cmd/compile: add ABI register definations for loong64

Updates #40724

Co-authored-by: Xiaolin Zhao <[email protected]>
Change-Id: I56f7382dda58a565b8c3256f1c7845a3031f67de
Reviewed-on: https://go-review.googlesource.com/c/go/+/521776
Reviewed-by: Cherry Mui <[email protected]>
Run-TryBot: David Chase <[email protected]>
Auto-Submit: David Chase <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: David Chase <[email protected]>

Author: 2 people

src/cmd/compile/abi-internal.md

@@ -633,6 +633,56 @@ modifying or saving the FPCR.
 Functions are allowed to modify it between calls (as long as they
 restore it), but as of this writing Go code never does.
 
+### loong64 architecture
+
+The loong64 architecture uses R4 – R19 for integer arguments and integer results.
+
+It uses F0 – F15 for floating-point arguments and results.
+
+Registers R20 - R21, R23 – R28, R30 - R31, F16 – F31 are permanent scratch registers.
+
+Register R2 is reserved and never used.
+
+Register R20, R21 is Used by runtime.duffcopy, runtime.duffzero.
+
+Special-purpose registers used within Go generated code and Go assembly code
+are as follows:
+
+| Register | Call meaning | Return meaning | Body meaning |
+| --- | --- | --- | --- |
+| R0 | Zero value | Same | Same |
+| R1 | Link register | Link register | Scratch |
+| R3 | Stack pointer | Same | Same |
+| R20,R21 | Scratch | Scratch | Used by duffcopy, duffzero |
+| R22 | Current goroutine | Same | Same |
+| R29 | Closure context pointer | Same | Same |
+| R30, R31 | used by the assembler | Same | Same |
+
+*Rationale*: These register meanings are compatible with Go’s stack-based
+calling convention.
+
+#### Stack layout
+
+The stack pointer, R3, grows down and is aligned to 8 bytes.
+
+A function's stack frame, after the frame is created, is laid out as
+follows:
+
+    +------------------------------+
+    | ... locals ...               |
+    | ... outgoing arguments ...   |
+    | return PC                    | ← R3 points to
+    +------------------------------+ ↓ lower addresses
+
+This stack layout is used by both register-based (ABIInternal) and
+stack-based (ABI0) calling conventions.
+
+The "return PC" is loaded to the link register, R1, as part of the
+loong64 `JAL` operation.
+
+#### Flags
+All bits in CSR are system flags and are not modified by Go.
+
 ### ppc64 architecture
 
 The ppc64 architecture uses R3 – R10 and R14 – R17 for integer arguments