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cmd/compile: document register-based ABI for ppc64
This adds the ppc64 information to the ABI doc. Change-Id: I55ce77a59a8b50309d02e3e0de0cea3fc62ad003 Reviewed-on: https://go-review.googlesource.com/c/go/+/342869 Trust: Lynn Boger <laboger@linux.vnet.ibm.com> Reviewed-by: Cherry Mui <cherryyz@google.com>
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@ -627,6 +627,105 @@ modifying or saving the FPCR.
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Functions are allowed to modify it between calls (as long as they
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restore it), but as of this writing Go code never does.
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### ppc64 architecture
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The ppc64 architecture uses R3 – R10 and R14 – R17 for integer arguments
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and results.
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It uses F1 – F12 for floating-point arguments and results.
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Register R31 is a permanent scratch register in Go.
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Special-purpose registers used within Go generated code and Go
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assembly code are as follows:
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| Register | Call meaning | Return meaning | Body meaning |
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| --- | --- | --- | --- |
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| R0 | Zero value | Same | Same |
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| R1 | Stack pointer | Same | Same |
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| R2 | TOC register | Same | Same |
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| R11 | Closure context pointer | Scratch | Scratch |
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| R12 | Function address on indirect calls | Scratch | Scratch |
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| R13 | TLS pointer | Same | Same |
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| R20,R21 | Scratch | Scratch | Used by duffcopy, duffzero |
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| R30 | Current goroutine | Same | Same |
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| R31 | Scratch | Scratch | Scratch |
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| LR | Link register | Link register | Scratch |
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*Rationale*: These register meanings are compatible with Go’s
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stack-based calling convention.
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The link register, LR, holds the function return
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address at the function entry and is set to the correct return
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address before exiting the function. It is also used
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in some cases as the function address when doing an indirect call.
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The register R2 contains the address of the TOC (table of contents) which
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contains data or code addresses used when generating position independent
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code. Non-Go code generated when using cgo contains TOC-relative addresses
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which depend on R2 holding a valid TOC. Go code compiled with -shared or
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-dynlink initializes and maintains R2 and uses it in some cases for
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function calls; Go code compiled without these options does not modify R2.
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When making a function call R12 contains the function address for use by the
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code to generate R2 at the beginning of the function. R12 can be used for
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other purposes within the body of the function, such as trampoline generation.
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R20 and R21 are used in duffcopy and duffzero which could be generated
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before arguments are saved so should not be used for register arguments.
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The Count register CTR can be used as the call target for some branch instructions.
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It holds the return address when preemption has occurred.
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On PPC64 when a float32 is loaded it becomes a float64 in the register, which is
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different from other platforms and that needs to be recognized by the internal
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implementation of reflection so that float32 arguments are passed correctly.
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Registers R18 - R29 and F13 - F31 are considered scratch registers.
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#### Stack layout
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The stack pointer, R1, grows down and is aligned to 8 bytes in Go, but changed
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to 16 bytes when calling cgo.
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A function's stack frame, after the frame is created, is laid out as
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follows:
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+------------------------------+
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| ... locals ... |
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| ... outgoing arguments ... |
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| 24 TOC register R2 save | When compiled with -shared/-dynlink
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| 16 Unused in Go | Not used in Go
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| 8 CR save | nonvolatile CR fields
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| 0 return PC | ← R1 points to
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+------------------------------+ ↓ lower addresses
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The "return PC" is loaded to the link register, LR, as part of the
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ppc64 `BL` operations.
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On entry to a non-leaf function, the stack frame size is subtracted from R1 to
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create its stack frame, and saves the value of LR at the bottom of the frame.
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A leaf function that does not require any stack space does not modify R1 and
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does not save LR.
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*NOTE*: We might need to save the frame pointer on the stack as
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in the PPC64 ELF v2 ABI so Go can inter-operate with platform debuggers
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and profilers.
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This stack layout is used by both register-based (ABIInternal) and
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stack-based (ABI0) calling conventions.
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#### Flags
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The condition register consists of 8 condition code register fields
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CR0-CR7. Go generated code only sets and uses CR0, commonly set by
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compare functions and use to determine the target of a conditional
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branch. The generated code does not set or use CR1-CR7.
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The floating point status and control register (FPSCR) is initialized
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to 0 by the kernel at startup of the Go program and not changed by
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the Go generated code.
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## Future directions
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### Spill path improvements
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