Application Binary Interface
A D implementation that conforms to the D ABI (Application Binary Interface) will be able to generate libraries, DLLs, etc., that can interoperate with D binaries built by other implementations.
C ABI
The C ABI referred to in this specification means the C Application Binary Interface of the target system. C and D code should be freely linkable together, in particular, D code shall have access to the entire C ABI runtime library.
Endianness
The endianness (byte order) of the layout of the data will conform to the endianness of the target machine. The Intel x86 CPUs are little endian meaning that the value 0x0A0B0C0D is stored in memory as: 0D 0C 0B 0A.
Basic Types
bool | 8 bit byte with the values 0 for false and 1 for true |
byte | 8 bit signed value |
ubyte | 8 bit unsigned value |
short | 16 bit signed value |
ushort | 16 bit unsigned value |
int | 32 bit signed value |
uint | 32 bit unsigned value |
long | 64 bit signed value |
ulong | 64 bit unsigned value |
cent | 128 bit signed value |
ucent | 128 bit unsigned value |
float | 32 bit IEEE 754 floating point value |
double | 64 bit IEEE 754 floating point value |
real | implementation defined floating point value, for x86 it is 80 bit IEEE 754 extended real |
Delegates
Delegates are fat pointers with two parts:
offset | property | contents |
---|---|---|
0 | .ptr | context pointer |
ptrsize | .funcptr | pointer to function |
The context pointer can be a class this reference, a struct this pointer, a pointer to a closure (nested functions) or a pointer to an enclosing function's stack frame (nested functions).
Structs
Conforms to the target's C ABI struct layout.
Classes
An object consists of:
size | property | contents |
---|---|---|
ptrsize | .__vptr | pointer to vtable |
ptrsize | .__monitor | monitor |
ptrsize... | vptrs for any interfaces implemented by this class in left to right, most to least derived, order | |
... | ... | super's non-static fields and super's interface vptrs, from least to most derived |
... | named fields | non-static fields |
The vtable consists of:
size | contents |
---|---|
ptrsize | pointer to instance of TypeInfo |
ptrsize... | pointers to virtual member functions |
Casting a class object to an interface consists of adding the offset of the interface's corresponding vptr to the address of the base of the object. Casting an interface ptr back to the class type it came from involves getting the correct offset to subtract from it from the object.Interface entry at vtbl[0]. Adjustor thunks are created and pointers to them stored in the method entries in the vtbl[] in order to set the this pointer to the start of the object instance corresponding to the implementing method.
An adjustor thunk looks like:
ADD EAX,offset JMP method
The leftmost side of the inheritance graph of the interfaces all share their vptrs, this is the single inheritance model. Every time the inheritance graph forks (for multiple inheritance) a new vptr is created and stored in the class' instance. Every time a virtual method is overridden, a new vtbl[] must be created with the updated method pointers in it.
The class definition:
class XXXX
{
....
};
Generates the following:
- An instance of Class called ClassXXXX.
- A type called StaticClassXXXX which defines all the static members.
- An instance of StaticClassXXXX called StaticXXXX for the static members.
Interfaces
An interface is a pointer to a pointer to a vtbl[]. The vtbl[0] entry is a pointer to the corresponding instance of the object.Interface class. The rest of the vtbl[1..$] entries are pointers to the virtual functions implemented by that interface, in the order that they were declared.
A COM interface differs from a regular interface in that there is no object.Interface entry in vtbl[0]; the entries vtbl[0..$] are all the virtual function pointers, in the order that they were declared. This matches the COM object layout used by Windows.
A C++ interface differs from a regular interface in that it matches the layout of a C++ class using single inheritance on the target machine.
Arrays
A dynamic array consists of:
offset | property | contents |
---|---|---|
0 | .length | array dimension |
size_t | .ptr | pointer to array data |
A dynamic array is declared as:
type[] array;whereas a static array is declared as:
type[dimension] array;
Thus, a static array always has the dimension statically available as part of the type, and so it is implemented like in C. Static arrays and Dynamic arrays can be easily converted back and forth to each other.
Associative Arrays
Associative arrays consist of a pointer to an opaque, implementation defined type.
The current implementation is contained in and defined by rt/aaA.d.
Reference Types
D has reference types, but they are implicit. For example, classes are always referred to by reference; this means that class instances can never reside on the stack or be passed as function parameters.
