[FFmpeg-devel] pngdsp add bytes padding

[Michael Niedermayer]

On Thu, Feb 18, 2016 at 05:53:46PM +0100, Clément Bœsch wrote:
> On Thu, Feb 18, 2016 at 12:42:59PM +0100, Michael Niedermayer wrote:
> > On Thu, Feb 18, 2016 at 11:36:18AM +0100, Clément Bœsch wrote:
> > > Hi,
> > > 
> > > While working on PNG DSP, I realized the filter up was doing extra
> > > overflow checks on the bytes non multiple of the long word size. This
> > > logic is present in both C and x86 ASM. Here is the current C code for
> > > reference:
> > > 
> > > -------------- 8< ----------------
> > > 
> > > // 0x7f7f7f7f or 0x7f7f7f7f7f7f7f7f or whatever, depending on the cpu's native arithmetic size
> > > #define pb_7f (~0UL / 255 * 0x7f)
> > > #define pb_80 (~0UL / 255 * 0x80)
> > > 
> > > static void add_bytes_l2_c(uint8_t *dst, uint8_t *src1, uint8_t *src2, int w)
> > > {
> > >     long i;
> > >     for (i = 0; i <= w - (int) sizeof(long); i += sizeof(long)) {
> > >         long a = *(long *)(src1 + i);
> > >         long b = *(long *)(src2 + i);
> > >         *(long *)(dst + i) = ((a & pb_7f) + (b & pb_7f)) ^ ((a ^ b) & pb_80);
> > >     }
> > >     for (; i < w; i++)
> > >         dst[i] = src1[i] + src2[i];
> > > }
> > > 
> > > -------------- 8< ----------------
> > > 
> > > The thing is, the buffers seem to be 0 padded to 16 (see
> > > av_fast_padded_malloc() calls). I'm assuming there are cases where it's
> > > not?
> > 
> > isnt dst the AVFrame in one call ?
> > so that this would write over width if it wrote beyond ?
> > 
> 
> but isn't frame->data[N] always padded as well...?

if a user app returns pointers to some memory for direct rendering
from get_buffer then it might be all kinds of stuff
for example it might be aligned but there might be data to the
right like window decorations/border

but maybe iam missing something

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

it is not once nor twice but times without number that the same ideas make
their appearance in the world. -- Aristotle
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