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cmd/compile: document register-based ABI for s390x

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This CL adds the s390x information to the ABI doc.

Update #40724

Cq-Include-Trybots: luci.golang.try:gotip-linux-s390x
Change-Id: I1b4b25ef1003e2ab011e1b808aeb1c02288095c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/719460
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Vishwanatha HD <vishwanatha.hd@ibm.com>
TryBot-Bypass: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
This commit is contained in:
Srinivas Pokala 2025-11-11 04:13:51 +01:00 • committed by Keith Randall
parent 8dd5b13abc
commit 2e5d12a277
1 changed files with 45 additions and 0 deletions
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src/cmd/compile/abi-internal.md
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@ -833,6 +833,51 @@ The riscv64 has Zicsr extension for control and status register (CSR) and
treated as scratch register.
All bits in CSR are system flags and are not modified by Go.
### s390x architecture
The s390x architecture uses R2 – R9 for integer arguments and integer results.
It uses F0 – F15 for floating-point arguments and results.
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 | Scratch | Scratch | Scratch |
| R10, R11 | used by the assembler | Same | Same |
| R12 | Closure context pointer | Same | Same |
| R13 | Current goroutine | Same | Same |
| R14 | Link register | Link register | Scratch |
| R15 | Stack pointer | Same | Same |
*Rationale*: These register meanings are compatible with Go’s stack-based
calling convention.
#### Stack layout
The stack pointer, R15, 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 | ← R15 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 R14, as part of the
s390x `BL` operation.
#### Flags
The s390x architecture maintains a single condition code (CC) field in the Program Status Word (PSW).
Go-generated code sets and tests this condition code to control conditional branches.
## Future directions
### Spill path improvements