Add pure expression inliner infrastructure for quantifier bodies#4567
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feliperodri wants to merge 4 commits intomodel-checking:mainfrom
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Add pure expression inliner infrastructure for quantifier bodies#4567feliperodri wants to merge 4 commits intomodel-checking:mainfrom
feliperodri wants to merge 4 commits intomodel-checking:mainfrom
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Add Expr::substitute_symbol() and inline_as_pure_expr() for inlining function calls as side-effect-free expression trees. This infrastructure is needed to generate CBMC quantifier bodies without StatementExpression nodes, which will enable nested quantifier support. Changes: - cprover_bindings: Expr::substitute_symbol() — recursive symbol replacement across all ExprValue variants, with 6 unit tests - goto_ctx.rs: inline_as_pure_expr() — inlines function calls by extracting return expressions, resolving intermediate variables, and substituting parameters. Handles StatementExpression flattening for checked arithmetic (drops Assert/Assume runtime checks). - docs/dev/pure-expression-inliner.md — developer documentation including soundness implications Soundness note: The pure inliner drops overflow and division-by-zero checks when flattening StatementExpression nodes from checked arithmetic. This is a known trade-off — CBMC requires pure expressions in quantifier bodies, and runtime checks are side effects. The existing handle_quantifiers post-pass is NOT modified — this is purely additive infrastructure. Signed-off-by: Felipe R. Monteiro <felisous@amazon.com>
Signed-off-by: Felipe R. Monteiro <felisous@amazon.com>
- substitute_symbol returns (Expr, bool) for reliable change detection instead of Debug-format string comparison - Graceful fallback for recursive functions (tracing::warn + return original) instead of assert! panic - Diagnostics for edge cases: multiple assignments to same variable, non-symbol return expressions, unknown UnaryOperator variants - Public API cleanup: inline_as_pure_expr_toplevel wrapper hides the visited set implementation detail - Document StatementExpression non-recursion in substitute_symbol Signed-off-by: Felipe R. Monteiro <felisous@amazon.com>
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| ## Soundness Implications | ||
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| **Checked arithmetic in quantifier bodies**: When flattening `StatementExpression` | ||
| nodes (e.g., from checked division or remainder), the pure inliner drops the | ||
| `Assert` and `Assume` statements that check for overflow and division by zero. | ||
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| - **Division by zero** inside a quantifier body will NOT be detected. | ||
| - **Arithmetic overflow** inside a quantifier body will NOT be detected. | ||
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| **Future improvement**: The dropped assertions could be hoisted outside the | ||
| quantifier as preconditions, preserving soundness while keeping the body pure. |
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@tautschnig this is similar to CBMC behavior, correct? Let me know if this raises any concerns.
- Replace destructuring assignment with .0 for substitute_symbol calls - Restore step-by-step 'How It Works' section in dev documentation Signed-off-by: Felipe R. Monteiro <felisous@amazon.com>
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What
Adds
Expr::substitute_symbol()andinline_as_pure_expr()— infrastructure for inlining function calls as side-effect-free expression trees. This is a prerequisite for generating CBMC quantifier bodies withoutStatementExpressionnodes, which will enable nested quantifier support.Why
CBMC rejects side effects inside quantifier expressions (
forall/exists). Today, Kani'shandle_quantifierspost-pass inlines function calls intoStatementExpressionnodes, which CBMC accepts viaconvert_statement_expression, but this breaks for nested quantifiers (inner quantifiers are treated as side effects, causing a CBMC invariant violation crash).The pure expression inliner resolves this by producing expression trees that contain no statements, no gotos, and no labels; only pure expressions that CBMC can handle directly, including in nested contexts.
How
Expr::substitute_symbol()(cprover_bindings/src/goto_program/expr.rs)inline_as_pure_expr()(kani-compiler/src/codegen_cprover_gotoc/context/goto_ctx.rs)StatementExpressionnodes (e.g., checked arithmetic).Soundness note
When flattening
StatementExpressionnodes from checked arithmetic (e.g., %, +), the inliner drops Assert/Assume statements that check for overflow and division by zero. This is a known trade-off documented in docs/dev/pure-expression-inliner.md: CBMC requires pure expressions in quantifier bodies, and runtime checks are inherently side effects. A future improvement could hoist these checks outside the quantifier as preconditions.By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.