When working with arbitrary precision arithmetic (e.g. 512-bit integers), is there any way to get GCC to use ADC and similar instructions without using inline assembly?
A first glance at GMP's sourcecode shows that they simply have assembly implementations for every supported platform.
Here is the test code I wrote, which adds two 128-bit numbers from the command line and prints the result. (Inspired by mini-gmp's add_n):
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
int main (int argc, char **argv)
{
uint32_t a[4];
uint32_t b[4];
uint32_t c[4];
uint32_t carry = 0;
for (int i = 0; i < 4; ++i)
{
a[i] = strtoul (argv[i+1], NULL, 16);
b[i] = strtoul (argv[i+5], NULL, 16);
}
for (int i = 0; i < 4; ++i)
{
uint32_t aa = a[i];
uint32_t bb = b[i];
uint32_t r = aa + carry;
carry = (r < carry);
r += bb;
carry += (r < bb);
c[i] = r;
}
printf ("%08X%08X%08X%08X + %08X%08X%08X%08X =\n", a[3], a[2], a[1], a[0], b[3], b[2], b[1], b[0]);
printf ("%08X%08X%08X%08X\n", c[3], c[2], c[1], c[0]);
return 0;
}
GCC -O3 -std=c99
Does not produce any adc
instructions, as checked by objdump
. My gcc version is i686-pc-mingw32-gcc (GCC) 4.5.2
.