## A Quick Guide to Go's Assembler

This document is a quick outline of the unusual form of assembly language used by the `gc` suite of Go compilers (`6g`, `8g`, etc.). The document is not comprehensive.

The assembler is based on the input to the Plan 9 assemblers, which is documented in detail on the Plan 9 site. If you plan to write assembly language, you should read that document although much of it is Plan 9-specific. This document provides a summary of the syntax and describes the peculiarities that apply when writing assembly code to interact with Go.

The most important thing to know about Go's assembler is that it is not a direct representation of the underlying machine. Some of the details map precisely to the machine, but some do not. This is because the compiler suite (see this description) needs no assembler pass in the usual pipeline. Instead, the compiler emits a kind of incompletely defined instruction set, in binary form, which the linker then completes. In particular, the linker does instruction selection, so when you see an instruction like `MOV` what the linker actually generates for that operation might not be a move instruction at all, perhaps a clear or load. Or it might correspond exactly to the machine instruction with that name. In general, machine-specific operations tend to appear as themselves, while more general concepts like memory move and subroutine call and return are more abstract. The details vary with architecture, and we apologize for the imprecision; the situation is not well-defined.

The assembler program is a way to generate that intermediate, incompletely defined instruction sequence as input for the linker. If you want to see what the instructions look like in assembly for a given architecture, say amd64, there are many examples in the sources of the standard library, in packages such as `runtime` and `math/big`. You can also examine what the compiler emits as assembly code:

    $ cat x.go
    package main

    func main() {
    	println(3)
    }
    $ go tool 6g -S x.go        # or: go build -gcflags -S x.go

    --- prog list "main" ---
    0000 (x.go:3) TEXT    main+0(SB),$8-0
    0001 (x.go:3) FUNCDATA $0,gcargs·0+0(SB)
    0002 (x.go:3) FUNCDATA $1,gclocals·0+0(SB)
    0003 (x.go:4) MOVQ    $3,(SP)
    0004 (x.go:4) PCDATA  $0,$8
    0005 (x.go:4) CALL    ,runtime.printint+0(SB)
    0006 (x.go:4) PCDATA  $0,$-1
    0007 (x.go:4) PCDATA  $0,$0
    0008 (x.go:4) CALL    ,runtime.printnl+0(SB)
    0009 (x.go:4) PCDATA  $0,$-1
    0010 (x.go:5) RET     ,
    ...

The `FUNCDATA` and `PCDATA` directives contain information for use by the garbage collector; they are introduced by the compiler.

### Symbols

Some symbols, such as `PC`, `R0` and `SP`, are predeclared and refer to registers. There are two other predeclared symbols, `SB` (static base) and `FP` (frame pointer). All user-defined symbols other than jump labels are written as offsets to these pseudo-registers.

The `SB` pseudo-register can be thought of as the origin of memory, so the symbol `foo(SB)` is the name `foo` as an address in memory. This form is used to name global functions and data. Adding `<>` to the name, as in `foo<>(SB)`, makes the name visible only in the current source file, like a top-level `static` declaration in a C file.

The `FP` pseudo-register is a virtual frame pointer used to refer to function arguments. The compilers maintain a virtual frame pointer and refer to the arguments on the stack as offsets from that pseudo-register. Thus `0(FP)` is the first argument to the function, `8(FP)` is the second (on a 64-bit machine), and so on. When referring to a function argument this way, it is conventional to place the name at the beginning, as in `first_arg+0(FP)` and `second_arg+8(FP)`. Some of the assemblers enforce this convention, rejecting plain `0(FP)` and `8(FP)`. For assembly functions with Go prototypes, `go` `vet` will check that the argument names and offsets match. On 32-bit systems, the low and high 32 bits of a 64-bit value are distinguished by adding a `_lo` or `_hi` suffix to the name, as in `arg_lo+0(FP)` or `arg_hi+4(FP)`. If a Go prototype does not name its result, the expected assembly name is `ret`.

The `SP` pseudo-register is a virtual stack pointer used to refer to frame-local variables and the arguments being prepared for function calls. It points to the top of the local stack frame, so references should use negative offsets in the range [−framesize, 0): `x-8(SP)`, `y-4(SP)`, and so on. On architectures with a real register named `SP`, the name prefix distinguishes references to the virtual stack pointer from references to the architectural `SP` register. That is, `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.

Instructions, registers, and assembler directives are always in UPPER CASE to remind you that assembly programming is a fraught endeavor. (Exception: the `g` register renaming on ARM.)

In Go object files and binaries, the full name of a symbol is the package path followed by a period and the symbol name: `fmt.Printf` or `math/rand.Int`. Because the assembler's parser treats period and slash as punctuation, those strings cannot be used directly as identifier names. Instead, the assembler allows the middle dot character U+00B7 and the division slash U+2215 in identifiers and rewrites them to plain period and slash. Within an assembler source file, the symbols above are written as `fmt·Printf` and `math∕rand·Int`. The assembly listings generated by the compilers when using the `-S` flag show the period and slash directly instead of the Unicode replacements required by the assemblers.

