asm declaration

 

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Overview declaration syntax decl-specifier-seq declarator Specifiers typedef inline virtual function specifier explicit function specifier    friend constexpr (C++11) consteval (C++20) constinit (C++20) storage class specifiers translation-unit-local (C++20) class/struct union enum decltype (C++11) auto (C++11) alignas (C++11) constvolatile elaborated type specifier attributes (C++11)

Declarators reference pointer array Block declarations simple-declaration → structured binding declaration (C++17) alias declaration(C++11) namespace alias definition using-declaration using-directive static_assert declaration (C++11) asm-declaration opaque enum declaration(C++11) Other declarations namespace definition function declaration class template declaration function template declaration explicit template instantiation(C++11) explicit template specialization linkage specification attribute declaration (C++11) empty declaration

 

asm-declaration gives the ability to embed assembly language source code within a C++ program. This declaration is conditionally-supported and implementation defined, meaning that it may not be present and, even when provided by the implementation, it does not have a fixed meaning.

Syntax

attr(optional) asm ( string-literal ) ;

attr	-	(since C++11) any number of attributes
string-literal	-	a string literal

Explanation

The string-literal is typically a short program written in assembly language, which is executed whenever this declaration is executed. Different C++ compilers have wildly varying rules for asm-declarations, and different conventions for the interaction with the surrounding C++ code.

As other block declarations, this declaration can appear inside a block (a function body or another compound statement), and, as all other declarations, this declaration can also appear outside a block.

Notes

Feature-test macro	Value	Std	Comment
__cpp_constexpr	201907L	(C++20)	Trivial default initialization and asm-declaration in constexpr functions

Examples

Demonstrates two kinds of inline assembly syntax offered by the GCC compiler. This program will only work correctly on x86_64 platform under Linux.

Run this code

#include <iostream>
 
extern "C" int func();
// the definition of func is written in assembly language
// raw string literal could be very useful
asm(R"(
.globl func
    .type func, @function
    func:
    .cfi_startproc
    movl $7, %eax
    ret
    .cfi_endproc
)");
 
int main()
{
    int n = func();
    // extended inline assembly
    asm ("leal (%0,%0,4),%0"
         : "=r" (n)
         : "0" (n));
    std::cout << "7*5 = " << n << std::endl; // flush is intentional
 
    // standard inline assembly
    asm ("movq $60, %rax\n\t" // the exit syscall number on Linux
         "movq $2,  %rdi\n\t" // this program returns 2
         "syscall");
}

Output:

7*5 = 35

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 195	C++98	it was required to support all asm declarations	made conditionally-supported
CWG 2262	C++11	attributes could not be applied to asm declarations	allowed

External links

1.	GCC Inline Assembly HOWTO
2.	IBM XL C/C++ Inline Assembly
3.	Intel C++ Inline Assembly
4.	Visual Studio Inline Assembler
5.	Sun Studio 12 Asm Statements
6.	Inline assembly for Itanium-based HP-UX