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Assert type consistency in the simplifier #2064

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Merged
merged 3 commits into from
Mar 7, 2019
Merged

Assert type consistency in the simplifier #2064

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bbc3f63
Note type inconsistencies in solver axioms and avoid simplifying
tautschnig Apr 19, 2018
c564c2c
Expression simplification must not alter the type
tautschnig Apr 18, 2018
8616586
Simplifier: As we now assert type equality we can remove a safety net
tautschnig Feb 13, 2019
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2 changes: 2 additions & 0 deletions src/solvers/strings/string_constraint_generator_main.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -120,6 +120,8 @@ array_string_exprt array_poolt::make_char_array_for_char_pointer(
if_expr.cond(),
to_array_type(t.type()).size(),
to_array_type(f.type()).size()));
// BEWARE: this expression is possibly type-inconsistent and must not be
// used for any purposes other than passing it to concretisation
return to_array_string_expr(if_exprt(if_expr.cond(), t, f, array_type));
}
const bool is_constant_array =
Expand Down
48 changes: 47 additions & 1 deletion src/solvers/strings/string_refinement.cpp
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Expand Up @@ -854,6 +854,49 @@ decision_proceduret::resultt string_refinementt::dec_solve()
return resultt::D_ERROR;
}

/// In a best-effort manner, try to clean up the type inconsistencies introduced
/// by \ref array_poolt::make_char_array_for_char_pointer, which creates
/// conditional expressions for the size of arrays. The cleanup is achieved by
/// removing branches that are found to be infeasible, and by simplifying the
/// conditional size expressions previously generated.
/// \param expr: Expression to be cleaned
/// \param ns: Namespace
/// \return Cleaned expression
static exprt adjust_if_recursive(exprt expr, const namespacet &ns)
{
for(auto it = expr.depth_begin(); it != expr.depth_end();)
{
if(it->id() == ID_if)
{
if_exprt if_expr = to_if_expr(*it);
const exprt simp_cond = simplify_expr(if_expr.cond(), ns);
if(simp_cond.is_true())
{
it.mutate() = adjust_if_recursive(if_expr.true_case(), ns);
it.next_sibling_or_parent();
}
else if(simp_cond.is_false())
{
it.mutate() = adjust_if_recursive(if_expr.false_case(), ns);
it.next_sibling_or_parent();
}
else if(
it->type().id() == ID_array &&
to_array_type(it->type()).size().id() == ID_if)
{
simplify(to_array_type(it.mutate().type()).size(), ns);
++it;
}
else
++it;
}
else
++it;
}

return expr;
}

/// Add the given lemma to the solver.
/// \param lemma: a Boolean expression
/// \param simplify_lemma: whether the lemma should be simplified before being
Expand All @@ -869,7 +912,10 @@ void string_refinementt::add_lemma(

exprt simple_lemma = lemma;
if(simplify_lemma)
{
simple_lemma = adjust_if_recursive(std::move(simple_lemma), ns);
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This will probably not get rid of all the inconsistencies, so I'm not sure about how useful it is. Ideally we should get rid of the introduction of type inconsistencies, but I'm not sure about the best way to do that.

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I recall having tried removing them completely (which would be done by using type casts), but that did not seem to work very well. It's the long-term solution, and maybe that's what we should actually invest in instead of this hack. Not sure though, I'm certainly curious about the results with TG!

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Everything is green on our side, so nothing blocking merging it.

simplify(simple_lemma, ns);
}

if(simple_lemma.is_true())
{
Expand Down Expand Up @@ -917,7 +963,7 @@ static optionalt<exprt> get_array(
{
const auto eom = messaget::eom;
const exprt &size = arr.length();
exprt arr_val = simplify_expr(super_get(arr), ns);
exprt arr_val = simplify_expr(adjust_if_recursive(super_get(arr), ns), ns);
exprt size_val = super_get(size);
size_val = simplify_expr(size_val, ns);
const typet char_type = arr.type().subtype();
Expand Down