Inline Assembler

D, being a systems programming language, provides an inline assembler. The inline assembler is standardized for D implementations across the same CPU family, for example, the Intel Pentium inline assembler for a Win32 D compiler will be syntax compatible with the inline assembler for Linux running on an Intel Pentium.

Implementations of D on different architectures, however, are free to innovate upon the memory model, function call/return conventions, argument passing conventions, etc.

This document describes the x86 and x86_64 implementations of the inline assembler. The inline assembler platform support that a compiler provides is indicated by the D_InlineAsm_X86 and D_InlineAsm_X86_64 version identifiers, respectively.

Asm statement

AsmStatement:
    asm FunctionAttributesopt { AsmInstructionListopt }

AsmInstructionList:
    AsmInstruction ;
    AsmInstruction ; AsmInstructionList

Assembler instructions must be located inside an asm block. Like functions, asm statements must be anotated with adequate function attributes to be compatible with the caller. Asm statements attributes must be explicitly defined, they are not inferred.

@safe is not allowed as an attribute, as the compiler does no safety checking of assembly statements - use @trusted instead.

void ok() pure nothrow @safe @nogc
{
    asm pure nothrow @trusted @nogc
    {}
}

void error() @safe @nogc
{
    asm @nogc // Error: asm statement is assumed to be @system - mark it with '@trusted' if it is not
    {}
    asm @safe @nogc // Deprecation: asm statement cannot be @safe, use @trusted instead
    {}
}

Asm instruction

AsmInstruction:
    Identifier : AsmInstruction
    align IntegerExpression
    even
    naked
    db Operands
    ds Operands
    di Operands
    dl Operands
    df Operands
    dd Operands
    de Operands
    db StringLiteral
    ds StringLiteral
    di StringLiteral
    dl StringLiteral
    dw StringLiteral
    dq StringLiteral
    Opcode
    Opcode Operands

Opcode:
    Identifier
    int
    in
    out

Operands:
    Operand
    Operand , Operands

Labels

Assembler instructions can be labeled just like other statements. They can be the target of goto statements. For example:

void *pc;
asm
{
    call L1          ;
  L1:                ;
    pop  EBX         ;
    mov  pc[EBP],EBX ; // pc now points to code at L1
}

align IntegerExpression

IntegerExpression:
    IntegerLiteral
    Identifier

Causes the assembler to emit NOP instructions to align the next assembler instruction on an IntegerExpression boundary. IntegerExpression must evaluate at compile time to an integer that is a power of 2.

Aligning the start of a loop body can sometimes have a dramatic effect on the execution speed.

even

Causes the assembler to emit NOP instructions to align the next assembler instruction on an even boundary.

naked

Causes the compiler to not generate the function prolog and epilog sequences. This means such is the responsibility of inline assembly programmer, and is normally used when the entire function is to be written in assembler.

db, ds, di, dl, df, dd, de

These pseudo ops are for inserting raw data directly into the code. db is for bytes, ds is for 16 bit words, di is for 32 bit words, dl is for 64 bit words, df is for 32 bit floats, dd is for 64 bit doubles, and de is for 80 bit extended reals. Each can have multiple operands. If an operand is a string literal, it is as if there were length operands, where length is the number of characters in the string. One character is used per operand. For example:

asm
{
    db 5,6,0x83;   // insert bytes 0x05, 0x06, and 0x83 into code
    ds 0x1234;     // insert bytes 0x34, 0x12
    di 0x1234;     // insert bytes 0x34, 0x12, 0x00, 0x00
    dl 0x1234;     // insert bytes 0x34, 0x12, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
    df 1.234;      // insert float 1.234
    dd 1.234;      // insert double 1.234
    de 1.234;      // insert real 1.234
    db "abc";      // insert bytes 0x61, 0x62, and 0x63
    ds "abc";      // insert bytes 0x61, 0x00, 0x62, 0x00, 0x63, 0x00
}

Opcodes

A list of supported opcodes is at the end.

The following registers are supported. Register names are always in upper case.

