[FFmpeg-devel] libswscale.c : ff_xyz12Torgb48 expensive unaligned 16 byte accesses

[Chitra Dey Sarkar]

H
We have been profiling FFmpeg at Microsoft and have identified that ff_xyz12ToRgb48 has a high sample count ( profiled every 1ms )

It seems like ff_xyz12ToRgb48 has performance penalty for

  1.  Unaligned read and write access
  2.  Access to xyz2rgb_matrix
  3.  Multiplication

I would be interested in optimizing this code , wanted to check if there is an existing optimized version of this function, or any recommended approach to improve it(?
I can move the repeated access to xyz2rgb_matrix outside the inner loop and load a full cache line at once to extract the X, Y, and Z values more efficiently-but I wanted to start by getting some initial feedback or thoughts before proceeding further


File : FFmpeg/libswscale/swscale.c at a79720e10f30e9fd18bd78242ce96dde06461343 * FFmpeg/FFmpeg<https://github.com/FFmpeg/FFmpeg/blob/a79720e10f30e9fd18bd78242ce96dde06461343/libswscale/swscale.c#L739>

void ff_xyz12Torgb48(const SwsInternal *c, uint8_t *dst, int dst_stride,
                     const uint8_t *src, int src_stride, int w, int h)
{
.......... Unaligned read .......................
               x = AV_RL16(src16 + xp + 0);
                y = AV_RL16(src16 + xp + 1);
                z = AV_RL16(src16 + xp + 2);


.......... DRAM Access and multiply  .......................

            // convert from XYZlinear to sRGBlinear
            r = c->xyz2rgb_matrix[0][0] * x +
                c->xyz2rgb_matrix[0][1] * y +
                c->xyz2rgb_matrix[0][2] * z >> 12;
            g = c->xyz2rgb_matrix[1][0] * x +
                c->xyz2rgb_matrix[1][1] * y +
                c->xyz2rgb_matrix[1][2] * z >> 12;
            b = c->xyz2rgb_matrix[2][0] * x +
                c->xyz2rgb_matrix[2][1] * y +
                c->xyz2rgb_matrix[2][2] * z >> 12;

.......... RMW Access .......................
                AV_WL16(dst16 + xp + 0, c->rgbgamma[r] << 4);
                AV_WL16(dst16 + xp + 1, c->rgbgamma[g] << 4);
                AV_WL16(dst16 + xp + 2, c->rgbgamma[b] << 4);


Regards
Chitra