Re: Re: dmix plugin

[Jaroslaw Sobierski <fycio@gucio.com>]

Quoting Jaroslav Kysela <perex@suse.cz>:

> I don't think so. It seems that my brain still remembers assembler ;-)
...
>  			sample = *sum;
>  			s16 s;
> -			if (unlikely(sample & 0xffff0000))
> +			if (unlikely(sample & 0x7fff0000))
>  				s = sample > 0 ? 0x7fff : -0x8000;
>  			else
>  				s = sample;

I think I remember some of the x86 assembly myself and this correction
does not fix the problem. This code will still "saturate" all negative
samples to -8000. You cannot detect an overflow into the upper half of
the register with a simple bitwise and. The actual test should be as
follows : 
- extend the sign of the lower half
- check if the upper half is the same as the effect of expansion
 if it is - there is no overflow
 if it differs - there was overflow and you need to saturate.
examples : 
value 0x 0000 0335
ext   0x 0000 0335
  -> no overflow

value 0x 0002 43b1
ext   0x 0000 43b1
  -> overflow

value 0x ffff f25b
ext   0x ffff f25b
  -> no overflow

value 0x ff1c 35c9
ext   0x 0000 35c9
  -> overflow

to put it in asm:

mov ebx,eax
cwde
cmp eax,ebx

The problem is cwde operates only on ax/eax.
This may sound complicated but in fact it amounts to a very simple
question : does the sample fit in a 16 bit int, or does it not, so
I guess in C it could look something like :

    s16 s=sample;
    if (unlikely(sample != (s32)s))

The cast is just there for clarity I believe it would be done
implicitly anyway. But don't take my word for it - I did not
test this.

--------------
Fycio (J.Sobierski)
 fycio@gucio.com


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