Bug 7844 – implement loop invariant code motion for pure functions

I have recently made a program that finds the sum of all primes (they are 11) that truncable from left to right and from right to left. I compiled it with -O -release -inline -noboundscheck The results were: real 0m1.659s user 0m1.652s sys 0m0.000s I made a few changes to make it compile as C code, and compiled in gcc with -O5 -lm -std=c99. The results: real 0m0.764s user 0m0.760s sys 0m0.004s There's ~900 ms difference here, which is big. The problem is in a function I used, "isPrime(ulong)" /// returns true if n is a prime number bool isPrime(ulong n) { // 0 and 1 aren't primes if( n < 2 ) return false; if( n % 2 == 0 && n != 2) return false; // an even number can't be a prime (except 2) // check only if it's odd for(ulong i = 2; i <= cast (ulong)sqrt(cast(double)n+1); ++i) if( n % i == 0 ) return false; return true; } As you can see, in the for loop, sqrt() is called in every iteration and that's what makes it slower. When changed to: ulong limit = cast (ulong)sqrt(cast(double)n+1) for(ulong i = 2; i <= limit; ++i) It performs in 400ms!!! Somehow the C version manages to do that sort of optimizations. I expect DMD to do it as well, as it makes the code A LOT faster.

This optimization is called loop invariant code motion.

*** This issue has been marked as a duplicate of issue 4453 ***