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 ? [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB No great genius has ever existed without some touch of madness. -- Aristotle
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