swift-mirror/docs/ABI/RegisterUsage.md

64-Bit Architecture Register Usage

From Swift 5, the calling convention register allocation for 64-bit architectures is largely based on existing standards, with the addition of the context and error return registers discussed in the ABI stability manifesto:

Register Purpose	ARM64	x86_64
Context register (self)	x20	r13
Error return register	x21	r12
Struct return pointer	x8	rax
Float call arguments	v0 … v7	xmm0 … xmm7
Integer call arguments	x0 … x7	rdi, rsi, rdx, rcx, r8, r9
Float return	v0 … v3	xmm0 … xmm3
Integer return	x0 … x3	rax, rdx, rcx, r8