Re: [hatari-devel] DSP Mandelbrot bug

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Andreas.... another small optimization for the NeXT :)

You can use the knowledge that a) the first iteration won't escape, and b) that the beginning of the second iteration already gives a clue to where in the set you are (e.g. inside the trap zone) so you can perform a quick bounding test and eliminate (total-1) iterations for maybe 15% of the interior of the set - which is where most of the wasted iterations go.

...but fully prepared to admit to this being an imperfect hack for purists, because configured like this it will steal the very tip of the set boundary at one specific point, at the closest possible zoomlevels. For realtime hackery though it's good :)

If you're determined you can unroll the whole thing and find an arbitrary bound at each iteration but I won't bother here.

In any case I've thrashed this to death by now - well offtopic - so I'll leave it here!

    ; bounding iteration #1
    mpy        y0,y0,b        a,x0
    mac        x0,x0,b
    mpy        y0,x0,b        y1,a
    asl        b
    addl        a,b
    mpy        -y0,y0,a    b,y0
    mac        x0,x0,a        x1,b
    addl        b,a   

    ; bounding iteration #2
    mpy        y0,y0,b        a,x0
    mac        x0,x0,b        #<$2,a
    cmp        a,b
    jlt        <_iters_bound  ; 99.999% certain we're trapped! shortcut!
    mpy        y0,x0,b        y1,a
    asl        b
    addl        a,b
    mpy        -y0,y0,a    b,y0
    mac        x0,x0,a        x1,b
    addl        b,a   
    do        #mand_iters-2,_iters

    mpy        y0,y0,b        a,x0
    mac        x0,x0,b
    jes        <_stop
    mpy        y0,x0,b        y1,a
    asl        b
    addl        a,b
    mpy        -y0,y0,a    b,y0
    mac        x0,x0,a        x1,b
    addl        b,a


On 23 November 2015 at 09:15, Douglas Little <doug694@xxxxxxxxxxxxxx> wrote:

I was able to run a test of the NeXT mandelbrot routine by tidying up the debug log and extracting the inner loop instructions (which my last post was based on), before trying to optimize it.

From what I could see, there were actually no bad pixels in the output. This test was run inside Hatari (i.e. not on real HW - didn't bother trying that since I didn't see a problem).

Of course I don't have the setup code for the NeXT routine, only the inner part. So it may be something to do with initial conditions, or what happens after escaping (calculation from LC).

TBH I have myself seen an odd bug in my own code when doing calculations from LC remainder. And it doesn't involve mandelbrot - something else.

I have not figured out what that issue is yet - assuming for now its just a bug in my code, but there's a small chance these things are related? I'll need to check that bug on real HW soon - just haven't had time.


On 22 November 2015 at 22:29, Andreas Grabher <andreas.grabher@xxxxxxxxxxxx> wrote:
Laurent and Douglas, you are of course right. This should be checked on real hardware. I try to find someone to test it for me. But until then, given the sense for perfection of all NeXT stuff, i think it is quite unlikely they shipped it with that obvious bug. There are also obvious issues (black line in the middle), if the default parameters are used.
I'll report back as soon as is have information from real hardware.

Laurent, i tried to answer your questions below.

Douglas, your optimizations are interesting. I think NeXT would have been interested in them, because the main purpose of the Mandelbrot demo was to demonstrate the performance advantage of the DSP over the CPU for certain tasks  ;-)
I'll have a look if i can isolate the complete DSP application. It seems to be embedded into the binary. Maybe i'll just print if from the DSP RAM ...

Am 22.11.2015 um 12:06 schrieb Laurent Sallafranque <laurent.sallafranque@xxxxxxx>:

Hi Andreas,

First, are you sure the resulting bad pixels don't appear on the real computer too ?
(just to be sure).

