asm declaration

asm-declaration gives the ability to embed assembly language source code within a C++ program. This declaration is conditionally-supported and (since C++11)implementation defined, meaning that it may not be present and, even when provided by the implementation, (since C++11)it does not have a fixed meaning.

[edit] Syntax

[edit] Explanation

The balanced-token-seq is typically a string literal that represents 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.

[edit] Notes

[edit] Keywords

asm

[edit] Example

#include <iostream>
 
extern "C" int func(int x);
// 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 %edi, %eax /* x is in RDI, see x86-64 calling convention */
 addl 1,ドル %eax
 ret
 .cfi_endproc
)");
 
int main()
{
 int n = func(0110);
 // formerly non-standard inline assembly, made comforming by P2361R6
 asm ("leal (%0,%0,4),%0"
 : "=r" (n)
 : "0" (n));
 std::cout << "73*5 = " << n << std::endl ; // flush is intentional
 
 // standard inline assembly
 asm ("movq 60,ドル %rax\n" // the exit syscall number on Linux
 "movq 2,ドル  %rdi\n" // this program returns 2
 "syscall");
}

73*5 = 365

[edit] Defect reports

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

[edit] References

[edit] See also

• C++ ABIs

[edit] External links

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