Z3_simplifier_and_then(ctx, s1, s2);
Z3_simplifier_inc_ref(ctx, composed1);

Z3_simplifier composed2 = Z3_simplifier_and_then(ctx, composed1, s3);
Z3_simplifier_inc_ref(ctx, composed2);

// Use composed simplifier
Z3_solver solver = Z3_mk_solver(ctx);
Z3_solver new_solver = Z3_solver_add_simplifier(ctx, solver, composed2);

// Cleanup
Z3_simplifier_dec_ref(ctx, composed2);
Z3_simplifier_dec_ref(ctx, composed1);
Z3_simplifier_dec_ref(ctx, s3);
Z3_simplifier_dec_ref(ctx, s2);
Z3_simplifier_dec_ref(ctx, s1);

Building a Simplification Pipeline

Z3_simplifier build_simplification_pipeline(Z3_context ctx) {
    // Stage 1: Propagate known values
    Z3_simplifier s1 = Z3_mk_simplifier(ctx, "propagate-values");
    Z3_simplifier_inc_ref(ctx, s1);
    
    // Stage 2: Solve for variables
    Z3_simplifier s2 = Z3_mk_simplifier(ctx, "solve-eqs");
    Z3_simplifier_inc_ref(ctx, s2);
    
    // Stage 3: Eliminate unconstrained
    Z3_simplifier s3 = Z3_mk_simplifier(ctx, "elim-uncnstr");
    Z3_simplifier_inc_ref(ctx, s3);
    
    // Stage 4: Final simplification
    Z3_simplifier s4 = Z3_mk_simplifier(ctx, "simplify");
    Z3_simplifier_inc_ref(ctx, s4);
    
    // Compose pipeline
    Z3_simplifier p1 = Z3_simplifier_and_then(ctx, s1, s2);
    Z3_simplifier_inc_ref(ctx, p1);
    Z3_simplifier p2 = Z3_simplifier_and_then(ctx, p1, s3);
    Z3_simplifier_inc_ref(ctx, p2);
    Z3_simplifier result = Z3_simplifier_and_then(ctx, p2, s4);
    Z3_simplifier_inc_ref(ctx, result);
    
    // Cleanup intermediate simplifiers
    Z3_simplifier_dec_ref(ctx, p2);
    Z3_simplifier_dec_ref(ctx, p1);
    Z3_simplifier_dec_ref(ctx, s4);
    Z3_simplifier_dec_ref(ctx, s3);
    Z3_simplifier_dec_ref(ctx, s2);
    Z3_simplifier_dec_ref(ctx, s1);
    
    return result;
}

Configuring Simplifiers with Parameters

Setting Simplifier Parameters

// Create simplifier
Z3_simplifier simp = Z3_mk_simplifier(ctx, "simplify");
Z3_simplifier_inc_ref(ctx, simp);

// Create parameters
Z3_params params = Z3_mk_params(ctx);
Z3_params_inc_ref(ctx, params);

// Configure simplifier behavior
Z3_symbol max_steps = Z3_mk_string_symbol(ctx, "max_steps");
Z3_params_set_uint(ctx, params, max_steps, 10000);

Z3_symbol max_memory = Z3_mk_string_symbol(ctx, "max_memory");
Z3_params_set_uint(ctx, params, max_memory, 100);

// Create parameterized simplifier
Z3_simplifier param_simp = Z3_simplifier_using_params(ctx, simp, params);
Z3_simplifier_inc_ref(ctx, param_simp);

// Use parameterized simplifier
Z3_solver solver = Z3_mk_solver(ctx);
Z3_solver new_solver = Z3_solver_add_simplifier(ctx, solver, param_simp);

// Cleanup
Z3_simplifier_dec_ref(ctx, param_simp);
Z3_params_dec_ref(ctx, params);
Z3_simplifier_dec_ref(ctx, simp);

Discovering Simplifier Parameters

Z3_simplifier simp = Z3_mk_simplifier(ctx, "simplify");
Z3_simplifier_inc_ref(ctx, simp);

// Get parameter descriptions
Z3_param_descrs descrs = Z3_simplifier_get_param_descrs(ctx, simp);
Z3_param_descrs_inc_ref(ctx, descrs);

printf("Parameters for 'simplify' simplifier:\n");
unsigned size = Z3_param_descrs_size(ctx, descrs);
for (unsigned i = 0; i < size; i++) {
    Z3_symbol name = Z3_param_descrs_get_name(ctx, descrs, i);
    Z3_param_kind kind = Z3_param_descrs_get_kind(ctx, descrs, name);
    Z3_string doc = Z3_param_descrs_get_documentation(ctx, descrs, name);
    
    printf("  %s: %s\n", Z3_get_symbol_string(ctx, name), doc);
}

Z3_param_descrs_dec_ref(ctx, descrs);
Z3_simplifier_dec_ref(ctx, simp);