When I have a closer look at the trace, I can read :

; Previous loop
p:0099  0af0a5 0000a1  (06 cyc)  jec p:$00a1                 

; New loop that seems to bug                     

p:00a1  2000d8         (02 cyc)  mpy +y0,x0,b                                     
    Reg: b   $00:4a0b9a:44a2c4 -> $00:2355a0:579062
    Reg: sr  $8040 -> $8050
p:00a2  20003a         (02 cyc)  asl b                                            
    Reg: b   $00:2355a0:579062 -> $00:46ab40:af20c4
    Reg: sr  $8050 -> $8040

; Is the next one correct ? (we have b = $00:8...   ) ?
I think this is all correct. It gets back to 00:7... later just before the jec.

p:00a3  20003a         (02 cyc)  asl b                                            
    Reg: b   $00:46ab40:af20c4 -> $00:8d5681:5e4188
    Reg: sr  $8040 -> $8060

; What worth Y1 value just below ? (I haven't found it in the trace)
; The problem may be here
y1 is 0x2c28f8, it gets set up at the beginning and does not get changed during the calculation.

p:00a4  200078         (02 cyc)  add y1,b                                         
    Reg: b   $00:8d5681:5e4188 -> $00:ae4c44:5e4188

; Just below, we've got the "overflow" (7fffff) (into y0)
; The problem seem to be that the program copy $00:ae4c44:5e4188 into y0 and set it to $7fffff
I think that overflow is normal behavior. But i'm not sure about it.

p:00a5  21e696         (02 cyc)  mac -y0,y0,a b,y0                                
    Reg: y0  $4e71df -> $7fffff
    Reg: a   $00:19f86d:2f6242 -> $ff:e9e540:1a21c0
    Reg: sr  $8060 -> $8058

p:00a6  200032         (02 cyc)  asl a                                            
    Reg: a   $ff:e9e540:1a21c0 -> $ff:d3ca80:344380
    Reg: sr  $8058 -> $8059
p:00a7  200060         (02 cyc)  add x1,a                                         
    Reg: a   $ff:d3ca80:344380 -> $ff:fff376:344380
    Reg: sr  $8059 -> $8058
p:0096  21c498         (02 cyc)  mpy +y0,y0,b a,x0                                
    Reg: x0  $39a7ef -> $fff376
    Reg: b   $00:ae4c44:5e4188 -> $00:7ffffe:000002
    Reg: sr  $8058 -> $8040
p:0097  200080         (02 cyc)  mpy +x0,x0,a                                     
    Reg: a   $ff:fff376:344380 -> $00:000001:3a74c8
    Reg: sr  $8040 -> $8050

p:0098  200018         (02 cyc)  add a,b                                          
    Reg: b   $00:7ffffe:000002 -> $00:7fffff:3a74ca                    <-- Here KO ?
The problem seems to that unlike with the "good" pixels, a is too small to get to 00:8...
    Reg: sr  $8050 -> $8040
p:0099  0af0a5 0000a1  (06 cyc)  jec p:$00a1                                    


Le 22/11/2015 10:24, Andreas Grabher a écrit :

Looking at the values of b at time of final jec it seems that it might also be some kind of rounding issue:

00:800d9f:248aca <--- 3 pixels before
00:7fffff:3a74ca <--- bad pixel
00:800e89:ff27ca <--- 3 pixels after

Anfang der weitergeleiteten Nachricht:

Von: Andreas Grabher <andreas.grabher@xxxxxxxxxxxxx>
Betreff: [hatari-devel] DSP Mandelbrot bug
Datum: 22. November 2015 09:53:17 MEZ

Hello Hatari Community,

i am experiencing a hard to find DSP bug here with Previous. Luckily it can be made "visible" using NeXTstep's included Mandelbrot demo. It might also be responsible for some distorted audio in other applications.

I appended a screenshot of the mandelbrot application where the effect of the bug is clearly visible. I pointed to one failing pixel. I also appended some debugging output containing the calculation of the failing pixel and one pixel before and one after the failing one. The last file i appended contains an overview about the variables during calculation to get a better overview.

Short overview on the calculation:
Every pixel is calculated separately. The visible pixel color is derived from the remaining loop count of some calculation. The higher the remaining count, the lower is the output value of the function. The loop exits using a jec instruction (check extension bit, exit if false).
For the "good" pixels it exits after the second run, because the upper 9 bits of b are no longer all 0. For the "bad" pixel it does not exit, because these bits are still all 0. The most suspect part of the calculation seems to be mpy +x0,x0,a at p:0097. The value of a after the third call of that instruction seems to not fit into the pattern.

Can someone with more DSP experience see the bug? It might or might not be in dsp_mul56.

Any help is greatly appreciated!


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