Signed saturated add of 64-bit ints?

I'm looking for some C code for signed saturated 64-bit addition that compiles to efficient x86-64 code with the gcc optimizer. Portable code would be ideal, although an asm solution could be used if necessary.

static const int64 kint64max = 0x7fffffffffffffffll;
static const int64 kint64min = 0x8000000000000000ll;

int64 signed_saturated_add(int64 x, int64 y) {
  bool x_is_negative = (x & kint64min) != 0;
  bool y_is_negative = (y & kint64min) != 0;
  int64 sum = x+y;
  bool sum_is_negative = (sum & kint64min) != 0;
  if (x_is_negative != y_is_negative) return sum;  // can't overflow
  if (x_is_negative && !sum_is_negative) return kint64min;
  if (!x_is_negative && sum_is_negative) return kint64max;
  return sum;
}

The function as written produces a fairly lengthy assembly output with several branches. Any tips on optimization? Seems like it ought to be be implementable with just an ADD with a few CMOV instructions but I'm a little bit rusty with this stuff.