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Count leading zeros operation is often implemented using a special
instruction for it on various architectures (at least this is true
for ARM and x86). Using __builtin_clz gcc intrinsic allows to
eliminate innermost loop in scale factors calculation and improve
performance. Also scale factors calculation can be optimized even
more using SIMD instructions.
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Channels deinterleaving, endian conversion and samples reordering
is done in one pass, avoiding the use of intermediate buffer. Also
this code is implemented as a new "performance primitive", which
allows further platform specific optimizations (ARMv6 and ARM NEON
should gain quite a lot from assembly optimizations here).
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Multiplying the first part of the analysis filter constant tables
by some coefficients and dividing the second part by the same
coefficients is a transformation which should produce the same
results if rounding errors are not taken into account. These
additional C0/C1/... coefficients can be varied in a certain
range (the requirement is that we still do not get overflows).
The 'magic' values for these coefficients are selected in such
a way that the rounding errors are minimized (rounding errors
are unavoidable when putting all the floating constants into
16-bit tables and losing some of the fractional part).
Also non-SIMD variant of the analysis filter is dropped because
keeping it would require applying a similar change to its tables,
which is a bit tricky and just increases maintenance overhead.
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Added SIMD-friendly C implementation of SBC analysis filter (the
structure of code had to be changed a bit and constants in the
tables reordered). This code can be used as a reference for
developing platform specific SIMD optimizations. These functions
are put into a new file 'sbc_primitives.c', which is going to
contain all the basic stuff for SBC codec.
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