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If the condition is true, then the lines of code inside the loop are executed. The advancement to the next iteration part is performed exactly once every time the loop ends. The loop is then repeated if the condition evaluates to true. Here is an example of the C-style traditional for-loop in Java.
load a reference onto the stack from a local variable #index: aload_0 2a 0010 1010 → objectref load a reference onto the stack from local variable 0 aload_1 2b 0010 1011 → objectref load a reference onto the stack from local variable 1 aload_2 2c 0010 1100 → objectref load a reference onto the stack from local variable 2 aload_3 2d 0010 1101
For example, PostScript has a return stack and an operand stack, and also has a graphics state stack and a dictionary stack. Many virtual machines are also stack-oriented, including the p-code machine and the Java Virtual Machine .
The following example is done in Ada which supports both early exit from loops and loops with test in the middle. Both features are very similar and comparing both code snippets will show the difference: early exit must be combined with an if statement while a condition in the middle is a self-contained construct.
Typical Java interpreters do not buffer the top-of-stack this way, however, because the program and stack have a mix of short and wide data values. If the hardwired stack machine has 2 or more top-stack registers, or a register file, then all memory access is avoided in this example and there is only 1 data cache cycle.
In addition, the loop control variables and number of operations inside the unrolled loop structure have to be chosen carefully so that the result is indeed the same as in the original code (assuming this is a later optimization on already working code). For example, consider the implications if the iteration count were not divisible by 5.
Nested functions can be used for unstructured control flow, by using the return statement for general unstructured control flow.This can be used for finer-grained control than is possible with other built-in features of the language – for example, it can allow early termination of a for loop if break is not available, or early termination of a nested for loop if a multi-level break or ...
Java bytecode is the instruction set of the Java virtual machine (JVM), the language to which Java and other JVM-compatible source code is compiled. [1] Each instruction is represented by a single byte , hence the name bytecode , making it a compact form of data .