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.
--
Tomasz Stachowiak
http://h3.team0xf.com/
h3/h3r3tic on #D freenode
