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Long Pham
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Multidimensional decreases clauses
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-66
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14 files changed

+324
-66
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‎regression/contracts/variant_multidimensional_ackermann/main.c

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#include <stdio.h>
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int ackermann(int m, int n);
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5+
int main()
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{
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int m = 3;
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int n = 5;
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int result = ackermann(m, n);
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printf("Result of the Ackermann function: %d\n", result);
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return 0;
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}
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int ackermann(int m, int n)
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// clang-format off
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__CPROVER_requires(0 <= m && 0 <= n)
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__CPROVER_ensures(__CPROVER_return_value >= 0)
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// clang-format on
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{
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while(m > 0)
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// clang-format off
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__CPROVER_loop_invariant(0 <= m && 0 <= n)
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__CPROVER_decreases(m, n)
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// clang-format on
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{
27+
if(n == 0)
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{
29+
m--;
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n = 1;
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}
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else
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{
34+
n = ackermann(m, n - 1);
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m--;
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}
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}
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return n + 1;
40+
}

‎regression/contracts/variant_multidimensional_ackermann/test.desc

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CORE
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main.c
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--apply-loop-contracts --replace-all-calls-with-contracts
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activate-multi-line-match
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^\[precondition.1\] file main.c .* Check requires clause: SUCCESS$
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^\[precondition.2\] file main.c .* Check requires clause: SUCCESS$
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^\[ackermann.1\] .* Check loop invariant before entry: SUCCESS$
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^\[ackermann.2\] .* Check that loop invariant is preserved: SUCCESS$
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^\[ackermann.3\] .* Check decreases clause on loop iteration: SUCCESS$
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^.*0 of 5 failed \(1 iterations\)$
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^VERIFICATION SUCCESSFUL$
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^EXIT=0$
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^SIGNAL=0$
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--
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--
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This test succeeds. It tests whether we can prove (only partially) the
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termination of the Ackermann function using a multidimensional decreases clause.
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Note that this particular implementation of the Ackermann function contains
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both a while-loop and recursion. Therefore, to fully prove the termination of
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the Ackermann function, we must prove both
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(i) the termination of the while-loop and
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(ii) the termination of the recursion.
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Because CBMC does not support termination proofs of recursions (yet), we cannot
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prove the latter, but the former. Hence, the termination proof in the code is
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only "partial."
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Furthermore, the Ackermann function has a function contract that the result
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is always non-negative. This post-condition is necessary for establishing
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the loop invariant. However, in this test, we do not enforce the function
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contract. Instead, we assume that the function contract is correct and use it
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(i.e. replace a recursive call of the Ackermann function with its contract).
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We cannot verify/enforce the function contract of the Ackermann function, since
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CBMC does not support function contracts for recursively defined functions.
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As of now, CBMC only supports function contracts for non-recursive functions.

‎regression/contracts/variant_multidimensional_nested/main.c

Lines changed: 23 additions & 0 deletions
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@@ -0,0 +1,23 @@
1+
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int main()
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{
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int i = 0;
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int j = 0;
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int N = 10;
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while(i < N)
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// clang-format off
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__CPROVER_loop_invariant(0 <= i && i <= N && 0 <= j && j <= N)
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__CPROVER_decreases(N - i, N - j)
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// clang-format on
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{
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if(j < N)
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{
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j++;
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}
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else
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{
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i++;
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j = 0;
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}
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}
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}

‎regression/contracts/variant_multidimensional_nested/test.desc

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CORE
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main.c
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--apply-loop-contracts
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activate-multi-line-match
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^\[main.1\].*Check loop invariant before entry: SUCCESS$
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^\[main.2\].*Check that loop invariant is preserved: SUCCESS$
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^\[main.3\].*Check decreases clause on loop iteration: SUCCESS$
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^EXIT=0$
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^SIGNAL=0$
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--
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--
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This test succeeds. It tests whether a multidimensional decreases clause works
13+
properly.

‎regression/contracts/variant_multidimensional_not_decreasing_fail/main.c

Lines changed: 23 additions & 0 deletions
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1+
2+
int main()
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{
4+
int i = 0;
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int j = 0;
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int N = 10;
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while(i < N)
8+
// clang-format off
9+
__CPROVER_loop_invariant(0 <= i && i <= N && 0 <= j && j <= N)
10+
__CPROVER_decreases(N - i, j)
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// clang-format on
12+
{
13+
if(j < N)
14+
{
15+
j++;
16+
}
17+
else
18+
{
19+
i++;
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j = 0;
21+
}
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}
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}

‎regression/contracts/variant_multidimensional_not_decreasing_fail/test.desc

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1+
CORE
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main.c
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--apply-loop-contracts
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activate-multi-line-match
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^\[main.1\] .* Check loop invariant before entry: SUCCESS$
6+
^\[main.2\] .* Check that loop invariant is preserved: SUCCESS$
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^\[main.3\] .* Check decreases clause on loop iteration: FAILURE$
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^.*1 of 3 failed \(2 iterations\)$
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^VERIFICATION FAILED$
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^EXIT=10$
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^SIGNAL=0$
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--
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--
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This test fails because the given multidimensional decreases clause does not
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monotonically decrease. A mistake is in the second component of the
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decreases clause: j. It should instead be N - j.

