FYI vien [0] figured out that simply compiling with "-static -fno-pie" and _exit(0)-ing at the end puts the solution presented here to 15000 points and hence #4 on the leaderboard. Pretty funny.
#pragma clang loop vectorize(enable)
#pragma clang loop interleave(enable)
for (; offset < length; offset += 4) {
const auto x = ((uint32_t\*)start)[offset / 4];
count += ((x & 0xFF) == 0x7F);
count += ((x & 0xFF00) == 0x7F00);
count += ((x & 0xFF0000) == 0x7F0000);
count += ((x & 0xFF000000) == 0x7F000000);
}