Intel® oneAPI DPC++/C++ Compiler Developer Guide and Reference

ID 767253
Date 3/22/2024
Public

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Document Table of Contents

C/C++/SYCL 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

The following table summarizes the supported calling conventions on Linux:

Calling Convention

Compiler Option

Description

__attribute((cdecl))

None

Default calling convention for C/C++/SYCL 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__((regcall))

-regcall specifies that __regcall is the default calling convention for functions in the compilation, unless another calling convention is specified on a declaration.

Calling convention that specifies as many arguments as possible should be 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++/SYCL 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.

Calling convention that specifies as many arguments as possible should be 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 oneAPI DPC++/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

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.

Type (Signed and Unsigned)

Intel® 64

bool, char, int, enum, _Decimal32, long, pointer

GPR

short, __mmask{8,16,64}

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 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.

__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 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

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.