Matrix mul

[Bill Baxter]

Doh!  The NaNs strike again.

--bb

On Sun, Nov 23, 2008 at 3:40 PM, Tom S <h3r3tic at remove.mat.uni.torun.pl> 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
>
>
> --
> Tomasz Stachowiak
> http://h3.team0xf.com/
> h3/h3r3tic on #D freenode
>