Go’s Assembler

  • Go’s assembler is that it is not a direct representation of the underlying machine: It is chosen to match what the compiler generates

Tools

View compiler asm (semi-abstract instruction set):

$ GOOS=linux GOARCH=amd64 go tool compile -S x.go

View executable asm (machine instruction set): (r u sure???)

$ go build -o x.o x.go
$ go tool objdump -s main.main x.exe

Syntax

Symbols

Registers

Like R1, LR, which depends on the architecture.

All pre-defined symbols in the assembler are upper-case:

  • Data registers are R0 through R7

  • Address registers are A0 through A7

  • Floating-point registers are F0 through F7

Pseudo Registers
FP

Frame pointer: arguments and locals

PC

Program counter: jumps and branches

SB

Static base pointer: global symbols

SP

Stack pointer: the highest address within the local stack frame (Top of stack?)

All user-defined symbols are written as offsets to the pseudo-registers FP and SB.

Global Symbols
foo(SB)

Address of global symbol foo

foo<>(SB)

Same as above, but visible only in the current source file

foo+4(SB)

With 4 bytes offset

Function Arguments
foo+0(FP)

The first function argument named “foo”

bar+8(FP)

The second Function argument named “bar” (on a 64-bit machine),

注解

The meaning of the offset – offset from the frame pointer – distinct from its use with SB, where it is an offset from the symbol

Local Symbols
x-8(SP)

The local variable named “x”

提示

Use negative offsets (- but not +) because SP points to the highest address within the local stack frame

注解

On architectures with a hardware register named SP, x-8(SP) and -8(SP) are different memory locations:

  • the first refers to the virtual stack pointer pseudo-register

  • while the second refers to the hardware’s SP register

让人容易混淆的约定……

Branch

Branches and direct jumps are always written as offsets to the PC, or as jumps to labels.

Directives

FUNCDATA, PCDATA

Contain information for use by the garbage collector; they are introduced by the compiler.

TEXT

Define function symbol:

TEXT runtime·profileloop(SB),NOSPLIT,$8
        MOVQ      $runtime·profileloop1(SB), CX         ; Body of the function
        MOVQ      CX, 0(SP)
        CALL      runtime·externalthreadhandler(SB)
        RET                                        ; Last instruction
runtime·profileloop(SB)

Global symbol name

NOSPLIT

Flag. If NOSPLIT is not specified, the argument size (see below )must be provided.

$8

Usually in form $FLAME_SIZE-ARG_SIZE, which live on the caller’s frame, in this example, ARG_SIZE is not provided.

RET

The last instruction in a TEXT block must be some sort of jump, usually a RET (pseudo-)instruction. (If it’s not, the linker will append a jump-to-itself instruction; there is no fallthrough in TEXTs.)

DATA

Define a data symbols:

DATA  symbol+offset(SB)/width, value      ; Global
; or
DATA  symbol<>+offset(SB)/width, value    ; Local

Which initializes the symbol memory at the given offset and width with the given value.

GLOBAL

Declares a symbol to be global.

The arguments are optional flags and the size of the data being declared as a global.

Flags

See textflag.h

列一些我们关心的:

NOSPLIT = 4

(For TEXT items.) Don’t insert the preamble to check if the stack must be split. The frame for the routine, plus anything it calls, must fit in the spare space remaining in the current stack segment. Used to protect routines such as the stack splitting code itself.

运行时不进行栈扩展(有人用溢出,这可能引发歧义)。 对需要扩展栈的函数(stack frame 过大)使用此 flag,会导致编译失败

NOPTR = 16

(For DATA and GLOBL items.) This data contains no pointers and therefore does not need to be scanned by the garbage collector.

Instructions

Like GAS, Left-to-right assignment.

MOVE

Does not distinguish between the various forms of MOVE instruction: move quick, move address, etc

NOP

It is a pseudo-instruction means NO INSTRUCTION AT ALL, rather than an instruction that does nothing.

Interacting with Go

If a package has any .s files, then go build will direct the compiler to emit a special header called go_asm.h, which the .s files can then #include. The file contains symbolic #define constants for the offsets of Go struct fields, the sizes of Go struct types, and most Go const declarations defined in the current package.

  • Constants are of the form const_name: const bufSize = 1024 -> const_bufSize

  • Field offsets are of the form type_field. type reader struct { r int } -> reader_r

Runtime Coordination

待处理

TODO

Architecture-specific Details

amd64

  • Uses MOVQ rather than MOVL

  • Register BP is callee-save. Using BP as a general purpose register is allowed, however it can interfere with sampling-based profiling.