
"AArch64" and "ARM64" refer to the same thing.
AArch64 is the 64-bit state introduced in the [Armv8-A
architecture](https://en.wikipedia.org/wiki/ARM_architecture_family#Armv8-A).
The 32-bit state which is backwards compatible with Armv7-A and previous
32-bit Arm architectures is referred to as AArch32. Therefore the GNU
triplet for the 64-bit ISA is aarch64. The Linux kernel community
[chose](https://lkml.org/lkml/2012/7/6/624) to call their port of the
kernel to this architecture arm64 rather than aarch64, so that's where
some of the arm64 usage comes from.
The Apple-developed backend for AArch64 was called "ARM64" whereas the
LLVM community-developed backend was called "AArch64" (as it is the
canonical name for the 64-bit ISA). The two were merged [in
2014](https://www.phoronix.com/news/MTY5ODk) and the backend now is
called "AArch64".
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