C/C++ Calling Conventions

Intel® C++ Compiler Classic Developer Guide and Reference

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Date 12/16/2022

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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 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 Classic 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 use vector registers.

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.
`__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 Classic 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.
`__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 use 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:

Linux

__attribute__((regcall)) foo (int I, int j);

Windows

__regcall int 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.

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 (Signed and Unsigned)	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

Types that are smaller in size 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

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Intel® C++ Compiler Classic Developer Guide and Reference

C/C++ Calling Conventions

Calling Conventions on Linux and macOS

Calling Conventions on Windows

The __regcall Calling Convention

Available __regcall Registers

__regcall Data Type Classification

__regcall Structured Data Type Classification Rules

__regcall Placement in Registers or on the Stack

__regcall Registers That Preserve Their Values

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