double-precision numbers in inline assembly (GCC, IA-32)
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 don't 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",开发者_高级运维
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?
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).
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?
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