lavfi/gradfun: fix dithering in MMX code.

Current dithering only uses the first 4 instead of the whole 8 random values.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavfilter/x86/vf_gradfun.c

@@ -46,8 +46,33 @@ static void gradfun_filter_line_mmxext(uint8_t *dst, uint8_t *src, uint16_t *dc,
         "pxor       %%mm7, %%mm7 \n"
         "pshufw $0, %%mm5, %%mm5 \n"
         "movq          %6, %%mm6 \n"
-        "movq          %5, %%mm4 \n"
+        "movq          (%5), %%mm3 \n"
+        "movq         8(%5), %%mm4 \n"
+
         "1: \n"
+        "movd     (%2,%0), %%mm0 \n"
+        "movd     (%3,%0), %%mm1 \n"
+        "punpcklbw  %%mm7, %%mm0 \n"
+        "punpcklwd  %%mm1, %%mm1 \n"
+        "psllw         $7, %%mm0 \n"
+        "pxor       %%mm2, %%mm2 \n"
+        "psubw      %%mm0, %%mm1 \n" // delta = dc - pix
+        "psubw      %%mm1, %%mm2 \n"
+        "pmaxsw     %%mm1, %%mm2 \n"
+        "pmulhuw    %%mm5, %%mm2 \n" // m = abs(delta) * thresh >> 16
+        "psubw      %%mm6, %%mm2 \n"
+        "pminsw     %%mm7, %%mm2 \n" // m = -max(0, 127-m)
+        "pmullw     %%mm2, %%mm2 \n"
+        "paddw      %%mm3, %%mm0 \n" // pix += dither
+        "psllw         $2, %%mm1 \n" // m = m*m*delta >> 14
+        "pmulhw     %%mm2, %%mm1 \n"
+        "paddw      %%mm1, %%mm0 \n" // pix += m
+        "psraw         $7, %%mm0 \n"
+        "packuswb   %%mm0, %%mm0 \n"
+        "movd       %%mm0, (%1,%0) \n" // dst = clip(pix>>7)
+        "add           $4, %0 \n"
+        "jnl 2f \n"
+
         "movd     (%2,%0), %%mm0 \n"
         "movd     (%3,%0), %%mm1 \n"
         "punpcklbw  %%mm7, %%mm0 \n"
@@ -70,10 +95,12 @@ static void gradfun_filter_line_mmxext(uint8_t *dst, uint8_t *src, uint16_t *dc,
         "movd       %%mm0, (%1,%0) \n" // dst = clip(pix>>7)
         "add           $4, %0 \n"
         "jl 1b \n"
+
+        "2: \n"
         "emms \n"
         :"+r"(x)
         :"r"(dst+width), "r"(src+width), "r"(dc+width/2),
-         "rm"(thresh), "m"(*dithers), "m"(*pw_7f)
+         "rm"(thresh), "r"(dithers), "m"(*pw_7f)
         :"memory"
     );
 }