# C/C++ 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++ programs. It can be specified on a function with variable arguments.
__stdcall | /Gz | Standard calling convention used for Win32 API functions.
__fastcall | /Gr | Fast calling convention that specifies that arguments are passed in registers rather than on the stack.
__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® 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* and macOS

The following table summarizes the supported calling conventions on Linux* and macOS:

Calling Convention | Compiler Option | Description
--- | --- | ---
__attribute((cdecl)) | none | Default calling convention for C/C++ 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® 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® 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* and macOS
    __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 in the table.

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

## __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.

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.

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.

## __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) 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:

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

All other registers do not preserve their values across this call.

Parent topic: Compiler Reference

## See Also

Structured Data Type Classification Rules
Data Type Classifications
Available Registers