C/C++/DPC++ Calling Conventions

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 DPC++/C++ Compiler calling convention that specifies that as many arguments as possible are passed in registers; likewise, `__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, download the Vector Function Application Binary Interface PDF. 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 DPC++/C++ Compiler calling convention that specifies that as many arguments as possible are passed in registers; likewise, `__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 DPC++/C++ Compiler and requires some additional explanation.

To use __regcall, place the keyword before a function declaration. For example:

Example
__regcall int foo (int i, int j);
// Linux __attribute__((regcall)) foo (int I, int j);

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.

Note

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`
long long, __int64	See Structured Data Type Classification Rules	`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	See Structured Data Type Classification Rules	See Structured Data Type Classification Rules
Note For the purpose of structured types, the classification of `GPR` class is used.
Note 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	See Structured Data Type Classification Rules
Note 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.
Note
This content is specific to C++; it does not apply to DPC++.
Classification is performed on eight-bytes.
Note
This content is specific to DPC++.

__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.

C/C++/DPC++ Calling Conventions

Calling Conventions on Windows*

Calling Conventions on Linux*

The __regcall Calling Convention

Available __regcall Registers

__regcall Data Type Classification

Note

Note

Note

Note

__regcall Structured Data Type Classification Rules

Note

Note

__regcall Placement in Registers or on the Stack

__regcall Registers that Preserve Their Values

See Also