Name Mangling
D accomplishes typesafe linking by mangling a D identifier to include scope and type information.
MangledName: _D QualifiedName Type _D QualifiedName Z // Internal
The Type above is the type of a variable or the return type of a function. This is never a TypeFunction, as the latter can only be bound to a value via a pointer to a function or a delegate.
QualifiedName: SymbolFunctionName SymbolFunctionName QualifiedName SymbolFunctionName: SymbolName SymbolName TypeFunctionNoReturn SymbolName M TypeModifiersopt TypeFunctionNoReturn
The M means that the symbol is a function that requires a this pointer. Class or struct fields are mangled without M. To disambiguate M from being a Parameter with modifier scope, the following type needs to be checked for being a TypeFunction.
SymbolName: LName TemplateInstanceName IdentifierBackRef 0 // anonymous symbols
Template Instance Names have the types and values of its parameters encoded into it:
TemplateInstanceName: TemplateID LName TemplateArgs Z TemplateID: __T __U // for symbols declared inside template constraint TemplateArgs: TemplateArg TemplateArg TemplateArgs TemplateArg: TemplateArgX H TemplateArgX
If a template argument matches a specialized template parameter, the argument is mangled with prefix H.
TemplateArgX: T Type V Type Value S QualifiedName X Number ExternallyMangledName
ExternallyMangledName can be any series of characters allowed on the current platform, e.g. generated by functions with C++ linkage or annotated with pragma(mangle,...).
Values: Value Value Values Value: n i Number N Number e HexFloat c HexFloat c HexFloat CharWidth Number _ HexDigits A Number Values S Number Values f MangledName HexFloat: NAN INF NINF N HexDigits P Exponent HexDigits P Exponent Exponent: N Number Number HexDigits: HexDigit HexDigit HexDigits HexDigit: Digit A B C D E F CharWidth: a w d
- n
- is for null arguments.
- i Number
- is for positive numeric literals (including character literals).
- N Number
- is for negative numeric literals.
- e HexFloat
- is for real and imaginary floating point literals.
- c HexFloat c HexFloat
- is for complex floating point literals.
- CharWidth Number _ HexDigits
- CharWidth is whether the characters are 1 byte (a), 2 bytes (w) or 4 bytes (d) in size. Number is the number of characters in the string. The HexDigits are the hex data for the string.
- A Number Values
- An array or asssociative array literal. Number is the length of the array. Value is repeated Number times for a normal array, and 2 * Number times for an associative array.
- S Number Values
- A struct literal. Value is repeated Number times.
Name: Namestart Namestart Namechars Namestart: _ Alpha Namechar: Namestart Digit Namechars: Namechar Namechar Namechars
A Name is a standard D identifier.
LName: Number Name Number: Digit Digit Number Digit: 0 1 2 3 4 5 6 7 8 9
An LName is a name preceded by a Number giving the number of characters in the Name.
Back references
Any LName or non-basic Type (i.e. any type that does not encode as a fixed one or two character sequence) that has been emitted to the mangled symbol before will not be emitted again, but is referenced by a special sequence encoding the relative position of the original occurrence in the mangled symbol name.
Numbers in back references are encoded with base 26 by upper case letters A - Z for higher digits but lower case letters a - z for the last digit.
TypeBackRef: Q NumberBackRef IdentifierBackRef: Q NumberBackRef NumberBackRef: lower-case-letter upper-case-letter NumberBackRef
To distinguish between the type of the back reference a look-up of the back referenced character is necessary: An identifier back reference always points to a digit 0 to 9, while a type back reference always points to a letter.
Type Mangling
Types are mangled using a simple linear scheme:
Type: TypeModifiersopt TypeX TypeBackRef TypeX: TypeArray TypeStaticArray TypeAssocArray TypePointer TypeFunction TypeIdent TypeClass TypeStruct TypeEnum TypeTypedef TypeDelegate TypeVoid TypeByte TypeUbyte TypeShort TypeUshort TypeInt TypeUint TypeLong TypeUlong TypeCent TypeUcent TypeFloat TypeDouble TypeReal TypeIfloat TypeIdouble TypeIreal TypeCfloat TypeCdouble TypeCreal TypeBool TypeChar TypeWchar TypeDchar TypeNull TypeTuple TypeVector TypeModifiers: Const Wild Wild Const Shared Shared Const Shared Wild Shared Wild Const Immutable Shared: O Const: x Immutable: y Wild: Ng TypeArray: A Type TypeStaticArray: G Number Type TypeAssocArray: H Type Type TypePointer: P Type TypeVector: Nh Type TypeFunction: TypeFunctionNoReturn Type TypeFunctionNoReturn: CallConvention FuncAttrsopt Parametersopt ParamClose CallConvention: F // D U // C W // Windows R // C++ Y // Objective-C FuncAttrs: FuncAttr FuncAttr FuncAttrs FuncAttr: FuncAttrPure FuncAttrNothrow FuncAttrRef FuncAttrProperty FuncAttrNogc FuncAttrReturn FuncAttrScope FuncAttrTrusted FuncAttrSafe FuncAttrLive
Function attributes are emitted in the order as listed above.
