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.
asm ( string_literal ) ; |
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.
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
asm(".globl func\n\t"
".type func, @function\n\t"
"func:\n\t"
".cfi_startproc\n\t"
"movl $7, %eax\n\t"
"ret\n\t"
".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
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http://en.cppreference.com/w/cpp/language/asm