Getting Simplifier Help

Z3_simplifier simp = Z3_mk_simplifier(ctx, "simplify");
Z3_simplifier_inc_ref(ctx, simp);

// Get help text
Z3_string help = Z3_simplifier_get_help(ctx, simp);
printf("Simplifier help:\n%s\n", help);

Z3_simplifier_dec_ref(ctx, simp);

Domain-Specific Simplification

Bit-Vector Simplification

Z3_simplifier build_bv_pipeline(Z3_context ctx) {
    // Bit-vector specific simplifications
    Z3_simplifier s1 = Z3_mk_simplifier(ctx, "simplify");
    Z3_simplifier_inc_ref(ctx, s1);
    
    Z3_simplifier s2 = Z3_mk_simplifier(ctx, "propagate-values");
    Z3_simplifier_inc_ref(ctx, s2);
    
    Z3_simplifier result = Z3_simplifier_and_then(ctx, s1, s2);
    Z3_simplifier_inc_ref(ctx, result);
    
    Z3_simplifier_dec_ref(ctx, s2);
    Z3_simplifier_dec_ref(ctx, s1);
    
    return result;
}

Arithmetic Simplification

Z3_simplifier build_arithmetic_pipeline(Z3_context ctx) {
    // Arithmetic-focused pipeline
    Z3_simplifier s1 = Z3_mk_simplifier(ctx, "normalize-bounds");
    Z3_simplifier_inc_ref(ctx, s1);
    
    Z3_simplifier s2 = Z3_mk_simplifier(ctx, "propagate-ineqs");
    Z3_simplifier_inc_ref(ctx, s2);
    
    Z3_simplifier s3 = Z3_mk_simplifier(ctx, "solve-eqs");
    Z3_simplifier_inc_ref(ctx, s3);
    
    Z3_simplifier p1 = Z3_simplifier_and_then(ctx, s1, s2);
    Z3_simplifier_inc_ref(ctx, p1);
    
    Z3_simplifier result = Z3_simplifier_and_then(ctx, p1, s3);
    Z3_simplifier_inc_ref(ctx, result);
    
    Z3_simplifier_dec_ref(ctx, p1);
    Z3_simplifier_dec_ref(ctx, s3);
    Z3_simplifier_dec_ref(ctx, s2);
    Z3_simplifier_dec_ref(ctx, s1);
    
    return result;
}

Performance Considerations

Simplifier Overhead

// Measure simplification overhead
void measure_simplification(Z3_context ctx, Z3_ast formula) {
    // Without simplification
    Z3_solver s1 = Z3_mk_solver(ctx);
    Z3_solver_inc_ref(ctx, s1);
    Z3_solver_assert(ctx, s1, formula);
    
    clock_t start1 = clock();
    Z3_solver_check(ctx, s1);
    clock_t end1 = clock();
    double time1 = (double)(end1 - start1) / CLOCKS_PER_SEC;
    
    // With simplification
    Z3_simplifier simp = Z3_mk_simplifier(ctx, "simplify");
    Z3_simplifier_inc_ref(ctx, simp);
    
    Z3_solver s2 = Z3_mk_solver(ctx);
    Z3_solver s2_simp = Z3_solver_add_simplifier(ctx, s2, simp);
    Z3_solver_inc_ref(ctx, s2_simp);
    Z3_solver_assert(ctx, s2_simp, formula);
    
    clock_t start2 = clock();
    Z3_solver_check(ctx, s2_simp);
    clock_t end2 = clock();
    double time2 = (double)(end2 - start2) / CLOCKS_PER_SEC;
    
    printf("Without simplification: %.3f seconds\n", time1);
    printf("With simplification: %.3f seconds\n", time2);
    printf("Speedup: %.2fx\n", time1 / time2);
    
    // Cleanup
    Z3_solver_dec_ref(ctx, s2_simp);
    Z3_simplifier_dec_ref(ctx, simp);
    Z3_solver_dec_ref(ctx, s1);
}

Best Practices

When to Use Simplifiers

Use simplifiers when:

Formulas contain obvious redundancies
Constants can be propagated
Variables can be eliminated
Bounds can be normalized
You want to improve performance without changing solver strategy

Avoid simplifiers when:

Formulas are already in optimal form
Simplification overhead exceeds benefits
Debugging and need to preserve original formula structure

Recommended Pipelines

Lightweight preprocessing:

// Fast, minimal overhead
Z3_simplifier s1 = Z3_mk_simplifier(ctx, "propagate-values");
Z3_simplifier s2 = Z3_mk_simplifier(ctx, "simplify");
Z3_simplifier pipeline = Z3_simplifier_and_then(ctx, s1, s2);

Aggressive preprocessing:

// More thorough, higher overhead
Z3_simplifier s1 = Z3_mk_simplifier(ctx, "propagate-values");
Z3_simplifier s2 = Z3_mk_simplifier(ctx, "solve-eqs");
Z3_simplifier s3 = Z3_mk_simplifier(ctx, "elim-uncnstr");
Z3_simplifier s4 = Z3_mk_simplifier(ctx, "simplify");
// Compose all four...

