Java: How does the VM handle a 64bit `long` on a 32bit processor

Question

How does the JVM handle a primitive "long", which is 64bits, on a 32bit processor?

Can it utilise mulitple cores in parallel when on a Multi-Core 32bit machine?
How much slower are 64bit operations on a 32bit machine?

Answer 1

It may use multiple cores to run different threads, but it does not use them in parallel for 64 bit calculations. A 64 bit long is basically stored as two 32 bit ints. In order to add them, two additions are needed, keeping track of the carry bit. Multiplication is kind of like multiplying two two-digit numbers, except each digit is in base 2^32 instead of base 10. So on for other arithmetic operations.

Edit about speed: I can only guess about the speed difference. An addition requires two adds instead of one, and a multiplication would (I think) require four multiplies instead of one. However, I suspect that if everything can be kept in registers then the actual time for the computation would be dwarfed by the time required to go to memory twice for the read and twice for the write, so my guess is about twice as long for most operations. I imagine that it would depend on the processor, the particular JVM implementation, the phase of the moon, etc. Unless you are doing heavy number crunching, I wouldn't worry about it. Most programs spend most of their time waiting for IO to/from the disk or network.

Answer 2

From TalkingTree, and the Java HotSpot FAQ:

Generally, the benefits of being able to address larger amounts of memory come with a small performance loss in 64-bit VMs versus running the same application on a 32-bit VM. This is due to the fact that every native pointer in the system takes up 8 bytes instead of 4. The loading of this extra data has an impact on memory usage which translates to slightly slower execution depending on how many pointers get loaded during the execution of your Java program.
The good news is that with AMD64 and EM64T platforms running in 64-bit mode, the Java VM gets some additional registers which it can use to generate more efficient native instruction sequences. These extra registers increase performance to the point where there is often no performance loss at all when comparing 32 to 64-bit execution speed.

The performance difference comparing an application running on a 64-bit platform versus a 32-bit platform on SPARC is on the order of 10-20% degradation when you move to a 64-bit VM. On AMD64 and EM64T platforms this difference ranges from 0-15% depending on the amount of pointer accessing your application performs.