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Apply deemphasis to both channels in the same loop for the simple case

This makes the decoder ~2.5% faster on x86 because the stereo loop
takes the same processing time as one mono loop due to the dependency chain

--- a/celt/celt_decoder.c

+++ b/celt/celt_decoder.c

@@ -177,6 +177,36 @@

 }

 #endif /* CUSTOM_MODES */

+#ifndef CUSTOM_MODES

+/* Special case for stereo with no downsampling and no accumulation. This is

+   quite common and we can make it faster by processing both channels in the

+   same loop, reducing overhead due to the dependency loop in the IIR filter. */

+static void deemphasis_stereo_simple(celt_sig *in[], opus_val16 *pcm, int N, const opus_val16 coef0,

+      celt_sig *mem)

+{

+   celt_sig * OPUS_RESTRICT x0;

+   celt_sig * OPUS_RESTRICT x1;

+   celt_sig m0, m1;

+   int j;

+   x0=in[0];

+   x1=in[1];

+   m0 = mem[0];

+   m1 = mem[1];

+   for (j=0;j<N;j++)

+   {

+      celt_sig tmp0, tmp1;

+      /* Add VERY_SMALL to x[] first to reduce dependency chain. */

+      tmp0 = x0[j] + VERY_SMALL + m0;

+      tmp1 = x1[j] + VERY_SMALL + m1;

+      m0 = MULT16_32_Q15(coef0, tmp0);

+      m1 = MULT16_32_Q15(coef0, tmp1);

+      pcm[2*j  ] = SCALEOUT(SIG2WORD16(tmp0));

+      pcm[2*j+1] = SCALEOUT(SIG2WORD16(tmp1));

+   }

+   mem[0] = m0;

+   mem[1] = m1;

+}

+#endif

 #ifndef RESYNTH

 static

@@ -190,6 +220,14 @@

    opus_val16 coef0;

    VARDECL(celt_sig, scratch);

    SAVE_STACK;

+#ifndef CUSTOM_MODES

+   /* Short version for common case. */

+   if (downsample == 1 && C == 2 && !accum)

+   {

+      deemphasis_stereo_simple(in, pcm, N, coef[0], mem);

+      return;

+   }

+#endif

 #ifndef FIXED_POINT

    (void)accum;

    celt_assert(accum==0);