libsyn123: add SYN123_NO_CASES switch for mono and stereo

Using scripts/syn123-channelscaling.sh, I see about 15% benefit
for mono/stereo operation when treating it as special cases in
the resampler. Fun fact: Even the higher channel counts profit
a tiny bit (1% to 2%, not sure how significant) from the
separation.

Giving the compiler information about what can be likely values
for those inner loop rounds still makes sense.



git-svn-id: svn://scm.orgis.org/mpg123/trunk@4494 35dc7657-300d-0410-a2e5-dc2837fedb53

scripts/syn123-channelscaling.sh

@@ -0,0 +1,32 @@
+#!/bin/sh
+
+# This gives a view at the scaling of resampling runtime with channel count.
+# The tested resampling mode involves 2X upsampling, lowpass, and interpolation.
+
+set -e
+
+export LANG=C
+export LC_NUMERIC=C
+chan_min=1
+chan_max=10
+chan_fit=3
+
+out123=src/out123
+generate="--wave-freq 300 --inputrate 44100 --timelimit 4410000 -q"
+
+wd=$(mktemp -d channelscaling.XXXX)
+echo "workdir: $wd"
+for n in $(seq $chan_min $chan_max)
+do
+  printf "generate with %d channels\n" "$n" >&2
+  /usr/bin/time -f "$n\t%e" $out123 $generate -c $n --rate 44100 -t 2>&1
+done > $wd/generate.txd 
+for n in $(seq $chan_min $chan_max)
+do
+  printf "resample with %d channels\n" "$n" >&2
+  /usr/bin/time -f "$n\t%e" $out123 $generate -c $n --rate 44101 -t 2>&1
+done > $wd/resample.txd 
+txdcalc '[3]=[2]-[1,2]' $wd/generate.txd  < $wd/resample.txd > $wd/resampling-overhead.txd
+gpfit --plot -g=1 -r='[3:]' $wd/resampling-overhead.txd 
+
+echo "Check results in $wd/."