On Wed, Feb 3, 2010 at 7:32 PM, David Cleaver <[email protected]> wrote:
> 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 */
I don't have a 64-bit machine handy to test this at the moment, but it
could at least be more efficiently written with different constraints:
static u64_t mulmod64_asm(u64_t a, u64_t b, u64_t c)
{
u64_t d;
asm ("mul %2;"
"div %3;"
:"=d"(d)
:"a"(a), "rm"(b), "rm"(c)
);
return d;
}
Apologies if this doesn't really answer your question...
-- Daniel Verkamp
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