‎regression/contracts/variant_parsing_tuple_fail/main.c renamed to ‎regression/contracts/variant_parsing_multiple_clauses_fail/main.c

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -5,7 +5,8 @@ int main()
55
while(i != N)
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// clang-format off
77
__CPROVER_loop_invariant(0 <= i && i <= N)
8-
__CPROVER_decreases(N - i, 42)
8+
__CPROVER_decreases(N - i)
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__CPROVER_decreases(42)
910
// clang-format on
1011
{
1112
i++;

‎regression/contracts/variant_parsing_multiple_clauses_fail/test.desc

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CORE
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main.c
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--apply-loop-contracts
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activate-multi-line-match
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^main.c.*error: syntax error before '__CPROVER_decreases'\n.*__CPROVER_decreases\(42\)$
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^PARSING ERROR$
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^EXIT=(1|64)$
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^SIGNAL=0$
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--
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--
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This test fails because we have multiple decreases clauses. CBMC only admits
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one decreases clause for each loop. If one wants to write a multidimensional
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decreases clause, it should be placed inside a single __CPROVER_decreases(...),
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where each component of the multidimensional decreases clause is separated by a
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comma. Hence, in this test, the correct syntax is
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__CPROVER_decreases(N - i, 42).

‎regression/contracts/variant_parsing_tuple_fail/test.desc

Lines changed: 0 additions & 13 deletions
This file was deleted.

‎src/ansi-c/c_typecheck_code.cpp

Lines changed: 6 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -828,9 +828,11 @@ void c_typecheck_baset::typecheck_spec_decreases(codet &code)
828828
{
829829
if(code.find(ID_C_spec_decreases).is_not_nil())
830830
{
831-
exprt &variant = static_cast<exprt &>(code.add(ID_C_spec_decreases));
832-
833-
typecheck_expr(variant);
834-
implicit_typecast_arithmetic(variant);
831+
for(auto &decreases_clause_component :
832+
(static_cast<exprt &>(code.add(ID_C_spec_decreases)).operands()))
833+
{
834+
typecheck_expr(decreases_clause_component);
835+
implicit_typecast_arithmetic(decreases_clause_component);
836+
}
835837
}
836838
}

‎src/ansi-c/parser.y

Lines changed: 16 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -45,6 +45,8 @@ extern char *yyansi_ctext;
4545
// requires more stack space
4646
#define YYMAXDEPTH 25600
4747

48+
#include <stdio.h>
49+
4850
/*** token declaration **************************************************/
4951
%}
5052

@@ -509,10 +511,17 @@ cprover_contract_loop_invariant_list_opt:
509511
| cprover_contract_loop_invariant_list
510512
;
511513

514+
ACSL_binding_expression_list:
515+
ACSL_binding_expression
516+
{ init($$); mto($$, $1); }
517+
| ACSL_binding_expression_list ',' ACSL_binding_expression
518+
{ $$=$1; mto($$, $3); }
519+
;
520+
512521
cprover_contract_decreases_opt:
513522
/* nothing */
514523
{ init($$); parser_stack($$).make_nil(); }
515-
| TOK_CPROVER_DECREASES '(' ACSL_binding_expression ')'
524+
| TOK_CPROVER_DECREASES '(' ACSL_binding_expression_list ')'
516525
{ $$=$3; }
517526
;
518527

@@ -2449,8 +2458,8 @@ iteration_statement:
24492458
if(!parser_stack($6).operands().empty())
24502459
static_cast<exprt &>(parser_stack($$).add(ID_C_spec_loop_invariant)).operands().swap(parser_stack($6).operands());
24512460

2452-
if(parser_stack($7).is_not_nil())
2453-
parser_stack($$).add(ID_C_spec_decreases).swap(parser_stack($7));
2461+
if(!parser_stack($7).operands().empty())
2462+
static_cast<exprt &>(parser_stack($$).add(ID_C_spec_decreases)).operands().swap(parser_stack($7).operands());
24542463
}
24552464
| TOK_DO statement TOK_WHILE '(' comma_expression ')'
24562465
cprover_contract_assigns_opt
@@ -2464,8 +2473,8 @@ iteration_statement:
24642473
if(!parser_stack($8).operands().empty())
24652474
static_cast<exprt &>(parser_stack($$).add(ID_C_spec_loop_invariant)).operands().swap(parser_stack($8).operands());
24662475

2467-
if(parser_stack($9).is_not_nil())
2468-
parser_stack($$).add(ID_C_spec_decreases).swap(parser_stack($9));
2476+
if(!parser_stack($9).operands().empty())
2477+
static_cast<exprt &>(parser_stack($$).add(ID_C_spec_decreases)).operands().swap(parser_stack($9).operands());
24692478
}
24702479
| TOK_FOR
24712480
{
@@ -2495,8 +2504,8 @@ iteration_statement:
24952504
if(!parser_stack($10).operands().empty())
24962505
static_cast<exprt &>(parser_stack($$).add(ID_C_spec_loop_invariant)).operands().swap(parser_stack($10).operands());
24972506

2498-
if(parser_stack($11).is_not_nil())
2499-
parser_stack($$).add(ID_C_spec_decreases).swap(parser_stack($11));
2507+
if(!parser_stack($11).operands().empty())
2508+
static_cast<exprt &>(parser_stack($$).add(ID_C_spec_decreases)).operands().swap(parser_stack($11).operands());
25002509

25012510
if(PARSER.for_has_scope)
25022511
PARSER.pop_scope(); // remove the C99 for-scope

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