https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102974
--- Comment #12 from Mason <slash.tmp at free dot fr> ---
Actually, in this case, we don't need to propagate the carry over 3 limbs.
typedef unsigned int u32;
typedef unsigned long long u64;
/* u32 acc[2], a[1], b[1] */
static void mul_add_32x32(u32 *acc, const u32 *a, const u32 *b)
{
u64 res = (u64)a[0] * b[0];
u32 lo = res, hi = res >> 32;
asm("add %[LO], %[D0]\n\t" "adc %[HI], %[D1]" :
[D0] "+m" (acc[0]), [D1] "+m" (acc[1]) :
[LO] "r" (lo), [HI] "r" (hi) : "cc");
}
/* u32 acc[4], a[2], b[2] */
void mul_64x64_128(u32 *acc, const u32 *a, const u32 *b)
{
mul_add_32x32(acc+0, a+0, b+0);
mul_add_32x32(acc+1, a+0, b+1);
mul_add_32x32(acc+1, a+1, b+0);
mul_add_32x32(acc+2, a+1, b+1);
}
gcc-trunk -O3 -m32
mul_64x64_128:
pushl %esi
pushl %ebx
movl 16(%esp), %ebx
movl 20(%esp), %esi
movl 12(%esp), %ecx
movl (%esi), %eax
mull (%ebx)
add %eax, (%ecx)
adc %edx, 4(%ecx)
movl 4(%esi), %eax
mull (%ebx)
add %eax, 4(%ecx)
adc %edx, 8(%ecx)
movl (%esi), %eax
mull 4(%ebx)
add %eax, 4(%ecx)
adc %edx, 8(%ecx)
movl 4(%esi), %eax
mull 4(%ebx)
add %eax, 8(%ecx)
adc %edx, 12(%ecx)
popl %ebx
popl %esi
ret
