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Speeding up PVQ using unlikely() and moving first position out of the loop

Speeds up encoding by another ~1-2%

--- a/celt/arch.h

+++ b/celt/arch.h

@@ -46,6 +46,14 @@

 #  endif

 # endif

+#if OPUS_GNUC_PREREQ(3, 0)

+#define opus_likely(x)       (__builtin_expect(!!(x), 1))

+#define opus_unlikely(x)     (__builtin_expect(!!(x), 0))

+#else

+#define opus_likely(x)       (!!(x))

+#define opus_unlikely(x)     (!!(x))

+#endif

+

 #define CELT_SIG_SCALE 32768.f

 #define celt_fatal(str) _celt_fatal(str, __FILE__, __LINE__);

--- a/celt/vq.c

+++ b/celt/vq.c

@@ -165,7 +165,6 @@

    VARDECL(int, iy);

    VARDECL(opus_val16, signx);

    int i, j;

-   opus_val16 s;

    int pulsesLeft;

    opus_val32 sum;

    opus_val32 xy;

@@ -251,12 +250,12 @@

       pulsesLeft=0;

    }

-   s = 1;

    for (i=0;i<pulsesLeft;i++)

    {

+      opus_val16 Rxy, Ryy;

       int best_id;

-      opus_val32 best_num = -VERY_LARGE16;

-      opus_val16 best_den = 0;

+      opus_val32 best_num;

+      opus_val16 best_den;

 #ifdef FIXED_POINT

       int rshift;

 #endif

@@ -267,9 +266,22 @@

       /* The squared magnitude term gets added anyway, so we might as well

          add it outside the loop */

       yy = ADD16(yy, 1);

-      j=0;

+

+      /* Calculations for position 0 are out of the loop, in part to reduce

+         mispredicted branches (since the if condition is usually false)

+         in the loop. */

+      /* Temporary sums of the new pulse(s) */

+      Rxy = EXTRACT16(SHR32(ADD32(xy, EXTEND32(X[0])),rshift));

+      /* We're multiplying y[j] by two so we don't have to do it here */

+      Ryy = ADD16(yy, y[0]);

+

+      /* Approximate score: we maximise Rxy/sqrt(Ryy) (we're guaranteed that

+         Rxy is positive because the sign is pre-computed) */

+      Rxy = MULT16_16_Q15(Rxy,Rxy);

+      best_den = Ryy;

+      best_num = Rxy;

+      j=1;

       do {

-         opus_val16 Rxy, Ryy;

          /* Temporary sums of the new pulse(s) */

          Rxy = EXTRACT16(SHR32(ADD32(xy, EXTEND32(X[j])),rshift));

          /* We're multiplying y[j] by two so we don't have to do it here */

@@ -280,8 +292,11 @@

          Rxy = MULT16_16_Q15(Rxy,Rxy);

          /* The idea is to check for num/den >= best_num/best_den, but that way

             we can do it without any division */

-         /* OPT: Make sure to use conditional moves here */

-         if (MULT16_16(best_den, Rxy) > MULT16_16(Ryy, best_num))

+         /* OPT: It's not clear whether a cmov is faster than a branch here

+            since the condition is more often false than true and using

+            a cmov introduces data dependencies across iterations. The optimal

+            choice may be architecture-dependent. */

+         if (opus_unlikely(MULT16_16(best_den, Rxy) > MULT16_16(Ryy, best_num)))

          {

             best_den = Ryy;

             best_num = Rxy;

@@ -296,7 +311,7 @@

       /* Only now that we've made the final choice, update y/iy */

       /* Multiplying y[j] by 2 so we don't have to do it everywhere else */

-      y[best_id] += 2*s;

+      y[best_id] += 2;

       iy[best_id]++;

    }

--- a/silk/macros.h

+++ b/silk/macros.h

@@ -36,14 +36,6 @@

 #include "opus_defines.h"

 #include "arch.h"

-#if OPUS_GNUC_PREREQ(3, 0)

-#define opus_likely(x)       (__builtin_expect(!!(x), 1))

-#define opus_unlikely(x)     (__builtin_expect(!!(x), 0))

-#else

-#define opus_likely(x)       (!!(x))

-#define opus_unlikely(x)     (!!(x))

-#endif

-

 /* This is an OPUS_INLINE header file for general platform. */

 /* (a32 * (opus_int32)((opus_int16)(b32))) >> 16 output have to be 32bit int */