FuncAttrPure: Na FuncAttrNogc: Ni FuncAttrNothrow: Nb FuncAttrProperty: Nd FuncAttrRef: Nc FuncAttrReturn: Nj FuncAttrScope: Nl FuncAttrTrusted: Ne FuncAttrSafe: Nf FuncAttrLive: Nm Parameters: Parameter Parameter Parameters Parameter: Parameter2 M Parameter2 // scope Nk Parameter2 // return Parameter2: Type I Type // in J Type // out K Type // ref L Type // lazy ParamClose: X // variadic T t...) style Y // variadic T t,...) style Z // not variadic TypeIdent: I QualifiedName TypeClass: C QualifiedName TypeStruct: S QualifiedName TypeEnum: E QualifiedName TypeTypedef: T QualifiedName TypeDelegate: D TypeModifiersopt TypeFunction TypeVoid: v TypeByte: g TypeUbyte: h TypeShort: s TypeUshort: t TypeInt: i TypeUint: k TypeLong: l TypeUlong: m TypeCent: zi TypeUcent: zk TypeFloat: f TypeDouble: d TypeReal: e TypeIfloat: o TypeIdouble: p TypeIreal: j TypeCfloat: q TypeCdouble: r TypeCreal: c TypeBool: b TypeChar: a TypeWchar: u TypeDchar: w TypeNull: n TypeTuple: B Parameters Z
Function Calling Conventions
The extern (C) and extern (D) calling convention matches the C calling convention used by the supported C compiler on the host system. Except that the extern (D) calling convention for Windows x86 is described here.
Register Conventions
- EAX, ECX, EDX are scratch registers and can be destroyed by a function.
- EBX, ESI, EDI, EBP must be preserved across function calls.
- EFLAGS is assumed destroyed across function calls, except for the direction flag which must be forward.
- The FPU stack must be empty when calling a function.
- The FPU control word must be preserved across function calls.
- Floating point return values are returned on the FPU stack. These must be cleaned off by the caller, even if they are not used.
Return Value
- The types bool, byte, ubyte, short, ushort, int, uint, pointer, Object, and interfaces are returned in EAX.
- long and ulong are returned in EDX,EAX, where EDX gets the most significant half.
- float, double, real, ifloat, idouble, ireal are returned in ST0.
- cfloat, cdouble, creal are returned in ST1,ST0 where ST1 is the real part and ST0 is the imaginary part.
- Dynamic arrays are returned with the pointer in EDX and the length in EAX.
- Associative arrays are returned in EAX.
- References are returned as pointers in EAX.
- Delegates are returned with the pointer to the function in EDX and the context pointer in EAX.
- 1, 2 and 4 byte structs and static arrays are returned in EAX.
- 8 byte structs and static arrays are returned in EDX,EAX, where EDX gets the most significant half.
- For other sized structs and static arrays, the return value is stored through a hidden pointer passed as an argument to the function.
- Constructors return the this pointer in EAX.
Parameters
The parameters to the non-variadic function:
foo(a1, a2, ..., an);are passed as follows:
a1 |
a2 |
... |
an |
hidden |
this |
where hidden is present if needed to return a struct value, and this is present if needed as the this pointer for a member function or the context pointer for a nested function.
The last parameter is passed in EAX rather than being pushed on the stack if the following conditions are met:
- It fits in EAX.
- It is not a 3 byte struct.
- It is not a floating point type.
Parameters are always pushed as multiples of 4 bytes, rounding upwards, so the stack is always aligned on 4 byte boundaries. They are pushed most significant first. out and ref are passed as pointers. Static arrays are passed as pointers to their first element. On Windows, a real is pushed as a 10 byte quantity, a creal is pushed as a 20 byte quantity. On Linux, a real is pushed as a 12 byte quantity, a creal is pushed as two 12 byte quantities. The extra two bytes of pad occupy the ‘most significant’ position.
