C/C++/DPC++ Calling Conventions
/DPC++
Calling Conventions There are a number of calling conventions that set the rules on how arguments are passed to a function and how the values are returned from the function.
Calling Conventions on Windows*
The following table summarizes the supported calling conventions on Windows:
Calling Convention | Compiler Option | Description |
|---|---|---|
__cdecl | /Gd | This is the default calling convention for C/C++ /DPC++ programs. It can be specified on a function with variable arguments.
|
__stdcall
| /Gz | Standard calling convention used for Win32 API functions.
This content is specific to C++; it does not apply to
DPC++ . |
__fastcall
| /Gr | Fast calling convention that specifies that arguments are passed in registers rather than on the stack.
This content is specific to C++; it does not apply to
DPC++ . |
__regcall
| /Qregcall
specifies that
__regcall is the default calling convention for functions in the compilation, unless another calling convention is specified on a declaration.
| Intel® oneAPI
calling convention that specifies that as many arguments as possible are passed in registers; likewise,
DPC++/C++ Compiler__regcall uses registers whenever possible to return values. This calling convention is ignored if specified on a function with variable arguments.
For more information about the Intel-compatible vector functions ABI, see the article
Vector Function Application Binary Interface at
https://software.intel.com/content/www/us/en/develop/download/vector-simd-function-abi.html.
For more information about the GCC vector functions ABI, see the item Libmvec - vector math library document in the GLIBC wiki at sourceware.org.
|
__thiscall
| none | Default calling convention used by C++ member functions that do not use variable arguments.
|
__vectorcall
| /Gv | Calling convention that specifies that a function passing vector type arguments should utilize vector registers.
|
Calling Conventions on Linux*
The following table summarizes the supported calling conventions on Linux:
Calling Convention | Compiler Option | Description |
|---|---|---|
__attribute((cdecl)) | none | Default calling convention for C/C++/DPC++ programs. Can be specified on a function with variable arguments.
|
__attribute((stdcall)) | none | Calling convention that specifies the arguments are passed on the stack. Cannot be specified on a function with variable arguments.
|
__attribute((regparm (number))) | none | On systems based on IA-32 architecture, the regparm attribute causes the compiler to pass up to
number arguments in registers
EAX ,
EDX , and
ECX instead of on the stack. Functions that take a variable number of arguments will continue to pass all of their arguments on the stack.
|
__attribute__((regcall))
| -regcall specifies that
__regcall
is the default calling convention for functions in the compilation, unless another calling convention is specified on a declaration.
| Intel oneAPI
calling convention that specifies that as many arguments as possible are passed in registers; likewise,
DPC++/C++ Compiler__regcall uses registers whenever possible to return values. This calling convention is ignored if specified on a function with variable arguments.
|
__attribute__((vectorcall))
| none | Calling convention that specifies that a function passing vector type arguments should utilize vector registers.
|
The __regcall Calling Convention
The
__regcall
calling convention is unique to the
Intel oneAPI
and requires some additional explanation.
DPC++/C++
Compiler To use
__regcall
, place the keyword before a function declaration. For example:
Example
|
|---|
|
|
Available __regcall Registers
All registers in a
__regcall
function can be used for parameter passing/returning a value, except those that are reserved by the compiler. The following table lists the registers that are available in each register class depending on the default ABI for the compilation. The registers are used in the order shown below.
This content is specific to C++; it does not apply to
DPC++
. Register Class/Architecture
| IA-32 for Linux
| IA-32 for Windows
| Intel® 64 for Linux
| Intel® 64 for Windows
|
|---|---|---|---|---|
GPR | EAX, ECX, EDX, EDI, ESI
| ECX, EDX, EDI, ESI
| RAX, RCX, RDX, RDI, RSI, R8, R9, R10, R11, R12, R14, R15
| RAX, RCX, RDX, RDI, RSI, R8, R9, R11, R12, R14, R15
|
FP | ST0
| ST0
| ST0
| ST0
|
MMX | None
| None
| None
| None
|
XMM | XMM0 - XMM7
| XMM0 - XMM7
| XMM0 - XMM15
| XMM0 - XMM15
|
YMM | YMM0 - YMM7
| YMM0 - YMM7
| YMM0 - YMM15
| YMM0 - YMM15
|
ZMM | ZMM0 - ZMM7
| ZMM0 - ZMM7
| ZMM0 - YMM15
| ZMM0 - YMM15
|
This content is specific to
DPC++
. Register Class/Architecture
| Intel® 64 for Linux
| Intel® 64 for Windows
|
|---|---|---|
GPR | RAX, RCX, RDX, RDI, RSI, R8, R9, R10, R11, R12, R14, R15
| RAX, RCX, RDX, RDI, RSI, R8, R9, R11, R12, R14, R15
|
FP | ST0
| ST0
|
MMX | None
| None
|
XMM | XMM0 - XMM15
| XMM0 - XMM15
|
YMM | YMM0 - YMM15
| YMM0 - YMM15
|
ZMM | ZMM0 - YMM15
| ZMM0 - YMM15
|
__regcall Data Type Classification
Parameters and return values for
__regcall
are classified by data type and are passed in the registers of the classes shown in the following table.
