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
attr(optional) asm ( string-literal ) ;
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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.
This section is incomplete Reason: write a note on GCC extended assembly syntax, since it is now supported by Intel, IBM, Sun (as of v12), etc |
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
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