CODEWITHSUNDEEP
c++performance
Parad0x13
Parad0x13

Reputation: 2065

why a uint32_t vs uint64_t speed difference?

Trying to understand something about how g++/cpu processes integers at runtime.

I'm measuring how long the following function takes to run:

template<class T>
void speedTest() {
    for(T d=0;d<4294967295u;d++)int number;
}

This simple method will run a dumb loop the max value of uint32_t many times

and when I call:

speedTest<uint32_t>();

the software takes an average of 8.15 seconds but when I call:

speedTest<uint64_t>();

the software takes an average of 10.35 seconds.

Why is this happening?

Upvotes: 2

Views: 2872

Answers (1)

Sam
Sam

Reputation: 7868

Some possible reasons:

  • Larger data types require more memory bandwidth in general
  • Even if that loop counter is kept inside a register, the CPU is probably taking more time to do calculations with large values, especially, if it needs multiple registers (e.g. if your CPU has just 32bit wide registers)
  • The compiler would need to emit extra machine instructions to emulate any type not directly supported by the CPU
  • It also depends on optimization. Such a loop without side effects could be optimized out completely, regardless of int number; (could just be for(T d=0;d<4294967295u;d++);)

You could continue your investigation/exercise by providing some assembly.

Upvotes: 3

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