avcodec/dsd: Hoist branch out of loop

This is possible by not converting from LSBF to MSBF; instead
add LSBF LUTs.
This approach necessitates reversing the initial values.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/dsd.c

@@ -68,7 +68,8 @@ static const double htaps[HTAPS] = {
      3.423230509967409e-07,  1.244182214744588e-07,  3.130441005359396e-08
 };
 
-static double ctables[CTABLES][256];
+static double ctables_lsbf[CTABLES][256];
+static double ctables_msbf[CTABLES][256];
 
 static av_cold void dsd_ctables_tableinit(void)
 {
@@ -81,8 +82,10 @@ static av_cold void dsd_ctables_tableinit(void)
             for (t = 0; t < CTABLES; ++t)
                 acc[t] += sign * htaps[t * 8 + m];
         }
-        for (t = 0; t < CTABLES; ++t)
-            ctables[CTABLES - 1 - t][e] = acc[t];
+        for (t = 0; t < CTABLES; ++t) {
+            ctables_msbf[CTABLES - 1 - t][e] = acc[t];
+            ctables_lsbf[CTABLES - 1 - t][ff_reverse[e]] = acc[t];
+        }
     }
 }
 
@@ -100,13 +103,14 @@ void ff_dsd2pcm_translate(DSDContext* s, size_t samples, int lsbf,
     unsigned pos, i;
     uint8_t* p;
     double sum;
+    const double (*const ctables)[256] = lsbf ? ctables_lsbf : ctables_msbf;
 
     pos = s->pos;
 
     memcpy(buf, s->buf, sizeof(buf));
 
     while (samples-- > 0) {
-        buf[pos] = lsbf ? ff_reverse[*src] : *src;
+        buf[pos] = *src;
         src += src_stride;
 
         p = buf + ((pos - CTABLES) & FIFOMASK);

libavcodec/dsddec.c

@@ -56,7 +56,8 @@ static av_cold int decode_init(AVCodecContext *avctx)
     if (!s)
         return AVERROR(ENOMEM);
 
-    silence = DSD_SILENCE;
+    silence = avctx->codec_id == AV_CODEC_ID_DSD_LSBF_PLANAR ||
+              avctx->codec_id == AV_CODEC_ID_DSD_LSBF ? DSD_SILENCE_REVERSED : DSD_SILENCE;
     for (i = 0; i < avctx->ch_layout.nb_channels; i++) {
         s[i].pos = 0;
         memset(s[i].buf, silence, sizeof(s[i].buf));