Register:
    AL
    AH
    AX
    EAX

    BL
    BH
    BX
    EBX

    CL
    CH
    CX
    ECX

    DL
    DH
    DX
    EDX

    BP
    EBP

    SP
    ESP

    DI
    EDI

    SI
    ESI

    ES
    CS
    SS
    DS
    GS
    FS

    CR0
    CR2
    CR3
    CR4

    DR0
    DR1
    DR2
    DR3
    DR6
    DR7

    TR3
    TR4
    TR5
    TR6
    TR7

    ST

    ST(0)
    ST(1)
    ST(2)
    ST(3)
    ST(4)
    ST(5)
    ST(6)
    ST(7)

    MM0
    MM1
    MM2
    MM3
    MM4
    MM5
    MM6
    MM7

    XMM0
    XMM1
    XMM2
    XMM3
    XMM4
    XMM5
    XMM6
    XMM7

x86_64 adds these additional registers.

Register64:
    RAX
    RBX
    RCX
    RDX

    BPL
    RBP

    SPL
    RSP

    DIL
    RDI

    SIL
    RSI

    R8B
    R8W
    R8D
    R8

    R9B
    R9W
    R9D
    R9

    R10B
    R10W
    R10D
    R10

    R11B
    R11W
    R11D
    R11

    R12B
    R12W
    R12D
    R12

    R13B
    R13W
    R13D
    R13

    R14B
    R14W
    R14D
    R14

    R15B
    R15W
    R15D
    R15

    XMM8
    XMM9
    XMM10
    XMM11
    XMM12
    XMM13
    XMM14
    XMM15

    YMM0
    YMM1
    YMM2
    YMM3
    YMM4
    YMM5
    YMM6
    YMM7

    YMM8
    YMM9
    YMM10
    YMM11
    YMM12
    YMM13
    YMM14
    YMM15

Special Cases

lock, rep, repe, repne, repnz, repz

These prefix instructions do not appear in the same statement as the instructions they prefix; they appear in their own statement. For example:

asm
{
    rep   ;
    movsb ;
}

pause

This opcode is not supported by the assembler, instead use

asm
{
    rep  ;
    nop  ;
}

which produces the same result.

floating point ops

Use the two operand form of the instruction format;

fdiv ST(1);     // wrong
fmul ST;        // wrong
fdiv ST,ST(1);  // right
fmul ST,ST(0);  // right

Operands

Operand:
    AsmExp

AsmExp:
    AsmLogOrExp
    AsmLogOrExp ? AsmExp : AsmExp

AsmLogOrExp:
    AsmLogAndExp
    AsmLogOrExp || AsmLogAndExp

AsmLogAndExp:
    AsmOrExp
    AsmLogAndExp && AsmOrExp

AsmOrExp:
    AsmXorExp
    AsmOrExp | AsmXorExp

AsmXorExp:
    AsmAndExp
    AsmXorExp ^ AsmAndExp

AsmAndExp:
    AsmEqualExp
    AsmAndExp & AsmEqualExp

AsmEqualExp:
    AsmRelExp
    AsmEqualExp == AsmRelExp
    AsmEqualExp != AsmRelExp

AsmRelExp:
    AsmShiftExp
    AsmRelExp < AsmShiftExp
    AsmRelExp <= AsmShiftExp
    AsmRelExp > AsmShiftExp
    AsmRelExp >= AsmShiftExp

AsmShiftExp:
    AsmAddExp
    AsmShiftExp << AsmAddExp
    AsmShiftExp >> AsmAddExp
    AsmShiftExp >>> AsmAddExp

AsmAddExp:
    AsmMulExp
    AsmAddExp + AsmMulExp
    AsmAddExp - AsmMulExp

AsmMulExp:
    AsmBrExp
    AsmMulExp * AsmBrExp
    AsmMulExp / AsmBrExp
    AsmMulExp % AsmBrExp

AsmBrExp:
    AsmUnaExp
    AsmBrExp [ AsmExp ]

AsmUnaExp:
    AsmTypePrefix AsmExp
    offsetof AsmExp
    seg AsmExp
    + AsmUnaExp
    - AsmUnaExp
    ! AsmUnaExp
    ~ AsmUnaExp
    AsmPrimaryExp

AsmPrimaryExp:
    IntegerLiteral
    FloatLiteral
    __LOCAL_SIZE
    $
    Register
    Register : AsmExp
    Register64
    Register64 : AsmExp
    DotIdentifier
    this

DotIdentifier:
    Identifier
    Identifier . DotIdentifier
    FundamentalType . Identifier

The operand syntax more or less follows the Intel CPU documentation conventions. In particular, the convention is that for two operand instructions the source is the right operand and the destination is the left operand. The syntax differs from that of Intel's in order to be compatible with the D language tokenizer and to simplify parsing.

