64 bit assignment trouble on a 32 bit platform

64 bit assignment trouble on a 32 bit platform

[kum]

hi,

UINT64 a64; // UINT64 has been typedefed as long long unsigned int
UINT32 a32, b32;
a32 = x; // some value
b32 = y; // some value

a64 = a32 * b32;
a64 += a32 + b32;

Is it necessary to type-cast both a32 and b32 to make above statements
to work as expected? Without casting, I find that addition works while
the multiplication does not. Is there a compiler option to make this
work without casting? I am using gcc 4.1.1 on an xscale platform.

Thanks,
kumaresh

Re: 64 bit assignment trouble on a 32 bit platform

[Samuel Tardieu]

[this is not a question for the GCC developers list]

Kumaresh> Is it necessary to type-cast both a32 and b32 to make above
Kumaresh> statements to work as expected?

Yes.

Kumaresh> Without casting, I find that addition works while the
Kumaresh> multiplication does not.

Addition is not supposed to work either. I guess that your addition
just doesn't overflow the capacity of an UINT32 while your
multiplication does.

The following program shows it clearly:

#include <stdio.h>

typedef long long unsigned UINT64;
typedef long unsigned UINT32;

int main(void)
{
   UINT32 a = 0xffffffff;
   UINT32 b = a;
   printf ("a+b = 0x%016llx\n", (UINT64) (a+b));
   return 0;
}

The output will be:

  a+b = 0x00000000fffffffe

instead of your expected

  a+b = 0x00000001fffffffe

  Sam
-- 
Samuel Tardieu -- sam@rfc1149.net -- http://www.rfc1149.net/

Re: 64 bit assignment trouble on a 32 bit platform

[Rask Ingemann Lambertsen]

On Mon, Oct 29, 2007 at 03:05:32PM +0530, kum wrote:
> 
> Is it necessary to type-cast both a32 and b32 to make above statements
> to work as expected? Without casting, I find that addition works while
> the multiplication does not. Is there a compiler option to make this
> work without casting? I am using gcc 4.1.1 on an xscale platform.

   This is the wrong mailing list for such a question, please ask on
gcc-help@. Also please tell us what "work as expected" means. Otherwise, we
can't really answer your question.

-- 
Rask Ingemann Lambertsen
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