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CONSTRAINT GUARDS

Constraint guards are predicate expressions that function as guards against the creation of constraints, and not as logical relations to be satisfied by the solver. These predicate expressions &&,|| and ! are evaluated before the constraints are solved. This enables users to write constraints that avoid errors due to nonexistent object handles or array indices out of bounds.

There are 4 states when a sub expression is evlauated.

0 FALSE Subexpression evaluates to FALSE.

1 TRUE Subexpression evaluates to TRUE.

E ERROR Subexpression causes an evaluation error like null pointer.

R RANDOM Expression includes random variables and cannot be evaluated.

If any of subexpression results ERROR,then randomization fails.

EXAMPLE:1
class SList_97;
rand int n;
rand Slist_97 next;
constraint sort { n < next.n; }
endclass

In the Example 1, while sorting the elements of array in ascending order, if next is null i.e for last element randomization fails.

EXAMPLE:2
class SList_98;
rand int n;
rand Slist_98 next;
constraint sort { if( next != null ) n < next.n; }
endclass

In Example 2, Even if next is null, constraint wont be generated so randomization will never fail.

EXAMPLE:3
class D;
int x;
endclass
class C;
rand int x, y;
D a, b;
constraint c1 { (x < y || a.x > b.x || a.x == 5 ) -> x+y == 10; }
endclass

In Example 3, the predicate subexpressions are (x < y), (a.x > b.x), and (a.x == 5), which are all
connected by disjunction. Some possible cases are as follows:

Case 1: a is non-null, b is null, a.x is 5.
Because (a.x==5) is true, the fact that b.x generates an error does not result in an error.
The unconditional constraint (x+y == 10) is generated.

Case 2: a is null.
This always results in error, irrespective of the other conditions.

Case 3: a is non-null, b is non-null, a.x is 10, b.x is 20.
All the guard subexpressions evaluate to FALSE.
The conditional constraint (x<y) -> (x+y == 10) is generated.

EXAMPLE:4
class D;
int x;
endclass
class C;
rand int x, y;
D a, b;
constraint c1 { (x < y && a.x > b.x && a.x == 5 ) -> x+y == 10; }
endclass

In Example 4, the predicate subexpressions are (x < y), (a.x > b.x), and (a.x == 5), which are all
connected by conjunction. Some possible cases are as follows:

Case 1: a is non-null, b is null, a.x is 6.
Because (a.x==5) is false, the fact that b.x generates an error does not result in an error.
The constraint is eliminated.

Case 2: a is null
This always results in error, irrespective of the other conditions.

Case 3: a is non-null, b is non-null, a.x is 5, b.x is 2.
All the guard subexpressions evaluate to TRUE, producing constraint (x<y) -> (x+y == 10).

EXAMPLE:5
class D;
int x;
endclass
class C;
rand int x, y;
D a, b;
constraint c1 { (x < y && (a.x > b.x || a.x ==5)) -> x+y == 10; }
endclass

In Example 5, the predicate subexpressions are (x < y) and (a.x > b.x || a.x == 5), which are connected
by disjunction. Some possible cases are as follows:

Case 1: a is non-null, b is null, a.x is 5.
The guard expression evaluates to (ERROR || a.x==5), which evaluat es to (ERROR || TRUE)
The guard subexpression evaluates to TRUE.
The conditional constraint (x<y) -> (x+y == 10) is generated.

Case 2: a is non-null, b is null, a.x is 8.
The guard expression evaluates to (ERROR || FALSE) and generates an error.

Case 3: a is null
This always results in error, irrespective of the other conditions.

Case 4: a is non-null, b is non-null, a.x is 5, b.x is 2.
All the guard subexpressions evaluate to TRUE.
The conditional constraint (x<y) -> (x+y == 10) is generated.

EXAMPLE:6

class A_108;
rand integer arr[];
constraint c { foreach( arr[i]) arr[i] == arr[i+1] ;}
endclass

In Example 6, generates an error when i is the last element in the array.

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