64 bit assignment on a 32 bit platform

[Rask Ingemann Lambertsen]

On Mon, Oct 29, 2007 at 03:37:04PM +0530, kum wrote:
> 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 yield correct 64 bit
> results?

"a64 = a32 * b32" multiplies two 32-bit numbers to give a 32-bit result,
which is then zero-extended to 64 bits. If you want an expanding
multiplication, use "a64 = (UINT64) a32 * (UINT64) b32".

> Without casting, I find that addition (even if the result
> overflows) works while the multiplication does not.

   Which of the two additions are you talking about? Could you perhaps post
some assembly (-O2 -S -dp) of this code?

#include <stdint.h>

uint64_t a;

void addtest1 (uint32_t b, uint32_t c)
{
  a += b + c;
}

> Is there a
> compiler option to make this work without casting? I am using gcc
> 4.1.1 on an xscale platform.

   It would appear to work the way it is supposed to and I don't know of any
option to change that.

-- 
Rask Ingemann Lambertsen
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