dmd codegen improvements

[Kagamin via Digitalmars-d]

On Friday, 21 August 2015 at 09:17:28 UTC, Iain Buclaw wrote:
> There's a paper somewhere about optimisations on Intel 
> processors that says that -O2 produces overall better results 
> than -O3 (I'll have to dig it out).

That being said, recently I compared performance of the datetime 
library using different algorithms. One function of interest was 
computing year from raw time: D1 had an implementation based on 
loop - it iterated over years until it matched the source raw 
time; and currently phobos has implementation without loop, which 
carefully reduces the time to year. I wrote two tests iterating 
over days and calling date-time conversion functions, the test 
which invoked yearFromDays directly showed that implementation 
without loop is faster, but the bigger test that called full 
conversion between date and time showed that version with loop is 
faster by 5%. Quite unintuitive. Could it be due to cache 
problems? The function with loop is smaller, but the whole 
executable is only 15kb - should fit in the processor cache 
entirely.