Swap with push / assignment / pop in GNU C inline assembly?

Question

I was reading some answers and questions on here and kept coming up with this suggestion but I noticed no one ever actually explained "exactly" what you need to do to do it, On Windows using Intel and GCC compiler. Commented below is exactly what I am trying to do.

#include <stdio.h>

int main()
{
    int x = 1;
    int y = 2;
    //assembly code begin
    /*
      push x into stack; < Need Help
      x=y;               < With This
      pop stack into y;  < Please
    */
    //assembly code end
    printf("x=%d,y=%d",x,y);
    getchar();
    return 0;
}

Answer 1

You can't just push/pop safely from inline asm, if it's going to be portable to systems with a red-zone. That includes every non-Windows x86-64 platform. (There's no way to tell gcc you want to clobber it). Well, you could add rsp, -128 first to skip past the red-zone before pushing/popping anything, then restore it later. But then you can't use an "m" constraints, because the compiler might use RSP-relative addressing with offsets that assume RSP hasn't been modified.

But really this is a ridiculous thing to be doing in inline asm.

Here's how you use inline-asm to swap two C variables:

#include <stdio.h>

int main()
{
    int x = 1;
    int y = 2;

    asm(""                  // no actual instructions.
        : "=r"(y), "=r"(x)   // request both outputs in the compiler's choice of register
        :  "0"(x),  "1"(y)   // matching constraints: request each input in the same register as the other output
        );
    // apparently "=m" doesn't compile: you can't use a matching constraint on a memory operand

    printf("x=%d,y=%d\n",x,y);
    // getchar();  // Set up your terminal not to close after the program exits if you want similar behaviour: don't embed it into your programs
    return 0;
}

gcc -O3 output (targeting the x86-64 System V ABI, not Windows) from the Godbolt compiler explorer:

.section .rodata
.LC0:
    .string "x=%d,y=%d"
.section .text
main:
    sub     rsp, 8
    mov     edi, OFFSET FLAT:.LC0
    xor     eax, eax
    mov     edx, 1
    mov     esi, 2
#APP
# 8 "/tmp/gcc-explorer-compiler116814-16347-5i3lz1/example.cpp" 1
            # I used "\n" instead of just "" so we could see exactly where our inline-asm code ended up.

# 0 "" 2
#NO_APP
    call    printf
    xor     eax, eax
    add     rsp, 8
    ret

C variables are a high level concept; it doesn't cost anything to decide that the same registers now logically hold different named variables, instead of swapping the register contents without changing the varname->register mapping.

When hand-writing asm, use comments to keep track of the current logical meaning of different registers, or parts of a vector register.

---

The inline-asm didn't lead to any extra instructions outside the inline-asm block either, so it's perfectly efficient in this case. Still, the compiler can't see through it, and doesn't know that the values are still 1 and 2, so further constant-propagation would be defeated. https://gcc.gnu.org/wiki/DontUseInlineAsm

Answer 2

You can use extended inline assembly. It is a compiler feature whicg allows you to write assembly instructions within your C code. A good reference for inline gcc assembly is available here.

The following code copies the value of x into y using pop and push instructions.
( compiled and tested using gcc on x86_64 )

This is only safe if compiled with -mno-red-zone, or if you subtract 128 from RSP before pushing anything. It will happen to work without problems in some functions: testing with one set of surrounding code is not sufficient to verify the correctness of something you did with GNU C inline asm.

  #include <stdio.h>

    int main()
    {
        int x = 1;
        int y = 2;

   asm volatile ( 
        "pushq  %%rax\n"          /* Push x into the stack */ 
        "movq   %%rbx, %%rax\n"   /* Copy y into x         */ 
        "popq   %%rbx\n"          /* Pop  x into y         */
      : "=b"(y), "=a"(x)          /* OUTPUT values         */ 
      : "a"(x),  "b"(y)           /* INPUT  values         */
      :    /*No need for the clobber list, since the compiler knows
             which registers have been modified            */
    ); 


        printf("x=%d,y=%d",x,y);
        getchar();
        return 0;
    }

Result x=2 y=1, as you expected.

The intel compiler works in a similar way, I think you have just to change the keyword asm to __asm__. You can find info about inline assembly for the INTEL compiler here.

Answer 3

#include <stdio.h>

int main()
{
    int x=1;
    int y=2;
    printf("x::%d,y::%d\n",x,y);
    __asm__( "movl %1, %%eax;"
             "movl %%eax, %0;"
             :"=r"(y)
             :"r"(x)
             :"%eax"
            );
    printf("x::%d,y::%d\n",x,y);
    return 0;
}

/* Load x to eax
Load eax to y */

If you want to exchange the values, it can also be done using this way. Please note that this instructs GCC to take care of the clobbered EAX register. For educational purposes, it is okay, but I find it more suitable to leave micro-optimizations to the compiler.