Are there performance advantages of 32 bit apps over 64 bit ones, on x86-64?

Question

I know the advantages of 64 bit over 32 bit, but except for compatibility, are there any advantages of 32 bit applications over 64 bit ones that could make 32-bit application faster or otherwise more efficient?

Answer 1

There's one big advantage: 32-bit applications use significantly less memory (precisely because pointers are smaller). Not everything is a pointer, e.g. strings and numbers don't change their size, so the effective difference is not 2x. I happen to know about JavaScript engines specifically, where the 64-bit version typically uses around 50% more memory for the same workload than the 32-bit version of the same engine.

V8 has recently addressed this by implementing "pointer compression" in its 64-bit version. In theory, any C/C++ app could do the same thing, but it's a big engineering effort.

That said, this generally isn't a reason not to move to 64-bit, as other benefits (more registers, more address space) typically outweigh this drawback. But it does mean that if you're targeting devices/machines with less than 4GiB memory anyway, you might want to stick with 32-bit builds, if memory consumption is a concern.

(Performance, in my experience, is a mixed bag: smaller code and smaller data mean better cache utilization on 32-bit; OTOH having more and wider registers on 64-bit can save instructions there. In rare extreme cases, a 64-bit app can process twice as much data in the same time; most of the time the difference will only be in the range 1-5%, and can go in either direction: sometimes a 32-bit build is indeed a little faster than a 64-bit build; it really depends on what the app is doing.)