Question

The following GCC inline asm is taken from LuaJit’s coco library. Can someone provide a line by line explanation of what it does?

static inline void coco_switch(coco_ctx from, coco_ctx to)
{
  __asm__ __volatile__ (
    "movl $1f, (%0)\n\t" 
    "movl %%esp, 4(%0)\n\t" 
    "movl %%ebp, 8(%0)\n\t"
    "movl 8(%1), %%ebp\n\t" 
    "movl 4(%1), %%esp\n\t" 
    "jmp *(%1)\n" "1:\n"
    : "+S" (from), "+D" (to) : : "eax", "ebx", "ecx", "edx", "memory", "cc");
}

Thanks

Answer 1

My ASM is a bit fuzzy about the details, but I think I can give you a general idea.

ESP: Stack pointer, EBP: Base pointer.

movl $1f, (%0)

Move address of label 1 (defined on last line) into parameter 0 (from).

movl %%esp, 4(%0)

Move the content of register ESP into (from + 4).

movl %%ebp, 8(%0)

Move the content of register EBP into (from + 8).

movl 8(%1), %%ebp

Move the content of (to + 8) into register EBP.

movl 4(%1), %%esp

Move the content of (to + 4) into register ESP.

jmp *(%1)

Jump to address contained in (to).

The “1:” is a jump label.

“+S” declares a “source” (read) parameter, “+D” a destination (write) parameter. The list of registers at the end of the statement is the “clobber” list, a list of registers possibly modified by the ASM code, so the compiler can take steps to maintain consistency (i.e., not relying on e.g. ECX still containing the same value as before).

I guess that coco_ctx means “coco context”. So: The function saves the current stack frame in the “from” structure, and sets the stack frame to what’s saved in the “to” structure. Basically, it jumps from the current function into another function.