Most hand-written assembly files do not include the full package path in symbol names, because the linker inserts the package path of the current object file at the beginning of any name starting with a period: in an assembly source file within the math/rand package implementation, the package's Int function can be referred to as `·Int`. This convention avoids the need to hard-code a package's import path in its own source code, making it easier to move the code from one location to another.

### Directives

The assembler uses various directives to bind text and data to symbol names. For example, here is a simple complete function definition. The `TEXT` directive declares the symbol `runtime·profileloop` and the instructions that follow form the body of the function. 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`.) After the symbol, the arguments are flags (see below) and the frame size, a constant (but see below):

    TEXT runtime·profileloop(SB),NOSPLIT,$8
    	MOVQ	$runtime·profileloop1(SB), CX
    	MOVQ	CX, 0(SP)
    	CALL	runtime·externalthreadhandler(SB)
    	RET

In the general case, the frame size is followed by an argument size, separated by a minus sign. (It's not a subtraction, just idiosyncratic syntax.) The frame size `$24-8` states that the function has a 24-byte frame and is called with 8 bytes of argument, which live on the caller's frame. If `NOSPLIT` is not specified for the `TEXT`, the argument size must be provided. For assembly functions with Go prototypes, `go` `vet` will check that the argument size is correct.

Note that the symbol name uses a middle dot to separate the components and is specified as an offset from the static base pseudo-register `SB`. This function would be called from Go source for package `runtime` using the simple name `profileloop`.

Global data symbols are defined by a sequence of initializing `DATA` directives followed by a `GLOBL` directive. Each `DATA` directive initializes a section of the corresponding memory. The memory not explicitly initialized is zeroed. The general form of the `DATA` directive is

    DATA	symbol+offset(SB)/width, value

which initializes the symbol memory at the given offset and width with the given value. The `DATA` directives for a given symbol must be written with increasing offsets.

The `GLOBL` directive declares a symbol to be global. The arguments are optional flags and the size of the data being declared as a global, which will have initial value all zeros unless a `DATA` directive has initialized it. The `GLOBL` directive must follow any corresponding `DATA` directives.

For example,

    DATA divtab<>+0x00(SB)/4, $0xf4f8fcff
    DATA divtab<>+0x04(SB)/4, $0xe6eaedf0
    ...
    DATA divtab<>+0x3c(SB)/4, $0x81828384
    GLOBL divtab<>(SB), RODATA, $64

    GLOBL runtime·tlsoffset(SB), NOPTR, $4

declares and initializes `divtab<>`, a read-only 64-byte table of 4-byte integer values, and declares `runtime·tlsoffset`, a 4-byte, implicitly zeroed variable that contains no pointers.

There may be one or two arguments to the directives. If there are two, the first is a bit mask of flags, which can be written as numeric expressions, added or or-ed together, or can be set symbolically for easier absorption by a human. Their values, defined in the standard `#include` file `textflag.h`, are:

- `NOPROF` = 1
  (For `TEXT` items.) Don't profile the marked function. This flag is deprecated.
- `DUPOK` = 2
  It is legal to have multiple instances of this symbol in a single binary. The linker will choose one of the duplicates to use.
- `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 at the top of the stack segment. Used to protect routines such as the stack splitting code itself.
- `RODATA` = 8
  (For `DATA` and `GLOBL` items.) Put this data in a read-only section.
- `NOPTR` = 16
  (For `DATA` and `GLOBL` items.) This data contains no pointers and therefore does not need to be scanned by the garbage collector.
- `WRAPPER` = 32
  (For `TEXT` items.) This is a wrapper function and should not count as disabling `recover`.

### Runtime Coordination

For garbage collection to run correctly, the runtime must know the location of pointers in all global data and in most stack frames. The Go compiler emits this information when compiling Go source files, but assembly programs must define it explicitly.

A data symbol marked with the `NOPTR` flag (see above) is treated as containing no pointers to runtime-allocated data. A data symbol with the `RODATA` flag is allocated in read-only memory and is therefore treated as implicitly marked `NOPTR`. A data symbol with a total size smaller than a pointer is also treated as implicitly marked `NOPTR`. It is not possible to define a symbol containing pointers in an assembly source file; such a symbol must be defined in a Go source file instead. Assembly source can still refer to the symbol by name even without `DATA` and `GLOBL` directives. A good general rule of thumb is to define all non-`RODATA` symbols in Go instead of in assembly.

Each function also needs annotations giving the location of live pointers in its arguments, results, and local stack frame. For an assembly function with no pointer results and either no local stack frame or no function calls, the only requirement is to define a Go prototype for the function in a Go source file in the same package. The name of the assembly function must not contain the package name component (for example, function `Syscall` in package `syscall` should use the name `·Syscall` instead of the equivalent name `syscall·Syscall` in its `TEXT` directive). For more complex situations, explicit annotation is needed. These annotations use pseudo-instructions defined in the standard `#include` file `funcdata.h`.

