Первый вариант.
5.11.1 Await StatementIn Active Oberon, an AWAIT(b) statement, where
b is an arbitrary boolean expression, is compiled into a helper procedure and an
Await system call.
The helper procedure is equivalent to the following procedure, where the parameter specifies the frame pointer of the procedure containing the await statement and all references to local variables and object fields in the expression
b are addressed relative to this parameter. A new name is generated for every helper procedure compiled and it can only be called by the runtime system.
Код:
PROCEDURE $Condition(fp: ADDRESS): BOOLEAN;
BEGIN
RETURN ”boolean expression from AWAIT statement”
END $Condition;
At the location of the await statement, the equivalent of the following code is compiled to perform the
Await system call. The helper procedure ($Condition) and the current frame pointer (FP) are passed to the runtime system so that it can later re-evaluate the condition [31]. The self reference to the associated object instance is also passed as parameter.
Код:
IF NOT $Condition(FP) THEN
Await($Condition, FP, SELF)
END
The generation of the inline IF statement by the compiler is an optimization. Measurements made while the system was hosting its own development on a dual-processor machine have shown that this avoids on average 70-85% of
Await system calls. As a further optimization, the compiler could inline the condition in the IF statement, saving a procedure call at the cost of slightly more object code.
Атомарность последнего оператора IF и вызывает сомнения.
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