Re: else ladders practice

On 25/11/2024 18:49, Tim Rentsch wrote:

Bart <bc@freeuk.com> writes:
 

It's funny how nobody seems to care about the speed of compilers
(which can vary by 100:1), but for the generated programs, the 2:1
speedup you might get by optimising it is vital!

 I think most people would rather take this path (these times
are actual measured times of a recently written program):
      compile time:       1 second
     program run time:   ~7 hours
 than this path (extrapolated using the ratios mentioned above):
      compile time:       0.01 second
     program run time:   ~14 hours
 

I'm trying to think of some computationally intensive app that would run non-stop for several hours without interaction.
If you dig back throug the thread, you will see that I am not against compiling with optimisations for production code. But for very frequent routine builds I want it as fast as possible.
For such a task as your example might do, you would spend some time testing on shorter examples and getting the best algorithm. Once you feel it's the best, /then/ you can think about getting it optimised. It doesn't even matter how long it takes, if it's going to take hours anyway.
I thought of one artificial example, it's a C program to display the Fibonacci sequence 1 to 100 using the recursive function for each fib(i).
I compiled it with gcc-O3 and set going. While it was doing that, I set up the same test using my interpreted language. It was much slower obviously. So I added memoisation. Now it showed all 100 values instantly (the C version meanwhile is in the low 50s).
I noticed however that it overflowed the 64-bit range at around fib(93) (as the C version might do eventually). So I tweaked my 'slow' version to use bignum values. Then I tweaked it again to show the first 10,000 values.
At this point, the optimised C was still in the mid 50s.
The point is, for such a task as this, you do as much as you can to bring down the runtime, which could reduce it by a magnitude or two with the right choices.
Adding -O3 at the end is a nice bonus speedup, but that's all it is.