Recently the question came up what the difference is between the usual implication operator (|->
) and the implies
operator in SystemVerilog. Unfortunately I couldn't find a clear answer yet. However, I collected the following information:
From SystemVerilog LRM 1800-2012:
§ 16.12.7 Implies and iff properties:
property_expr1 implies property_expr2
A property of this form evaluates to true if, and only if, either property_expr1 evaluates to false or property_expr2 evaluates to true.
§ F.3.4.3.2 Derived Boolean operators:
p1 implies p2 ≡ (not p1 or p2)
§ F.3.4.3.4 Derived conditional operators:
(if(b) P) ≡ (b |-> P)
However, the LRM does not really point out what the actual difference is. I assume that they differ in the evaluation in case of a false antecedent (success vs. vacuous success), but I could not find any source or evidence for this assumption. Moreover, I know that the implies
operator is very common in combination with formal verification tools like OneSpin.
Could anyone help me out?
P.S.: It seems there is an answer to this question in the following book: SystemVerilog Assertions Handbook, 3rd Edition. But $155 is a bit too much for me just for getting the answer to this question :)
I think there is even a more significant difference. Assume that we have the following example:
property p1;
@ (posedge clk)
a ##1 b |-> c;
endproperty
property p2;
@ (posedge clk)
a ##1 b implies c;
endproperty
assert property (p1);
assert property (p2);
Both implication operators simply have different proving behavior. Property p1
will be triggered through a match of a ##1 b
and will look for a matching c
during the same clock tick as b
. However, property p2
is triggered by a ##1 b
and will check for a match of c
during the clock cycle of a
. This means the properties would pass for the following scenarios:
Property p1 passes and p2 fails: Property p2 passes and p1 fails:
A hint for this behavior can be found in the SystemVerilog LRM. The defined substitutions are:
(if(b) P) = (b |-> P)
p1 implies p2 = (not p1 or p2)
So all in all, if one uses the implies operator it becomes easier to define multi-cycle operations since antecedent and consequence have the same starting point for the evaluation.
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