On x86, it's just the extra registers, and indeed 32-bit can be faster. But AArch64 is essentially a completely different ISA from AArch32. Here's an old benc

On x86, it's just the extra registers, and indeed 32-bit can be faster. But AArch64 is essentially a completely different ISA from AArch32. Here's an old benchmark from Apple's first 64-bit CPU that shows a significant improvement on benchmarks in 64-bit versus 32-bit mode on the same device: http://www.anandtech.com/show/7335/the-iphone-5s-review/4 http://www.anandtech.com/show/7335/the-iphone-5s-review/4 Of course, this may be an idiosyncrasy of that processor implementation, or it may not generalize well to real applications which tend to stress the instruction cache more...