Re: [hatari-devel] EXA demo Entracte : Improvement in search

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

Le 05/02/2017 à 22:32, Laurent Sallafranque a écrit :
> I don't understand something :
>
> According to hatari debug mode, I can read :
>
> cpu video_cyc=  1034  10@  1 : 0004EB82 4a78 0468                TST.W
> $00000468
> cpu video_cyc=  1038  14@  1 : 0004EB86 6600 0006                BNE.W
> #$0006 == $0004eb8e (F)
> cpu video_cyc=  1046  22@  1 : 0004EB8A 5240                     ADD.W
> #$00000001,D0
> cpu video_cyc=  1048  24@  1 : 0004EB8C 60f4                     BT .B
> #$fffffff4 == $0004eb82 (T)
>
> If(s the CPU speed loop of EXA demos.
> The code runs into the instr cache.
>
> I've taken the 4th iteration (they all take the same time except the
> first one, which seems OK to me).
>
> If I can rely on the debugger, I read 14 cycles for this iteration.


Your count is wrong, 14 doesn't count the bra.s at $4eb8c ; if you count 
it, you get +6 and total is 20 cycles, which is what I see too and which 
is 1 cycle too slow on average that on real HW (if we want to get D0 
between $4100-$4200)

> If I compute the loop by hand, I get :
>
> tst.w     $0468.w        {2,    2,     8,1,0,0,    12,1,2,0}, // TST.W
> (xxx).W
> bne       $1cd2a         {6,    0,     6,0,0,0,  8,0,1,0},    // Bcc.W
> (not taken)
> addq.w    #1,d0          {2,    0,     2,0,0,0,  4,0,1,0},    // ADDQ.W
> #<data>,Dn
> bra.s     $1cd1e         {6,    0,     6,0,0,0, 12,0,2,0},    // Bcc
> (taken)
>
> Which is 8+6+2+6-2 cycles = 20 cycles.
>
> (-2 cycles because of the Head/tail -2 between the first and second
> instructions)
>
>
> So, it seems that the loop run faster under hatari than it should, but
> D0 value is lower than $4100 (it should be higher, no ?)

See above ; 20 is the theoretical value, but as discussed with Toni, the 
68030 has some complex pipeline and it seems some access are taking less 
time, but the rule is not known yet.