Common Pitfalls

Adding simplifier after assertions: Simplifiers must be added before assertions
Over-simplification: Too many simplifiers can increase overhead
Wrong simplifier order: Order matters in composition
Forgetting reference counting: Always pair inc_ref with dec_ref

Limitations

Simplifier Restrictions

// ERROR: Cannot add simplifier after assertions
Z3_solver solver = Z3_mk_solver(ctx);
Z3_solver_assert(ctx, solver, formula);  // Assertion added

Z3_simplifier simp = Z3_mk_simplifier(ctx, "simplify");
// This will fail:
Z3_solver new_solver = Z3_solver_add_simplifier(ctx, solver, simp);

Workaround: Add simplifier first, then make assertions:

Z3_solver solver = Z3_mk_solver(ctx);
Z3_simplifier simp = Z3_mk_simplifier(ctx, "simplify");
Z3_solver new_solver = Z3_solver_add_simplifier(ctx, solver, simp);
// Now add assertions
Z3_solver_assert(ctx, new_solver, formula);

API Reference

Simplifier Creation

Z3_mk_simplifier - Create simplifier by name (api_tactic.cpp:533)
Z3_simplifier_inc_ref - Increment reference counter (api_tactic.cpp:549)
Z3_simplifier_dec_ref - Decrement reference counter (api_tactic.cpp:557)

Simplifier Discovery

Z3_get_num_simplifiers - Get number of available simplifiers (api_tactic.cpp:565)
Z3_get_simplifier_name - Get simplifier name by index (api_tactic.cpp:573)
Z3_simplifier_get_descr - Get simplifier description (api_tactic.cpp:657)

Simplifier Composition

Z3_simplifier_and_then - Sequential composition (api_tactic.cpp:585)
Z3_simplifier_using_params - Add parameters to simplifier (api_tactic.cpp:601)

Simplifier Information

Z3_simplifier_get_help - Get help text (api_tactic.cpp:625)
Z3_simplifier_get_param_descrs - Get parameter descriptions (api_tactic.cpp:641)

Solver Integration

Z3_solver_add_simplifier - Attach simplifier to solver (api_solver.cpp:244)

Core API

Theory Solvers

Advanced

​Overview

​Understanding Simplifiers

​Simplifiers vs. Tactics

​When to Use Simplifiers

​Creating and Using Simplifiers

​Basic Simplifier Usage

​Listing Available Simplifiers

​Common Simplifiers

​Core Simplifiers

​simplify

​propagate-values

​solve-eqs

​elim-uncnstr

​Arithmetic Simplifiers

​normalize-bounds

​lia2pb

​Boolean Simplifiers

​propagate-ineqs

​ctx-solver-simplify

​Composing Simplifiers

​Sequential Composition

​Building a Simplification Pipeline

​Configuring Simplifiers with Parameters

​Setting Simplifier Parameters

​Discovering Simplifier Parameters

​Getting Simplifier Help

​Domain-Specific Simplification

​Bit-Vector Simplification

​Arithmetic Simplification

​Performance Considerations

​Simplifier Overhead

​Best Practices

​When to Use Simplifiers

​Recommended Pipelines

​Common Pitfalls

​Limitations

​Simplifier Restrictions

​API Reference

​Simplifier Creation

​Simplifier Discovery

​Simplifier Composition

​Simplifier Information

​Solver Integration

​See Also

Build docs developers (and LLMs) love

Overview

Understanding Simplifiers

Simplifiers vs. Tactics

When to Use Simplifiers

Creating and Using Simplifiers

Basic Simplifier Usage

Listing Available Simplifiers

Common Simplifiers

Core Simplifiers

`simplify`

`propagate-values`

`solve-eqs`

`elim-uncnstr`

Arithmetic Simplifiers

`normalize-bounds`

`lia2pb`

Boolean Simplifiers

`propagate-ineqs`

`ctx-solver-simplify`

Composing Simplifiers

Sequential Composition

Building a Simplification Pipeline

Configuring Simplifiers with Parameters

Setting Simplifier Parameters

Discovering Simplifier Parameters

Getting Simplifier Help

Domain-Specific Simplification

Bit-Vector Simplification

Arithmetic Simplification

Performance Considerations

Simplifier Overhead

Best Practices

When to Use Simplifiers

Recommended Pipelines

Common Pitfalls

Limitations

Simplifier Restrictions

API Reference

Simplifier Creation

Simplifier Discovery

Simplifier Composition

Simplifier Information

Solver Integration

See Also