aarch64/opusdsp: implement NEON accelerated postfilter and deemphasis

153372 UNITS in postfilter_c,   65536 runs,      0 skips
73164 UNITS in postfilter_neon,   65536 runs,      0 skips -> 2.1x speedup

80591 UNITS in deemphasis_c,  131072 runs,      0 skips
43969 UNITS in deemphasis_neon,  131072 runs,      0 skips -> 1.83x speedup

Total decoder speedup: ~15% on a Raspberry Pi 3 (from 28.1x to 33.5x realtime)

Deemphasis SIMD based on the following unrolling:
const float c1 = CELT_EMPH_COEFF, c2 = c1*c1, c3 = c2*c1, c4 = c3*c1;
float state = coeff;

for (int i = 0; i < len; i += 4) {
    y[0] = x[0] + c1*state;
    y[1] = x[1] + c2*state + c1*x[0];
    y[2] = x[2] + c3*state + c1*x[1] + c2*x[0];
    y[3] = x[3] + c4*state + c1*x[2] + c2*x[1] + c3*x[0];

    state = y[3];
    y += 4;
    x += 4;
}

Unlike the x86 version, duplication is used instead of pslldq so
the structure and tables are different.

1 files changed, 3 insertions, 0 deletions

diff --git a/libavcodec/opusdsp.c b/libavcodec/opusdsp.c
index 17e819f977..0e179c98c9 100644
--- a/libavcodec/opusdsp.c
+++ b/libavcodec/opusdsp.c
@@ -61,4 +61,7 @@ av_cold void ff_opus_dsp_init(OpusDSP *ctx)
 
     if (ARCH_X86)
         ff_opus_dsp_init_x86(ctx);
+
+    if (ARCH_AARCH64)
+        ff_opus_dsp_init_aarch64(ctx);
 }