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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.


Index
Constrained Random Verification
Verilog Crv
Systemverilog Crv
Randomizing Objects
Random Variables
Randomization Methods
Checker
Constraint Block
Inline Constraint
Global Constraint
Constraint Mode
External Constraints
Randomization Controlability
Static Constraint
Constraint Expression
Variable Ordering
Constraint Solver Speed
Randcase
Randsequence
Random Stability
Array Randomization
Constraint Guards
Titbits

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