Question

char buffer[424242] = {0};

Is buffer[index] as fast as *buffer?

for (int i = 0; i < SIZE; ++i) {
  buffer[i] = 42;
}

char* end = buffer + SIZE;
for (char* pos = buffer; pos != end; ++pos) {
  *pos = 42;
}

I guess my question is, is there any assembly instruction that can set a position in memory plus and offset to a given value in a single cycle?

LEA seems to load the address plus multiply it in this way.

Answer 1

*buffer is at least as fast as buffer[index]. Depends on index. If index == 0, they’ll be equally fast.

As per your edit:

char& end = buffer + SIZE;
for (char* pos; pos != end; ++pos) {
  *pos = 42;
}

invokes undefined behavior because you don’t initialize pos.

As per your second edit:

Write your code for readability first, profile & possibly change only after you have some concrete results.

for (int i = 0; i < SIZE; ++i) {
  buffer[i] = 42;
}

is way more readable, stick to it.