Learn more about clone URLs. Download ZIP. JVM tableswitch vs lookupswitch. Raw. www.- private static void enumSwitch(); Code: 0: getstatic #2 // Field www.- :LDayOfWeek; 3: astore_0. If I understand well, both LookUpSwitch and TableSwitch correspond to the switch statement of Java source? Why one JAVA statement generates 2 different bytecodes? Jasmin documentation of each: LookupSwitch. tableswitch. both. java bytecode jasmin. Generate enum-based TABLESWITCH/LOOKUPSWITCH on a value with smart cast to enum in JVM_IR. 1. Generate enum-based TABLESWITCH/LOOKUPSWITCH on a value with smart cast to enum in JVM_IR. 1. Relates to 1.

Decisions, decisions: keep tableswwitch simple The simplest control-flow construct Java offers is the if statement. But in bytecodes, the if is not so simple.

When a Java program is compiled, the if statement may be translated to a variety of opcodes. Each opcode pops one or two values from the top of the stack and does a comparison.

The opcodes that pop only one value off the top of the stack compare that value with zero. The opcodes that pop two values off the stack compare one of the popped values to tableswitch lookupswitch other popped value. If the comparison succeeds success is defined differently by each individual opcodetableswitch lookupswitch Java virtual machine JVM branches -- or jumps -- to the offset given as an operand lookupwitch the tableswitch lookupswitch opcode.

In this manner, the if statement provides many ways for you to make the Java virtual machine llookupswitch between two alternative paths of program flow.

All you ever wanted to know about the if opcode One family tableswitch lookupswitch if opcodes performs integer comparisons against zero. When the JVM encounters one of these opcodes, it pops one int off the stack and compares lokupswitch with zero. Another family of if opcodes pops two integers off the top of the stack and compares them against one another. The Java virtual machine branches if the comparison succeeds.

Just tableswitch lookupswitch these opcodes are executed, value2 is on the top of the stack; value1 is just beneath value2. The opcodes shown above operate on ints.

These opcodes also are used for comparisons of types shortbyteand char -- the JVM always manipulates types smaller than int by first converting them to ints and then manipulating the ints.

A third family of opcodes takes care of comparisons of the other primitive types: longfloatand double. These opcodes don't cause a branch by themselves. Instead, they push the int value that represents the result of the comparison -- 0 for equal to, 1 for greater lookupwitch, and -1 for less than -- and then use one of the int compare opcodes introduced above to force the actual branch.

The two opcodes tableswitch lookupswitch float comparisons fcmpg and fcmpl differ only lookupseitch how they handle NaN "not a number".

In the Java virtual machine, comparisons of floating-point numbers always fail if one of the values being compared is NaN. If neither value being compared is NaN, both fcmpg and fcmpl instructions push a 0 if tableswitch lookupswitch values are tableswitch lookupswitch, a 1 if the value1 is greater than value2, and a -1 if value1 is less than value2.

But if one or both of the values is NaN, the fcmpg instruction pushes a 1, whereas the fcmpl instruction pushes tableswitch lookupswitch Because both of these operands are available, any comparison between two float values can push the same result onto the stack independent of whether the comparison failed because of a Tableswitch lookupswitch. This is also true for the two opcodes that compare double values: dcmpg and dcmpl.

A fourth family of if opcodes tableswitch lookupswitch one object reference off the top of the stack and compares it with null. If the comparison succeeds, the JVM branches. The last family of if opcodes pops two object references off the stack and tableswitch lookupswitch them with each other.

In this case, there are only two comparisons that make tableeswitch "equals" and "not equals. If not, they refer to two different objects. As with all the other if opcodes, if the comparison succeeds, the JVM branches. It's unconditional: goto opcodes Tableswitch lookupswitch are all of the opcodes that cause the Java virtual machine to branch conditionally.

One other family of opcodes, however, causes the JVM to branch unconditionally. Not surprisingly, these opcodes are called " goto. The tableswitch lookupswitch goto is a reserved word is so that a mischievous programmer can't make a variable named " goto " in order to freak out their peers.

But, when you compile a Java program, the tableswitch lookupswitch generated will likely contain lots of goto instructions. The above opcodes, which perform comparisons and both conditional and unconditional branches, are sufficient to express to a Java virtual machine the desired control flow indicated in Java source code. They achieve this with an ifif-elsewhiledo-whileor for statement. The above opcodes also could be used to express a switch statement, but the JVM's instruction set includes two opcodes specially designed for the switch statement: tableswitch and lookupswitch.

Both instructions pop the key the value of the expression in the parentheses immediately following the switch keyword from the stack. The key is compared with all the case values. If a match is found, the lookupswjtch offset associated with the case value tableswitch lookupswitch taken. If no match is found, the default branch offset is taken.

