Coroutines, x64 and Visual Studio

I really like co-routines, finding them really useful for programming game AI and scripting.  Since visual studio has dropped support for inline assembly when compiling x64 code I thought I would be out of luck trying to implement co-routines for this target.  Fortunately visual studio can still compile assembly files using masm in x64 mode.  In order to implement co-routines I had to familiarities myself with x64 calling conventions, something that has changed quite a bit from x86.  The first 4 arguments are passed on the stack, there are many more callee save registers, and more obviously all registers and pointers have been extended to 64bits.

Below is a small demo showing how to implement co-routines in visual studio for x64 targets.


#include <stdio.h>

typedef unsigned long long u64;
typedef void * user_t;
typedef void *stack_t;

typedef void (*cofunc_t)( stack_t *token, user_t arg );

/* coroutine functions */
extern "C" void yield_( stack_t *token );
extern "C" void enter_( stack_t *token, user_t arg );

/* artificial stack */
const int nSize = 1024 * 1024;
static char stack[ nSize ] = { 0 };

/* prepare a coroutine stack */
void prepare( stack_t *token, void *stack, u64 size, cofunc_t func )
{
    u64 *s64     = (u64*)( (char*)stack + size);
         s64    -= 10;                 // 10 items exist on stack
         s64[0]  = 0;                  // R15
         s64[1]  = 0;                  // R14
         s64[2]  = 0;                  // R13
         s64[3]  = 0;                  // R12
         s64[4]  = 0;                  // RSI
         s64[5]  = 0;                  // RDI
         s64[6]  = (u64) s64 + 64;     // RBP
         s64[7]  = 0;                  // RBX
         s64[8]  = (u64) func;         // return address
         s64[9]  = (u64) yield_;       // coroutine return address
          *token = (stack_t) s64;      // save the stack for yield
}

/* coroutine function */
void threadFunc( stack_t *token, user_t arg )
{
    for ( int i=0; i<10; i++ )
    {
        printf( "  coroutine %d\n", i );
        yield_( token );
    }
}

/* program entry point */
int main( )
{
    stack_t token = nullptr;

    /* prepare the stack */
    prepare( &token, stack, nSize, threadFunc );
    /* enter the coroutine */
    enter_( &token, (void*)0x12345678 );
    /* simple test loop */
    for ( int i=0; i<10; i++ )
    {
        printf( "main thread %d\n", i );
        yield_( &token );
    }
    /* program done */
    printf( "program exit\n" );
    getchar( );
}

Coroutine.asm


.code 

;---- ---- ---- ---- ---- ---- ---- ----
; coroutine yield function
;
;   : void yield_( void * token );
;
;   'token' -&amp;gt; RCX
;
yield_ proc

    push RBX
    push RBP
    push RDI
    push RSI
    push R12
    push R13
    push R14
    push R15

    mov  RAX ,  RSP
    mov  RSP , [RCX]
    mov [RCX],  RAX

    pop R15
    pop R14
    pop R13
    pop R12
    pop RSI
    pop RDI
    pop RBP
    pop RBX

    ret

yield_ endp

;---- ---- ---- ---- ---- ---- ---- ----
; enter a co-routine
;
;   : void enter_( void * token, void * arg1, ... );
;
;   'token'     -&amp;gt; RCX
;   'arg1, ...' -&amp;gt; RDX, R8, and R9
;
enter_ proc

    jmp yield_

enter_ endp

end

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