From 9e60c371473ff15eb4b13981721b29f62e1d862c Mon Sep 17 00:00:00 2001 From: Lynn Boger Date: Tue, 17 Aug 2021 07:24:31 -0500 Subject: 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 Reviewed-by: Cherry Mui --- src/cmd/compile/abi-internal.md | 99 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 99 insertions(+) diff --git a/src/cmd/compile/abi-internal.md b/src/cmd/compile/abi-internal.md index 3619aea4aa..50d8ed9159 100644 --- a/src/cmd/compile/abi-internal.md +++ b/src/cmd/compile/abi-internal.md @@ -627,6 +627,105 @@ 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. +### ppc64 architecture + +The ppc64 architecture uses R3 – R10 and R14 – R17 for integer arguments +and results. + +It uses F1 – F12 for floating-point arguments and results. + +Register R31 is a permanent scratch register in Go. + +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 | Stack pointer | Same | Same | +| R2 | TOC register | Same | Same | +| R11 | Closure context pointer | Scratch | Scratch | +| R12 | Function address on indirect calls | Scratch | Scratch | +| R13 | TLS pointer | Same | Same | +| R20,R21 | Scratch | Scratch | Used by duffcopy, duffzero | +| R30 | Current goroutine | Same | Same | +| R31 | Scratch | Scratch | Scratch | +| LR | Link register | Link register | Scratch | +*Rationale*: These register meanings are compatible with Go’s +stack-based calling convention. + +The link register, LR, holds the function return +address at the function entry and is set to the correct return +address before exiting the function. It is also used +in some cases as the function address when doing an indirect call. + +The register R2 contains the address of the TOC (table of contents) which +contains data or code addresses used when generating position independent +code. Non-Go code generated when using cgo contains TOC-relative addresses +which depend on R2 holding a valid TOC. Go code compiled with -shared or +-dynlink initializes and maintains R2 and uses it in some cases for +function calls; Go code compiled without these options does not modify R2. + +When making a function call R12 contains the function address for use by the +code to generate R2 at the beginning of the function. R12 can be used for +other purposes within the body of the function, such as trampoline generation. + +R20 and R21 are used in duffcopy and duffzero which could be generated +before arguments are saved so should not be used for register arguments. + +The Count register CTR can be used as the call target for some branch instructions. +It holds the return address when preemption has occurred. + +On PPC64 when a float32 is loaded it becomes a float64 in the register, which is +different from other platforms and that needs to be recognized by the internal +implementation of reflection so that float32 arguments are passed correctly. + +Registers R18 - R29 and F13 - F31 are considered scratch registers. + +#### Stack layout + +The stack pointer, R1, grows down and is aligned to 8 bytes in Go, but changed +to 16 bytes when calling cgo. + +A function's stack frame, after the frame is created, is laid out as +follows: + + +------------------------------+ + | ... locals ... | + | ... outgoing arguments ... | + | 24 TOC register R2 save | When compiled with -shared/-dynlink + | 16 Unused in Go | Not used in Go + | 8 CR save | nonvolatile CR fields + | 0 return PC | ← R1 points to + +------------------------------+ ↓ lower addresses + +The "return PC" is loaded to the link register, LR, as part of the +ppc64 `BL` operations. + +On entry to a non-leaf function, the stack frame size is subtracted from R1 to +create its stack frame, and saves the value of LR at the bottom of the frame. + +A leaf function that does not require any stack space does not modify R1 and +does not save LR. + +*NOTE*: We might need to save the frame pointer on the stack as +in the PPC64 ELF v2 ABI so Go can inter-operate with platform debuggers +and profilers. + +This stack layout is used by both register-based (ABIInternal) and +stack-based (ABI0) calling conventions. + +#### Flags + +The condition register consists of 8 condition code register fields +CR0-CR7. Go generated code only sets and uses CR0, commonly set by +compare functions and use to determine the target of a conditional +branch. The generated code does not set or use CR1-CR7. + +The floating point status and control register (FPSCR) is initialized +to 0 by the kernel at startup of the Go program and not changed by +the Go generated code. + ## Future directions ### Spill path improvements -- cgit v1.2.3-54-g00ecf