Why Loops with one basic instruction works in O(1)

Asked Jan 16 '18 at 05:19

Active Jan 16 '18 at 05:20

Viewed 100 times

Why C++ compiler executes this code very fast? as if it's one statement.

long long sum = 0;
for (long long i = 1; i <= 100000000000; i++)
    sum ++;
cout << sum;

It seems like compilers optimize it as sum += 100000000000. so how? and what's the limitation of this?

edited Jan 16 '18 at 05:20

asked Jan 16 '18 at 05:19

seif gamal

What compiler are you using? – Asesh Jan 16 '18 at 05:20
gcc (GCC) 5.3.0 – seif gamal Jan 16 '18 at 05:22
Yes, GCC will do this [at -O1](https://godbolt.org/g/P2W5iM). – chris Jan 16 '18 at 05:30
VC++ produces SIMD instructions to execute that code and will loop your code to produce that output. That's another reason to switch to a better compiler. – Asesh Jan 16 '18 at 05:34
1

@Asesh My experiment shows the contrary. https://godbolt.org/g/rcu7Jn – Jan 16 '18 at 05:42
@NickyC I tried that code in VC++ 2015 (Release version). Good to know that it has been improved – Asesh Jan 16 '18 at 05:44

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