Hello, I'm not sure if this will be the proper place to ask this. If not,
could someone suggest a better place to ask.
I'm having a bit of trouble with a small function I've created. It is supposed
to multiply 2 64-bit numbers, divide the result (possibly up to a 128-bit
number) by another 64-bit number, and then return the 64-bit remainder after
that division (mulmod64).
I thought that I have compiled this successfully with Mingw64 in the past, but
I
can't seem to recreate that now. Now when I compile it, it silently crashes.
When I run it through gdb, it tells me I am getting a SIGFPE on a "mov"
instruction. If I hit continue, it just keeps repeating SIGILL after that.
I guess my question is, Is my code correct? Can what I'm trying to do be done
with Mingw64? Any help/advice would be greatly appreciated.
-David C.
Here is a small sample program:
#include <stdio.h>
typedef unsigned long long u64_t;
/***************************************************************/
static u64_t mulmod64_asm(u64_t a, u64_t b, u64_t c)
{
u64_t d; /* to hold the result of a*b mod c */
/* calculates a*b mod c, stores result in d */
asm ("mov %1, %%rax;" /* put a into rax */
"mul %2;" /* mul a*b -> rdx:rax */
"div %3;" /* (a*b)/c -> quot in rax rem in rdx */
"mov %%rdx, %0;" /* store result in d */
:"=r"(d) /* output */
:"r"(a), "r"(b), "r"(c) /* input */
:"%rax", "%rdx" /* clobbered registers */
);
return d;
}/* method mulmod64_asm */
/***************************************************************/
int main(int argc, char* argv[])
{
u64_t n = 0;
u64_t x = 18446744073709551615ULL;
u64_t y = 18446744073709551615ULL;
u64_t z = 18446744073709550000ULL; /* this gives n = 2608225 */
/* u64_t z = 10000; /* with this z, n = 8225 */
printf("Testing asm code...\n");
n = mulmod64_asm(x,y,z);
printf("x*y mod z = %I64u\n", n);
return 0;
}
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