Tom S wrote:
Andrei Alexandrescu wrote:
Tom S wrote:
Don wrote:
bearophile wrote:
While writing code that works on matrices I have found something
curious...
Here's what I think is going on. AFAIK, D hasn't got any special
code for initializing jagged arrays. So
auto A = new double[][](N, N);
involves N+1 memory allocations.
As well as being slow, this is going to give horrible locality of
reference in the memory allocation. You'll suffer cache misses on
almost every array access!
I don't think this has anything to do with bounds checking.
Nah, it's about NaNs :)
Version #1 initializes C to NaN, Version #2 initializes it to 0. The
'init mats randomly' loop doesn't touch C at all, thus all the latter
additions leave C at NaN, causing lots of FP exceptions.
We actually had something like this happen in Deadlock a few times in
the animation subsystem. Brought the framerate down to a single digit :D
I swear I wrote my previous message before reading this :o). Damn
you're right. I hadn't seen the bug in initialization. But since
version #2 uses malloc, isn't that memory garbage (possibly, but not
necessarily, zero)?
Damn, you're right too ;) I always forget whether malloc or calloc
initializes to zero. According to my tests, DMD/Tango's malloc returns
mostly zeroes in bearophile's test anyway.
In my experience malloc returns zeroed memory around the start of an
application (due to the OS applying simple security considerations).
Later on, as memory is reused, there's no more zeroes - it's whatever
you left in there.
Andrei
