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cmd/internal/obj/ppc64: update doc

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This updates the ppc64 asm doc file, including information on
updates to the objdump, correcting information on operand order,
and adding some information on shifts.

Change-Id: Ib8ed53eac86c2121ea5b657c361ad92aae31cb32
Reviewed-on: https://go-review.googlesource.com/c/go/+/238237
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Lynn Boger 2020-06-16 13:40:03 -04:00
parent d872bbcfec
commit 4379fa1740
1 changed files with 46 additions and 6 deletions
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52
src/cmd/internal/obj/ppc64/doc.go
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@ -4,9 +4,7 @@
/*
Package ppc64 implements a PPC64 assembler that assembles Go asm into
the corresponding PPC64 binary instructions as defined by the Power
ISA. Since POWER8 is the minimum instruction set used by GOARCHes ppc64le
and ppc64, refer to ISA 2.07B or later for details.
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.
@ -16,16 +14,24 @@ updates to the Go assembly language used mnemonics that are mostly similar if no
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. In general, the remaining operands are
in the same order except in a few special cases, especially those with 4 operands.
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, r3, r4
ADD R3, R4, R5 <=> add r5, r4, r3
2. Constant operands
@ -179,6 +185,40 @@ In the examples below, the Go assembly is on the left, PPC64 assembly on the rig
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