The seg means load the segment number that the symbol is in. This is not relevant for flat model code. Instead, do a move from the relevant segment register.

A dotted expression is evaluated during the compilation and then must either give a constant or indicate a higher level variable that fits in the target register or variable.

Operand Types

AsmTypePrefix:
    near ptr
    far ptr
    word ptr
    dword ptr
    qword ptr
    FundamentalType ptr

In cases where the operand size is ambiguous, as in:

add [EAX],3     ;

add  byte ptr [EAX],3 ;
add  int ptr [EAX],7  ;

far ptr is not relevant for flat model code.

Struct/Union/Class Member Offsets

To access members of an aggregate, given a pointer to the aggregate is in a register, use the .offsetof property of the qualified name of the member:

struct Foo { int a,b,c; }
int bar(Foo *f)
{
    asm
    {
        mov EBX,f                   ;
        mov EAX,Foo.b.offsetof[EBX] ;
    }
}
void main()
{
    Foo f = Foo(0, 2, 0);
    assert(bar(&f) == 2);
}

Alternatively, inside the scope of an aggregate, only the member name is needed:

struct Foo   // or class
{
    int a,b,c;
    int bar()
    {
        asm
        {
            mov EBX, this   ;
            mov EAX, b[EBX] ;
        }
    }
}
void main()
{
    Foo f = Foo(0, 2, 0);
    assert(f.bar() == 2);
}

Stack Variables

Stack variables (variables local to a function and allocated on the stack) are accessed via the name of the variable indexed by EBP:

int foo(int x)
{
    asm
    {
        mov EAX,x[EBP] ; // loads value of parameter x into EAX
        mov EAX,x      ; // does the same thing
    }
}

If the [EBP] is omitted, it is assumed for local variables. If naked is used, this no longer holds.

Special Symbols

$

Represents the program counter of the start of the next instruction. So,

jmp  $  ;

branches to the instruction following the jmp instruction. The $ can only appear as the target of a jmp or call instruction.

__LOCAL_SIZE

This gets replaced by the number of local bytes in the local stack frame. It is most handy when the naked is invoked and a custom stack frame is programmed.

Opcodes Supported

Pentium 4 (Prescott) Opcodes Supported

AMD Opcodes Supported

SIMD

SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and AVX are supported.

GCC syntax

The GNU D Compiler uses an alternative, GCC-based syntax for inline assembler:

GccAsmStatement:
    asm FunctionAttributesopt { GccAsmInstructionList }

GccAsmInstructionList:
    GccAsmInstruction ;
    GccAsmInstruction ; GccAsmInstructionList

GccAsmInstruction:
    GccBasicAsmInstruction
    GccExtAsmInstruction
    GccGotoAsmInstruction

GccBasicAsmInstruction:
    AssignExpression

GccExtAsmInstruction:
    AssignExpression : GccAsmOperandsopt
    AssignExpression : GccAsmOperandsopt : GccAsmOperandsopt
    AssignExpression : GccAsmOperandsopt : GccAsmOperandsopt : GccAsmClobbersopt

GccGotoAsmInstruction:
    AssignExpression : : GccAsmOperandsopt : GccAsmClobbersopt : GccAsmGotoLabelsopt

GccAsmOperands:
    GccSymbolicNameopt StringLiteral ( AssignExpression )
    GccSymbolicNameopt StringLiteral ( AssignExpression ) , GccAsmOperands

GccSymbolicName:
    [ Identifier ]

GccAsmClobbers:
    StringLiteral
    StringLiteral , GccAsmClobbers

GccAsmGotoLabels:
    Identifier
    Identifier , GccAsmGotoLabels