The callee cleans the stack.
The parameters to the variadic function:
void foo(int p1, int p2, int[] p3...) foo(a1, a2, ..., an);are passed as follows:
p1 |
p2 |
a3 |
hidden |
this |
The variadic part is converted to a dynamic array and the rest is the same as for non-variadic functions.
The parameters to the variadic function:
void foo(int p1, int p2, ...) foo(a1, a2, a3, ..., an);are passed as follows:
an |
... |
a3 |
a2 |
a1 |
_arguments |
hidden |
this |
The caller is expected to clean the stack. _argptr is not passed, it is computed by the callee.
Exception Handling
Windows
Conforms to the Microsoft Windows Structured Exception Handling conventions.
Linux, FreeBSD and OS X
Uses static address range/handler tables. It is not compatible with the ELF/Mach-O exception handling tables. The stack is walked assuming it uses the EBP/RBP stack frame convention. The EBP/RBP convention must be used for every function that has an associated EH (Exception Handler) table.
For each function that has exception handlers, an EH table entry is generated.
field | description |
---|---|
void* | pointer to start of function |
DHandlerTable* | pointer to corresponding EH data |
uint | size in bytes of the function |
The EH table entries are placed into the following special segments, which are concatenated by the linker.
Operating System | Segment Name |
---|---|
Windows | FI |
Linux | .deh_eh |
FreeBSD | .deh_eh |
OS X | __deh_eh, __DATA |
The rest of the EH data can be placed anywhere, it is immutable.
field | description |
---|---|
void* | pointer to start of function |
uint | offset of ESP/RSP from EBP/RBP |
uint | offset from start of function to return code |
uint | number of entries in DHandlerInfo[] |
DHandlerInfo[] | array of handler information |
field | description |
---|---|
uint | offset from function address to start of guarded section |
uint | offset of end of guarded section |
int | previous table index |
uint | if != 0 offset to DCatchInfo data from start of table |
void* | if not null, pointer to finally code to execute |
field | description |
---|---|
uint | number of entries in DCatchBlock[] |
DCatchBlock[] | array of catch information |
field | description |
---|---|
ClassInfo | catch type |
uint | offset from EBP/RBP to catch variable |
Garbage Collection
The interface to this is found in Druntime's gc/gcinterface.d.
Runtime Helper Functions
These are found in Druntime's rt/.
Module Initialization and Termination
All the static constructors for a module are aggregated into a single function, and a pointer to that function is inserted into the ctor member of the ModuleInfo instance for that module.
All the static destructors for a module are aggregated into a single function, and a pointer to that function is inserted into the dtor member of the ModuleInfo instance for that module.
Unit Testing
All the unit tests for a module are aggregated into a single function, and a pointer to that function is inserted into the unitTest member of the ModuleInfo instance for that module.
Symbolic Debugging
D has types that are not represented in existing C or C++ debuggers. These are dynamic arrays, associative arrays, and delegates. Representing these types as structs causes problems because function calling conventions for structs are often different than that for these types, which causes C/C++ debuggers to misrepresent things. For these debuggers, they are represented as a C type which does match the calling conventions for the type.
D type | C representation |
---|---|
dynamic array | unsigned long long |
associative array | void* |
delegate | long long |
dchar | unsigned long |
For debuggers that can be modified to accept new types, the following extensions help them fully support the types.
Codeview Debugger Extensions
The D dchar type is represented by the special primitive type 0x78.
D makes use of the Codeview OEM generic type record indicated by LF_OEM (0x0015). The format is:
field size | 2 | 2 | 2 | 2 | 2 | 2 |
D Type | Leaf Index | OEM Identifier | recOEM | num indices | type index | type index |
---|---|---|---|---|---|---|
dynamic array | LF_OEM | OEM | 1 | 2 | @index | @element |
associative array | LF_OEM | OEM | 2 | 2 | @key | @element |
delegate | LF_OEM | OEM | 3 | 2 | @this | @function |
OEM | 0x42 |
index | type index of array index |
key | type index of key |
element | type index of array element |
this | type index of context pointer |
function | type index of function |
These extensions can be pretty-printed by obj2asm. The Ddbg debugger supports them.