If a function has no arguments and no results, the pointer information can be omitted. This is indicated by an argument size annotation of `$n-0` on the `TEXT` instruction. Otherwise, pointer information must be provided by a Go prototype for the function in a Go source file, even for assembly functions not called directly from Go. (The prototype will also let `go` `vet` check the argument references.) At the start of the function, the arguments are assumed to be initialized but the results are assumed uninitialized. If the results will hold live pointers during a call instruction, the function should start by zeroing the results and then executing the pseudo-instruction `GO_RESULTS_INITIALIZED`. This instruction records that the results are now initialized and should be scanned during stack movement and garbage collection. It is typically easier to arrange that assembly functions do not return pointers or do not contain call instructions; no assembly functions in the standard library use `GO_RESULTS_INITIALIZED`.

If a function has no local stack frame, the pointer information can be omitted. This is indicated by a local frame size annotation of `$0-n` on the `TEXT` instruction. The pointer information can also be omitted if the function contains no call instructions. Otherwise, the local stack frame must not contain pointers, and the assembly must confirm this fact by executing the pseudo-instruction `NO_LOCAL_POINTERS`. Because stack resizing is implemented by moving the stack, the stack pointer may change during any function call: even pointers to stack data must not be kept in local variables.

## Architecture-specific details

It is impractical to list all the instructions and other details for each machine. To see what instructions are defined for a given machine, say 32-bit Intel x86, look in the top-level header file for the corresponding linker, in this case `8l`. That is, the file `$GOROOT/src/cmd/8l/8.out.h` contains a C enumeration, called `as`, of the instructions and their spellings as known to the assembler and linker for that architecture. In that file you'll find a declaration that begins

    enum	as
    {
    	AXXX,
    	AAAA,
    	AAAD,
    	AAAM,
    	AAAS,
    	AADCB,
    	...

Each instruction begins with a initial capital `A` in this list, so `AADCB` represents the `ADCB` (add carry byte) instruction. The enumeration is in alphabetical order, plus some late additions (`AXXX` occupies the zero slot as an invalid instruction). The sequence has nothing to do with the actual encoding of the machine instructions. Again, the linker takes care of that detail.

One detail evident in the examples from the previous sections is that data in the instructions flows from left to right: `MOVQ` `$0,` `CX` clears `CX`. This convention applies even on architectures where the usual mode is the opposite direction.

Here follows some descriptions of key Go-specific details for the supported architectures.

### 32-bit Intel 386

The runtime pointer to the `g` structure is maintained through the value of an otherwise unused (as far as Go is concerned) register in the MMU. A OS-dependent macro `get_tls` is defined for the assembler if the source includes an architecture-dependent header file, like this:

    #include "zasm_GOOS_GOARCH.h"

Within the runtime, the `get_tls` macro loads its argument register with a pointer to the `g` pointer, and the `g` struct contains the `m` pointer. The sequence to load `g` and `m` using `CX` looks like this:

    get_tls(CX)
    MOVL	g(CX), AX     // Move g into AX.
    MOVL	g_m(AX), BX   // Move g->m into BX.

### 64-bit Intel 386 (a.k.a. amd64)

The assembly code to access the `m` and `g` pointers is the same as on the 386, except it uses `MOVQ` rather than `MOVL`:

    get_tls(CX)
    MOVQ	g(CX), AX     // Move g into AX.
    MOVQ	g_m(AX), BX   // Move g->m into BX.

### ARM

The registers `R10` and `R11` are reserved by the compiler and linker.

`R10` points to the `g` (goroutine) structure. Within assembler source code, this pointer must be referred to as `g`; the name `R10` is not recognized.

To make it easier for people and compilers to write assembly, the ARM linker allows general addressing forms and pseudo-operations like `DIV` or `MOD` that may not be expressible using a single hardware instruction. It implements these forms as multiple instructions, often using the `R11` register to hold temporary values. Hand-written assembly can use `R11`, but doing so requires being sure that the linker is not also using it to implement any of the other instructions in the function.

When defining a `TEXT`, specifying frame size `$-4` tells the linker that this is a leaf function that does not need to save `LR` on entry.

The name `SP` always refers to the virtual stack pointer described earlier. For the hardware register, use `R13`.

### Unsupported opcodes

The assemblers are designed to support the compiler so not all hardware instructions are defined for all architectures: if the compiler doesn't generate it, it might not be there. If you need to use a missing instruction, there are two ways to proceed. One is to update the assembler to support that instruction, which is straightforward but only worthwhile if it's likely the instruction will be used again. Instead, for simple one-off cases, it's possible to use the `BYTE` and `WORD` directives to lay down explicit data into the instruction stream within a `TEXT`. Here's how the 386 runtime defines the 64-bit atomic load function.

    // uint64 atomicload64(uint64 volatile* addr);
    // so actually
    // void atomicload64(uint64 *res, uint64 volatile *addr);
    TEXT runtime·atomicload64(SB), NOSPLIT, $0-8
    	MOVL	ptr+0(FP), AX
    	LEAL	ret_lo+4(FP), BX
    	BYTE $0x0f; BYTE $0x6f; BYTE $0x00	// MOVQ (%EAX), %MM0
    	BYTE $0x0f; BYTE $0x7f; BYTE $0x03	// MOVQ %MM0, 0(%EBX)
    	BYTE $0x0F; BYTE $0x77			// EMMS
    	RET