CODEWITHSUNDEEP
c++
Nick
Nick

Reputation: 33

Are multiple sequential array access optimised?

Will C++ compilers automatically optimise the following code to calculate 3*i only once, and increment the result, or must the programmer code this?

void writeToArray(int i, int a, int b, int c)
{
   array[3*i][1]=a;
   array[3*i][2]=b;
   array[3*i][3]=c;
}

Upvotes: 3

Views: 136

Answers (4)

BjoernD
BjoernD

Reputation: 4780

With modern compilers you can most of the time rely on the compiler doing the clever job and not try anything yourself. See for instance http://dl.fefe.de/optimizer-isec.pdf

Upvotes: 3

Puppy
Puppy

Reputation: 146910

Common Subexpression Elimination is one of the easiest and most commonly applied optimizations. In general, you can rely on this being done.

Upvotes: 3

Chris Eberle
Chris Eberle

Reputation: 48775

Most optimizers will pick up on this, yes. However if you want to encourage the optimizer, here's how:

void writeToArray(int i, int a, int b, int c)
{
   const int offset = 3 * i;
   array[offset][1]=a;
   array[offset][2]=b;
   array[offset][3]=b;
}

Upvotes: 7

Ben Voigt
Ben Voigt

Reputation: 283624

When you enable optimization, almost all compilers will not only precompute the common index 3*i but the pointer array + (3*i)*sizeof (*array).

Upvotes: 5

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