All types assigned to
XMM
,
YMM
, or
ZMM
in a non-SSE target are passed in the stack.
This content is specific to C++; it does not apply to
DPC++
. Type (for both unsigned and signed types) | IA-32 | Intel® 64
|
|---|---|---|
bool, char, int, enum, _Decimal32, long, pointer
| GPR | GPR |
short, __mmask{8,16,32,64}
| GPR | GPR |
_Decimal64
| XMM | GPR |
long double
| FP | FP |
float, double, float128, _Decimal128
| XMM | XMM |
__m128, __m128i, __m128d
| XMM | XMM |
__m256, __m256i, __m256d
| YMM | YMM |
__m512, __m512i, __m512d
| ZMM | ZMM |
complex type, struct, union
| ||
For the purpose of structured types, the classification of
GPR class is used.
| ||
On systems based on IA-32 architecture, these 64-bit integer types (long long, __int64) get classified to the
GPR class and are passed in two registers, as if they were implemented as a structure of two 32-bit integer fields.
| ||
This content is specific to
DPC++
. Type (for both unsigned and signed types) | Intel® 64
|
|---|---|
bool, char, int, enum, _Decimal32, long, pointer
| GPR |
short, __mmask{8,16,32,64}
| GPR |
long long, __int64
| GPR |
_Decimal64
| GPR |
long double
| FP |
float, double, float128, _Decimal128
| XMM |
__m128, __m128i, __m128d
| XMM |
__m256, __m256i, __m256d
| YMM |
__m512, __m512i, __m512d
| ZMM |
complex type, struct, union
| |
For the purpose of structured types, the classification of
GPR class is used.
|
Types that are smaller in size than registers than registers of their associated class are passed in the lower part of those registers; for example, float is passed in the lower four bytes of an
XMM
register.
__regcall Structured Data Type Classification Rules
Structures/unions and complex types are classified similarly to what is described in the x86_64 ABI, with the following exceptions:
- There is no limitation on the overall size of a structure.
- The register classes for basic types are given in Data Type Classifications.
- For systems based on the IA-32 architecture, classification is performed on four-bytes. For systems based on other architectures, classification is performed on eight-bytes.This content is specific to C++; it does not apply toDPC++.
- Classification is performed on eight-bytes.This content is specific toDPC++.
__regcall Placement in Registers or on the Stack
After the classification described in
Data Type Classifications and
Structured Data Type Classification Rules,
__regcall
parameters and return values are either put into registers specified in
Available Registers or placed in memory, according to the following:
- Each chunk (eight bytes on systems based on Intel® 64 architecture or four-bytes on systems based on IA-32 architecture (IA-32 is for C++ only) of a value of Data Type is assigned a register class. If enough registers from Available Registers are available, the whole value is passed in registers, otherwise the value is passed using the stack.
- If the classification were to use one or more register classes, then the registers of these classes from the table in Available Registers are used, in the order given there.
- If no more registers are available in one of the required register classes, then the whole value is put on the stack.
__regcall Registers that Preserve Their Values
The following registers preserve their values across a
__regcall
call, as long as they were not used for passing a parameter or returning a value:
This content is specific to C++; it does not apply to
DPC++
. Register Class/ABI
| IA-32
| Intel® 64 for Linux
| Intel® 64 for Windows
|
|---|---|---|---|
GPR | ESI, EDI, EBX, EBP, ESP
| R12 - R15, RBX, RBP, RSP
| R12 - R15, RBX, RBP, RSP
|
FP | None
| None
| None
|
MMX | None
| None
| None
|
XMM | XMM4 - XMM7
| XMM8 - XMM15
| XMM8 - XMM15
|
YMM | XMM4 - XMM7
| XMM8 - XMM15
| XMM8 - XMM15
|
ZMM | XMM4 - XMM7
| XMM8 - XMM15
| XMM8 - XMM15
|
This content is specific to
DPC++
. Register Class/ABI
| Intel® 64 for Linux
| Intel® 64 for Windows
|
|---|---|---|
GPR | R12 - R15, RBX, RBP, RSP
| R12 - R15, RBX, RBP, RSP
|
FP | None
| None
|
MMX | None
| None
|
XMM | XMM8 - XMM15
| XMM8 - XMM15
|
YMM | XMM8 - XMM15
| XMM8 - XMM15
|
ZMM | XMM8 - XMM15
| XMM8 - XMM15
|
All other registers do not preserve their values across this call.