Re: Portable "add with carry" loop for arbitrary precision arithmetic

[Tom St Denis <tstdenis at ellipticsemi dot com>]

Alexander Konovalenko wrote:
> There is an "add with carry" instruction on some architectures which
> enables efficient implementation of arbitrary precision (bignum)
> integer addition. Is it possible to make gcc generate efficient
> machine code from pure C or C++ code for bignum addition on those
> architectures?
>
> It looks like the popular libraries like GMP and CLN use hand-coded
> assembly for that. I wonder if gcc can help rewrite that kind of code
> in a more portable way.
>   

GCC will use them in most cases.  For example,

extern unsigned long long a, b, c;
int main(void)
{
  c = a + b;
  return 0;
}

Produces this with "-O3 -fomit-frame-pointer"

main:
   leal    4(%esp), %ecx
   andl    $-16, %esp
   pushl    -4(%ecx)
   pushl    %ecx
   movl    a, %eax
   movl    a+4, %edx
   addl    b, %eax
   adcl    b+4, %edx
   movl    %eax, c
   xorl    %eax, %eax
   movl    %edx, c+4
   popl    %ecx
   leal    -4(%ecx), %esp
   ret

Tom