src/cmd/internal/obj/ppc64/doc.go

// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

/*
Package ppc64 implements a PPC64 assembler that assembles Go asm into
the corresponding PPC64 instructions as defined by the Power ISA 3.0B.

This document provides information on how to write code in Go assembler
for PPC64, focusing on the differences between Go and PPC64 assembly language.
It assumes some knowledge of PPC64 assembler. The original implementation of
PPC64 in Go defined many opcodes that are different from PPC64 opcodes, but
updates to the Go assembly language used mnemonics that are mostly similar if not
identical to the PPC64 mneumonics, such as VMX and VSX instructions. Not all detail
is included here; refer to the Power ISA document if interested in more detail.

Starting with Go 1.15 the Go objdump supports the -gnu option, which provides a
side by side view of the Go assembler and the PPC64 assembler output. This is
extremely helpful in determining what final PPC64 assembly is generated from the
corresponding Go assembly.

In the examples below, the Go assembly is on the left, PPC64 assembly on the right.

1. Operand ordering

  In Go asm, the last operand (right) is the target operand, but with PPC64 asm,
  the first operand (left) is the target. The order of the remaining operands is
  not consistent: in general opcodes with 3 operands that perform math or logical
  operations have their operands in reverse order. Opcodes for vector instructions
  and those with more than 3 operands usually have operands in the same order except
  for the target operand, which is first in PPC64 asm and last in Go asm.

  Example:
    ADD R3, R4, R5		<=>	add r5, r4, r3

2. Constant operands

  In Go asm, an operand that starts with '$' indicates a constant value. If the
  instruction using the constant has an immediate version of the opcode, then an
  immediate value is used with the opcode if possible.

  Example:
    ADD $1, R3, R4		<=> 	addi r4, r3, 1

3. Opcodes setting condition codes

  In PPC64 asm, some instructions other than compares have variations that can set
  the condition code where meaningful. This is indicated by adding '.' to the end
  of the PPC64 instruction. In Go asm, these instructions have 'CC' at the end of
  the opcode. The possible settings of the condition code depend on the instruction.
  CR0 is the default for fixed-point instructions; CR1 for floating point; CR6 for
  vector instructions.

  Example:
    ANDCC R3, R4, R5		<=>	and. r5, r3, r4 (set CR0)

4. Loads and stores from memory

  In Go asm, opcodes starting with 'MOV' indicate a load or store. When the target
  is a memory reference, then it is a store; when the target is a register and the
  source is a memory reference, then it is a load.

  MOV{B,H,W,D} variations identify the size as byte, halfword, word, doubleword.

  Adding 'Z' to the opcode for a load indicates zero extend; if omitted it is sign extend.
  Adding 'U' to a load or store indicates an update of the base register with the offset.
  Adding 'BR' to an opcode indicates byte-reversed load or store, or the order opposite
  of the expected endian order. If 'BR' is used then zero extend is assumed.

  Memory references n(Ra) indicate the address in Ra + n. When used with an update form
  of an opcode, the value in Ra is incremented by n.

  Memory references (Ra+Rb) or (Ra)(Rb) indicate the address Ra + Rb, used by indexed
  loads or stores. Both forms are accepted. When used with an update then the base register
  is updated by the value in the index register.

  Examples:
    MOVD (R3), R4		<=>	ld r4,0(r3)
    MOVW (R3), R4		<=>	lwa r4,0(r3)
    MOVWZU 4(R3), R4		<=>	lwzu r4,4(r3)
    MOVWZ (R3+R5), R4		<=>	lwzx r4,r3,r5
    MOVHZ  (R3), R4		<=>	lhz r4,0(r3)
    MOVHU 2(R3), R4		<=>	lhau r4,2(r3)
    MOVBZ (R3), R4		<=>	lbz r4,0(r3)

    MOVD R4,(R3)		<=>	std r4,0(r3)
    MOVW R4,(R3)		<=>	stw r4,0(r3)
    MOVW R4,(R3+R5)		<=>	stwx r4,r3,r5
    MOVWU R4,4(R3)		<=>	stwu r4,4(r3)
    MOVH R4,2(R3)		<=>	sth r4,2(r3)
    MOVBU R4,(R3)(R5)		<=>	stbux r4,r3,r5

4. Compares

  When an instruction does a compare or other operation that might
  result in a condition code, then the resulting condition is set
  in a field of the condition register. The condition register consists
  of 8 4-bit fields named CR0 - CR7. When a compare instruction
  identifies a CR then the resulting condition is set in that field
  to be read by a later branch or isel instruction. Within these fields,
  bits are set to indicate less than, greater than, or equal conditions.

  Once an instruction sets a condition, then a subsequent branch, isel or
  other instruction can read the condition field and operate based on the
  bit settings.

  Examples:
    CMP R3, R4			<=>	cmp r3, r4	(CR0 assumed)
    CMP R3, R4, CR1		<=>	cmp cr1, r3, r4

  Note that the condition register is the target operand of compare opcodes, so
  the remaining operands are in the same order for Go asm and PPC64 asm.
  When CR0 is used then it is implicit and does not need to be specified.

