[FFmpeg-devel] [PATCH 1/2] libavcodec: changed mathematical functions in aacpsy.c

[Bojan Zivkovic]

This patch changes existing mathematical functions with faster
ones. Speeds up encoding more than 10%. Tested on x86 and
MIPS platforms.

Signed-off-by: Bojan Zivkovic <bojan at mips.com>
---
 libavcodec/aacpsy.c |   29 ++++++++++++++++++-----------
 1 files changed, 18 insertions(+), 11 deletions(-)

diff --git a/libavcodec/aacpsy.c b/libavcodec/aacpsy.c
index d77b3de..cd70247 100644
--- a/libavcodec/aacpsy.c
+++ b/libavcodec/aacpsy.c
@@ -78,6 +78,9 @@
 
 #define PSY_3GPP_AH_THR_LONG    0.5f
 #define PSY_3GPP_AH_THR_SHORT   0.63f
+#define PSY_LN_10               log(10)
+#define PSY_3GPP_REDUCTION      0.1732868f    /* 0.25f * ln(0.5f) */
+#define PSY_3GPP_LN_2           log(2.0f)
 
 enum {
     PSY_3GPP_AH_NONE,
@@ -330,12 +333,12 @@ static av_cold int psy_3gpp_init(FFPsyContext *ctx) {
         for (g = 0; g < ctx->num_bands[j] - 1; g++) {
             AacPsyCoeffs *coeff = &coeffs[g];
             float bark_width = coeffs[g+1].barks - coeffs->barks;
-            coeff->spread_low[0] = pow(10.0, -bark_width * PSY_3GPP_THR_SPREAD_LOW);
-            coeff->spread_hi [0] = pow(10.0, -bark_width * PSY_3GPP_THR_SPREAD_HI);
-            coeff->spread_low[1] = pow(10.0, -bark_width * en_spread_low);
-            coeff->spread_hi [1] = pow(10.0, -bark_width * en_spread_hi);
+            coeff->spread_low[0] = exp(-bark_width * PSY_3GPP_THR_SPREAD_HI * PSY_LN_10);
+            coeff->spread_hi [0] = exp(-bark_width * PSY_3GPP_THR_SPREAD_HI * PSY_LN_10);
+            coeff->spread_low[1] = exp(-bark_width * en_spread_low * PSY_LN_10);
+            coeff->spread_hi [1] = exp(-bark_width * en_spread_hi  * PSY_LN_10);
             pe_min = bark_pe * bark_width;
-            minsnr = exp2(pe_min / band_sizes[g]) - 1.5f;
+            minsnr = expf(PSY_3GPP_LN_2 * pe_min / band_sizes[g]) - 1.5f;
             coeff->min_snr = av_clipf(1.0f / minsnr, PSY_SNR_25DB, PSY_SNR_1DB);
         }
         start = 0;
@@ -529,8 +532,8 @@ static float calc_reduction_3gpp(float a, float desired_pe, float pe,
     if(active_lines == 0.0)
         return 0;
 
-    thr_avg   = exp2f((a - pe) / (4.0f * active_lines));
-    reduction = exp2f((a - desired_pe) / (4.0f * active_lines)) - thr_avg;
+    thr_avg   = exp(PSY_3GPP_REDUCTION * (a - pe) / active_lines);
+    reduction = exp(PSY_3GPP_REDUCTION * (a - desired_pe) / active_lines) - thr_avg;
 
     return FFMAX(reduction, 0.0f);
 }
@@ -541,8 +544,10 @@ static float calc_reduced_thr_3gpp(AacPsyBand *band, float min_snr,
     float thr = band->thr;
 
     if (band->energy > thr) {
-        thr = powf(thr, 0.25f) + reduction;
-        thr = powf(thr, 4.0f);
+        thr = sqrtf(thr);
+        thr = sqrtf(thr) + reduction;
+        thr *= thr;
+        thr *= thr;
 
         /* This deviates from the 3GPP spec to match the reference encoder.
          * It performs min(thr_reduced, max(thr, energy/min_snr)) only for bands
@@ -582,13 +587,15 @@ static void psy_3gpp_analyze_channel(FFPsyContext *ctx, int channel,
             AacPsyBand *band = &pch->band[w+g];
 
             float form_factor = 0.0f;
+            float Temp;
             band->energy = 0.0f;
             for (i = 0; i < band_sizes[g]; i++) {
                 band->energy += coefs[start+i] * coefs[start+i];
                 form_factor  += sqrtf(fabs(coefs[start+i]));
             }
+            Temp = sqrtf((float)band_sizes[g] / band->energy);
             band->thr      = band->energy * 0.001258925f;
-            band->nz_lines = band->energy>0 ? form_factor / powf(band->energy / band_sizes[g], 0.25f) : 0;
+            band->nz_lines = form_factor * sqrtf(Temp);
 
             start += band_sizes[g];
         }
@@ -708,7 +715,7 @@ static void psy_3gpp_analyze_channel(FFPsyContext *ctx, int channel,
                         float delta_sfb_pe = band->norm_fac * norm_fac * delta_pe;
                         float thr = band->thr;
 
-                        thr *= exp2f(delta_sfb_pe / band->active_lines);
+                        thr *= exp(PSY_3GPP_LN_2 * delta_sfb_pe / band->active_lines);
                         if (thr > coeffs[g].min_snr * band->energy && band->avoid_holes == PSY_3GPP_AH_INACTIVE)
                             thr = FFMAX(band->thr, coeffs[g].min_snr * band->energy);
                         band->thr = thr;
-- 
1.7.3.4