Re: GCC has problems with 64-bit multiplication

[Hans Petter Selasky <hselasky at c2i dot net>]

On Thursday 08 February 2007 16:02, Graham Stott wrote:
> All,
>
> Not a bug in GCC the result is correct as you've only asked for a 32-bit
> multiply.

Hi again,

The problem was not that "mull" is used, but that "gcc 3.4.6" generates highly 
un-optimized code when I for example multiply a 16-bit integer by a 64-bit 
integer.

It actually generates code where three multiplications are used instead of 
two. That has a speed impact.

Code:

#include <stdio.h>
#include <sys/types.h>

int main() {

        int64_t temp;
        int16_t x;

        temp *= x;

        printf("%lld\n", temp);

        return 0;
}

objdump -D ./a.out

 8048507:       89 4d e8                mov    %ecx,0xffffffe8(%ebp)
 804850a:       31 c0                   xor    %eax,%eax
 804850c:       89 55 ec                mov    %edx,0xffffffec(%ebp)
 804850f:       f7 65 e8                mull   0xffffffe8(%ebp)
 8048512:       6b 7d ec 00             imul   $0x0,0xffffffec(%ebp),%edi
 8048516:       89 d6                   mov    %edx,%esi
 8048518:       89 c1                   mov    %eax,%ecx
 804851a:       01 fe                   add    %edi,%esi
 804851c:       6b 45 e8 00             imul   $0x0,0xffffffe8(%ebp),%eax
 8048520:       83 e4 f0                and    $0xfffffff0,%esp
 8048523:       83 ec 14                sub    $0x14,%esp
 8048526:       8d 1c 06                lea    (%esi,%eax,1),%ebx

--HPS