5. Branches

  Many branches are represented as a form of the BC instruction. There are
  other extended opcodes to make it easier to see what type of branch is being
  used.

  The following is a brief description of the BC instruction and its commonly
  used operands.

  BC op1, op2, op3

    op1: type of branch
        16 -> bctr (branch on ctr)
        12 -> bcr  (branch if cr bit is set)
        8  -> bcr+bctr (branch on ctr and cr values)
	4  -> bcr != 0 (branch if specified cr bit is not set)

	There are more combinations but these are the most common.

    op2: condition register field and condition bit

	This contains an immediate value indicating which condition field
	to read and what bits to test. Each field is 4 bits long with CR0
        at bit 0, CR1 at bit 4, etc. The value is computed as 4*CR+condition
        with these condition values:

        0 -> LT
        1 -> GT
        2 -> EQ
        3 -> OVG

	Thus 0 means test CR0 for LT, 5 means CR1 for GT, 30 means CR7 for EQ.

    op3: branch target

  Examples:

    BC 12, 0, target		<=>	blt cr0, target
    BC 12, 2, target		<=>	beq cr0, target
    BC 12, 5, target		<=>	bgt cr1, target
    BC 12, 30, target		<=>	beq cr7, target
    BC 4, 6, target		<=>	bne cr1, target
    BC 4, 1, target		<=>	ble cr1, target

    The following extended opcodes are available for ease of use and readability:

    BNE CR2, target		<=>	bne cr2, target
    BEQ CR4, target		<=>	beq cr4, target
    BLT target			<=>	blt target (cr0 default)
    BGE CR7, target		<=>	bge cr7, target

  Refer to the ISA for more information on additional values for the BC instruction,
  how to handle OVG information, and much more.

5. Align directive

  Starting with Go 1.12, Go asm supports the PCALIGN directive, which indicates
  that the next instruction should be aligned to the specified value. Currently
  8 and 16 are the only supported values, and a maximum of 2 NOPs will be added
  to align the code. That means in the case where the code is aligned to 4 but
  PCALIGN $16 is at that location, the code will only be aligned to 8 to avoid
  adding 3 NOPs.

  The purpose of this directive is to improve performance for cases like loops
  where better alignment (8 or 16 instead of 4) might be helpful. This directive
  exists in PPC64 assembler and is frequently used by PPC64 assembler writers.

  PCALIGN $16
  PCALIGN $8

  Functions in Go are aligned to 16 bytes, as is the case in all other compilers
  for PPC64.

6. Shift instructions

  The simple scalar shifts on PPC64 expect a shift count that fits in 5 bits for
  32-bit values or 6 bit for 64-bit values. If the shift count is a constant value
  greater than the max then the assembler sets it to the max for that size (31 for
  32 bit values, 63 for 64 bit values). If the shift count is in a register, then
  only the low 5 or 6 bits of the register will be used as the shift count. The
  Go compiler will add appropriate code to compare the shift value to achieve the
  the correct result, and the assembler does not add extra checking.

  Examples:

    SRAD $8,R3,R4		=>	sradi r4,r3,8
    SRD $8,R3,R4		=>	rldicl r4,r3,56,8
    SLD $8,R3,R4		=>	rldicr r4,r3,8,55
    SRAW $16,R4,R5		=>	srawi r5,r4,16
    SRW $40,R4,R5		=>	rlwinm r5,r4,0,0,31
    SLW $12,R4,R5		=>	rlwinm r5,r4,12,0,19

  Some non-simple shifts have operands in the Go assembly which don't map directly
  onto operands in the PPC64 assembly. When an operand in a shift instruction in the
  Go assembly is a bit mask, that mask is represented as a start and end bit in the
  PPC64 assembly instead of a mask. See the ISA for more detail on these types of shifts.
  Here are a few examples:

    RLWMI $7,R3,$65535,R6 	=>	rlwimi r6,r3,7,16,31
    RLDMI $0,R4,$7,R6 		=>	rldimi r6,r4,0,61

  More recently, Go opcodes were added which map directly onto the PPC64 opcodes. It is
  recommended to use the newer opcodes to avoid confusion.

    RLDICL $0,R4,$15,R6		=>	rldicl r6,r4,0,15
    RLDICR $0,R4,$15,R6		=>	rldicr r6.r4,0,15

Register naming

1. Special register usage in Go asm

  The following registers should not be modified by user Go assembler code.

  R0: Go code expects this register to contain the value 0.
  R1: Stack pointer
  R2: TOC pointer when compiled with -shared or -dynlink (a.k.a position independent code)
  R13: TLS pointer
  R30: g (goroutine)

  Register names:

  Rn is used for general purpose registers. (0-31)
  Fn is used for floating point registers. (0-31)
  Vn is used for vector registers. Slot 0 of Vn overlaps with Fn. (0-31)
  VSn is used for vector-scalar registers. V0-V31 overlap with VS32-VS63. (0-63)
  CTR represents the count register.
  LR represents the link register.

*/
package ppc64