Introduce value-set supported simplifier for goto-symex #8642
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tautschnig
requested review from
qinheping,
feliperodri,
remi-delmas-3000,
kroening and
peterschrammel
as code owners
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## develop #8642 +/- ##
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+ Coverage 80.37% 80.39% +0.01%
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Move `try_evaluate_pointer_comparison` to a simplifier that can eventually support more cases than just equalities in GOTO conditions. The initial change does not alter behaviour (except that previously `try_evaluate_pointer_comparison` was even used when simplification was disabled). A side-effect is that we can also clean up renamedt.
value-set based simplifications may be helpful well outside just GOTO conditions.
When all candidates in the value set have the same offset we can replace a pointer_offset expression by the offset value found in the value set.
The value set can help us infer that, e.g., a particular pointer cannot be among the ones assigned to __CPROVER_dead_object, whereby we can simplify R_OK/W_OK expressions.
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feliperodri
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Jun 18, 2025
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| simplify_exprt::resultt<> | ||
| simplify_expr_with_value_sett::simplify_pointer_offset( |
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I'd include a header comment here with your commit description
When all candidates in the value set have the same offset we can replace
a pointer_offset expression by the offset value found in the value set.
Since there is only one simplification applied. This is nitpick since one may infer from the code.
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| /// Try to evaluate pointer comparisons where they can be trivially determined | ||
| /// using the value-set. This is optional as all it does is allow symex to | ||
| /// resolve some comparisons itself and therefore create a simpler formula for | ||
| /// the SAT solver. | ||
| /// \param [in,out] condition: An L2-renamed expression with boolean type | ||
| /// \param value_set: The value-set for determining what pointer-typed symbols | ||
| /// might possibly point to | ||
| /// \param language_mode: The language mode | ||
| /// \param ns: A namespace | ||
| /// \return The possibly modified condition | ||
| renamedt<exprt, L2> try_evaluate_pointer_comparisons( | ||
| renamedt<exprt, L2> condition, | ||
| const value_sett &value_set, | ||
| const irep_idt &language_mode, | ||
| const namespacet &ns); | ||
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Nitpick: it'd be nice if the clean up was done on a separate commit.
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| // all pointed-to objects on the left-hand side are different from any of | ||
| // the pointed-to objects on the right-hand side | ||
| return expr.id() == ID_equal ? changed(static_cast<exprt>(false_exprt{})) | ||
| : changed(static_cast<exprt>(true_exprt{})); |
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If pointed-to objects are the same for LHS and RHS but for each object we have statically different symbolic offsets then we could conclude that the comparison is still always false ?
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I meant to say this still seems to rely only on object information, not really using offset info to the fullest ? but this might already be done somewhere else.
remi-delmas-3000
approved these changes
Jun 18, 2025
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Move
try_evaluate_pointer_comparisonto a simplifier that can eventually support more cases than just equalities in GOTO conditions. The initial change does not alter behaviour (except that previouslytry_evaluate_pointer_comparisonwas even used when simplification was disabled).A side-effect is that we can also clean up renamedt.