Matrix mul

[Tom S]

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