double-precision numbers in inline assembly (GCC, IA-32)

Question

I'm just starting to learn assembly and I want to round a floating-point value using a specified rounding mode. I've tried to implement this using fstcw, fldcw, and frndint.

Right now i get a couple of errors:

~ $ gc a02p
gcc -Wall -g a02p.c -o a02p
a02p.c: In function `roundD':
a02p.c:33: error: parse error before '[' token
a02p.c:21: warning: unused variable `mode'
~ $

I m not sure if i am even doing this right at all. I dont want to use any predefined functions. I want to use GCC inline assembly.

this is the code..

#include <stdio.h>
#include <stdlib.h>

#define PRECISION           3
#define RND_CTL_BIT_SHIFT   10

// floating point rounding modes: IA-32 Manual, Vol. 1, p. 4-20
typedef enum {
ROUND_NEAREST_EVEN =    0 << RND_CTL_BIT_SHIFT,
ROUND_MINUS_INF =       1 << RND_CTL_BIT_SHIFT,
ROUND_PLUS_INF =        2 << RND_CTL_BIT_SHIFT,
ROUND_TOWARD_ZERO =     3 << RND_CTL_BIT_SHIFT
} RoundingMode;

double roundD (double n, RoundingMode roundingMode)
{

short c;
short mode = (( c & 0xf3ff) | (roundingMode));


asm("fldcw    %[nIn] \n"
    "fstcw    %%eax  \n"             // not sure why i would need to store the CW
    "fldcw    %[modeIn] \n"
    "frndint            \n"
    "fistp    %[nOut]          \n"


     : [nOut]  "=m"          (n)
     : [nIn]   "m"           (n)
     : [modeIn]  "m"         (mode)

     );

 return n;


}

int main (int argc, char **argv)
{
double  n = 0.0;


if (argc > 1)
    n = atof(argv[1]);

printf("roundD even %.*f = %.*f\n",
       PRECISION, n, PRECISION, roundD(n, ROUND_NEAREST_EVEN));
printf("roundD down %.*f = %.*f\n",
       PRECISION, n, PRECISION, roundD(n, ROUND_MINUS_INF));
printf("roundD up   %.*f = %.*f\n",
       PRECISION, n, PRECISION, roundD(n, ROUND_PLUS_INF));
printf("roundD zero %.*f = %.*f\n",
       PRECISION, n, PRECISION, roundD(n, ROUND_TOWARD_ZERO));

return 0;
}

am i even remotely close to getting this right?

Answer 1

A better process is to write a simple function that rounds a floating point value. Next, instruct your compiler to print an assembly listing for the function. You may want to put the function in a separate file.

This process will show you the calling and exiting conventions used by the compiler. By placing the function in a separate file, you won't have to build other files. Also, it will give you the opportunity to replace the C language function with an assembly language function.

Although inline assembly is supported, I prefer to replace an entire function in assembly language and not use inline assembly (inline assembly isn't portable, so the source will have to be changed when porting to a different platform).

Answer 2

GCC's inline assembler syntax is arcane to say the least, and I do not claim to be an expert, but when I have used it I used this howto guide. In all examples all template markers are of the form %n where n is a number, rather then the %[ttt] form that you have used.

I also note that the line numbers reported in your error messages do not seem to correspond with the code you posted. So I wonder if they are in fact for this exact code?