asm 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

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.

Examples

#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");
}

7*5 = 35

External links

• GCC Inline Assembly HOWTO
• IBM XL C/C++ Inline Assembly
• Intel C++ Inline Assembly
• Visual Studio 2013 Inline Assembler
• Sun Studio 12 Asm Statements
• Inline assembly for Itanium-based HP-UX