Use of constant shift in SBC quantization code to make it faster

The result of 32x32->64 unsigned long multiplication is returned
in two registers (high and low 32-bit parts) for many 32-bit
architectures. For these architectures constant right shift by
32 bits is optimized out by the compiler to just taking the high
32-bit part. Also some data needed at the quantization stage is
precalculated beforehand to improve performance.

diff --git a/sbc/sbc.c b/sbc/sbc.c
index b349090..650bc2f 100644 (file)
--- a/sbc/sbc.c
+++ b/sbc/sbc.c
@@ -880,11 +880,12 @@ static int sbc_pack_frame(uint8_t *data, struct sbc_frame *frame, size_t len)
        uint8_t crc_header[11] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
        int crc_pos = 0;
 
-       uint16_t audio_sample;
+       uint32_t audio_sample;
 
        int ch, sb, blk;        /* channel, subband, block and bit counters */
        int bits[2][8];         /* bits distribution */
-       int levels[2][8];       /* levels are derived from that */
+       uint32_t levels[2][8];  /* levels are derived from that */
+       uint32_t sb_sample_delta[2][8];
 
        u_int32_t scalefactor[2][8];    /* derived from frame->scale_factor */
 
@@ -1017,8 +1018,14 @@ static int sbc_pack_frame(uint8_t *data, struct sbc_frame *frame, size_t len)
        sbc_calculate_bits(frame, bits);
 
        for (ch = 0; ch < frame->channels; ch++) {
-               for (sb = 0; sb < frame->subbands; sb++)
-                       levels[ch][sb] = (1 << bits[ch][sb]) - 1;
+               for (sb = 0; sb < frame->subbands; sb++) {
+                       levels[ch][sb] = ((1 << bits[ch][sb]) - 1) <<
+                               (32 - (frame->scale_factor[ch][sb] +
+                                       SCALE_OUT_BITS + 2));
+                       sb_sample_delta[ch][sb] = (uint32_t) 1 <<
+                               (frame->scale_factor[ch][sb] +
+                                       SCALE_OUT_BITS + 1);
+               }
        }
 
        for (blk = 0; blk < frame->blocks; blk++) {
@@ -1029,12 +1036,8 @@ static int sbc_pack_frame(uint8_t *data, struct sbc_frame *frame, size_t len)
                                        continue;
 
                                audio_sample = ((uint64_t) levels[ch][sb] *
-                                       (((uint32_t) 1 <<
-                                       (frame->scale_factor[ch][sb] +
-                                       SCALE_OUT_BITS + 1)) +
-                                       frame->sb_sample_f[blk][ch][sb])) >>
-                                               (frame->scale_factor[ch][sb] +
-                                               SCALE_OUT_BITS + 2);
+                                       (sb_sample_delta[ch][sb] +
+                                       frame->sb_sample_f[blk][ch][sb])) >> 32;
 
                                PUT_BITS(audio_sample, bits[ch][sb]);
                        }