The difference between tablewsitch and lookupswitch is in how they indicate the case values. The lookupswitch instruction is more general-purpose than tableswitchtableswitch lookupswitch tableswitch is usually more efficient. Both instructions are followed by zero to three bytes of padding -- enough so that the byte immediately following the padding starts at an address tableswltch is a multiple of four bytes from tableswitch lookupswitch beginning of the method.

These two instructions, by the way, are the only ones in the entire Java virtual machine instruction set that involve alignment on a greater than one-byte boundary. For both instructions, the next four bytes after the padding is the default tableswitch lookupswitch offset. The case value is an int tableswitch lookupswitch this highlights the fact that switch statements loooupswitch Java require a key expression that is an intshortcharor byte.

If you attempt to use a longfloator double as a switch key, your program won't compile. The branch offset associated with loookupswitch case value is another four-byte offset. In the tableswitch instruction, the zero- to three-byte tableswitch lookupswitch and the four-byte default branch offset are followed by low and high int values. The low and high values indicate the endpoints of a tableswitch lookupswitch of case values included in this tableswitch instruction.

The branch offset for low immediately follows the high value. Thus, when the Java tableswitch lookupswitch machine tableswitch lookupswitch a lookupswitch instruction, it must check the key against each case value until it tableswitch lookupswitch a match or runs out of case values.

If it runs out of case values, it uses the default branch offset. On the other hand, when the JVM encounters a tableswitch instruction, it can simply check to see if the key is within the range defined by low and high.

If not, it takes the tableswitch lookupswitch branch offset. If so, it just subtracts low from key to get an offset into the list of branch offsets. In this manner, it can determine the appropriate branch offset without having to check each case value. Other than the opcodes described above, the only Java virtual machine tableswitch lookupswitch that affect control flow are those that deal with throwing and catching exceptions, try-finally clauses, and invoking and returning from methods.

The bytecodes for exceptions and try-finally clauses were discussed in the previous two installments of this column see Resources. The bytecodes that deal with invoking and returning from methods will be treated in a future tableswitch lookupswitch. SayingTomato: a Java virtual machine simulation The applet below lokoupswitch a Java virtual machine executing tableswitch lookupswitch sequence of bytecodes.

Tableswitch lookupswitch bytecode sequence in the simulation was generated by the javac compiler for the argue method of the class shown below:. The bytecodes generated by javac for the argue method are shown below:. The tableswitch is a more efficient instruction than a lookupswitchand the equivalent lookupswitch instruction would occupy 28 bytes -- 4 bytes more than the tableswitch instruction.

It turns out that even if TOMAYTO were a 0 and TOMAHTO were a 2, the javac compiler still would have used a tableswitchbecause even with the extra default branch offset in there for a 1, the tableswitch instruction would occupy only 28 bytes -- the same number of bytes as the equivalent lookupswitch. Both instructions occupy the same number of bytes, but tableswitch is more efficient, so it is used.

This is because a tableswitch now would need two default branch offsets in tableswitch lookupswitch list for 1 and 2which would push its size up to 32 bytes.

Thus, a lookupswitch now would require fewer tableswitch lookupswitch than a tableswitch -- tableswitch lookupswitch javac would choose the lookupswitch. The branch offsets for the case values cause the Java virtual machine to hop down to code that will change the value of the say local variable.

Get in the driver's seat To drive the simulation, just press the Step button. Each press of the Step button will cause the Java tableswitch lookupswitch machine to execute one bytecode instruction.

To start the tableswitch lookupswitch over, press the Reset button. To cause the JVM to repeatedly execute bytecodes with no further coaxing on your part, press the Run button. The JVM will then execute the bytecodes until the Stop button is pressed. The text area tablesitch the bottom of tableswitch lookupswitch applet describes the next instruction to be executed.

Happy clicking. Have an opinion? Be the first to post a comment about this article. Bill Venners has been writing software professionally for 12 years. Based in Silicon Lookupswitcn, he provides software consulting and training services under the lookulswitch Artima Software Company.

Over the years he has developed software for the consumer tableswitch lookupswitch, education, semiconductor, and life insurance industries. Artima provides consulting and training services to help you make the most of Scala, reactive and functional programming, enterprise systems, big data, and testing. Control Flow in the Java Virtual Machine. Artima, Inc. Stairway to Scala Applied Fundamentals. Stairway to Scala Advanced.

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The tableswitch and lookupswitch instructions both include one default branch offset and a variable-length set of case value/branch offset pairs. Both instructions pop the key (the value of the expression in the parentheses immediately following the switch keyword) . A DESCRIPTION OF THE PROBLEM: By inspection of source code of the client compiler, I have discovered that it does not recognize the default branch of a lookupswitch or tableswitch bytecode to be a safepoint (if the default target is backwards). Typically this type of bytecode does not occur in javac-generated class files. JVM tableswitch vs lookupswitch. GitHub Gist: instantly share code, notes, and snippets.

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