Version II

Author: johnno1962

docs/ABI/RegisterAllocation.md

@@ -0,0 +1,13 @@
+## 64 bit Architecture Register Allocations
+
+From Swift 5, the calling convention register allocation for 64 architectures is largely based on [existing](https://developer.apple.com/library/content/documentation/Xcode/Conceptual/iPhoneOSABIReference/Articles/ARM64FunctionCallingConventions.html) [standards](https://developer.apple.com/library/content/documentation/DeveloperTools/Conceptual/LowLevelABI/140-x86-64_Function_Calling_Conventions/x86_64.html) with the addition of the context and error return registers discussed in the ABI manifesto:
+
+| Register Purpose | ARM64 | x86_64 |
+| ------------- |:-------------:| ----- |
+| Context register (self) | x20 | r13 |
+| Error return register | x21 | r12 |
+| Struct return pointer | x8 | rax |
+| Float call arguments | d0 - d7 | xmm0 - xmm7 |
+| Integer call arguments | x0 - x7 | rdi, rsi, rdx, rcx, r8, r9 |
+| Float return | d0, d1 | xmm0, xmm1 |
+| Integer return | x1, x2 | rax, rbx |

docs/ABIStabilityManifesto.md

@@ -337,21 +337,7 @@ Having the call context register be callee-saved is advantageous. It keeps the r
 
 Throwing functions communicate error values to their callers through the *error* register on some platforms. The error register holds a pointer to the error value if an error occurred, otherwise 0. The caller of a throwing function is expected to quickly check for 0 before continuing on with non-error code, otherwise branching to code to handle or propagate the error. Using a callee-saved register for the error register enables free conversion from non-throwing to throwing functions, which is required to honor the subtyping relationship.
 
-### Register Allocations
-
-From Swift 5, the register allocation for 64 architectures is as follows:
-
-| Register Purpose | ARM64 | x86_64 |
-| ------------- |:-------------:| ----- |
-| Context register (self) | x20 | r13 |
-| Error return register | x21 | r12 |
-| Struct return pointer | x8 | rax |
-| Float call arguments | d0 - d7 | xmm0 - xmm7 |
-| Integer call arguments | x0 - x7 | rdi, rsi, rdx, rcx, r8, r9 |
-| Float return | d0, d1 | xmm0, xmm1 |
-| Integer return | x1, x2 | rax, rbx |
-
-For more detail consult the specifications linked at the end of this document.
+The specific registers used in these roles is documented in [this document](ABI/RegisterAllocation.md).
 
 ### <a name="function-signature-lowering"></a>Function Signature Lowering