asm declaration

From cppreference.com
< cpp‎ | language
 
 
C++ language
General topics
Flow control
Conditional execution statements
if
Iteration statements (loops)
for
range-for (C++11)
Jump statements
Functions
Function declaration
Lambda function expression
inline specifier
Dynamic exception specifications (until C++20)
noexcept specifier (C++11)
Exceptions
Namespaces
Types
Specifiers
decltype (C++11)
auto (C++11)
alignas (C++11)
Storage duration specifiers
Initialization
Expressions
Alternative representations
Literals
Boolean - Integer - Floating-point
Character - String - nullptr (C++11)
User-defined (C++11)
Utilities
Attributes (C++11)
Types
typedef declaration
Type alias declaration (C++11)
Casts
Implicit conversions - Explicit conversions
static_cast - dynamic_cast
const_cast - reinterpret_cast
Memory allocation
Classes
Class-specific function properties
explicit (C++11)
static
Special member functions
Templates
Miscellaneous
Inline assembly
 
 

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.

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.

#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

See also