libavutil: add saturating addition functions

Fixed-point audio codecs often use saturating arithmetic, and
special instructions for these operations are common.

Signed-off-by: Mans Rullgard <mans@mansr.com>
celp: optimise ff_celp_lp_synthesis_filter()

Adding instead of subtracting the products in the loop allows the
compiler to generate more efficient multiply-accumulate instructions
when 16-bit multiply-subtract is not available. ARM has only
multiply-accumulate for 16-bit operands.  In general, if only one
variant exists, it is usually accumulate rather than subtract.

In the same spirit, using the dedicated saturation function enables
use of any special optimised versions of this.

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: deobfuscate "(x << 4) - x" to "15 * x"

The compiler performs this optimisation.

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: remove unnecessary argument 'shift' from dot_product()

The 'shift' argument is always 1 so there is no need to pass it
explicitly in every call.

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: remove useless uses of MUL64()

The operands in both cases are 16-bit so cannot overflow a 32-bit
destination.  In gain_scale() the inputs are reduced to 14-bit,
so even the shift cannot overflow.

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: optimise scale_vector()

Firstly, nothing in this function can overflow 32 bits so the use
of a 64-bit type is completely unnecessary.  Secondly, the scale
is either a power of two or 0x7fff.  Doing separate loops for these
cases avoids using multiplications.  Finally, since only the number
of bits, not the actual value, of the maximum value is needed, the
bitwise or of all the values serves the purpose while being faster.

It is worth noting that even if overflow could happen, it was not
handled correctly anyway.

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: use saturating addition functions

Use saturating addition functions instead of 64-bit intermediates
and separate clipping.  This is much faster when dedicated
instructions are available.

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: do not needlessly use int64_t

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: make autocorr_max() work on an arbitrary buffer

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: make scale_vector() output to a separate buffer

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: drop unnecessary variable buf_ptr in formant_postfilter()

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: make postfilter write directly to output buffer

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: avoid unnecessary memcpy() in residual_interp()

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: avoid saving/restoring excitation

Writing the scaled excitation to a scratch buffer (borrowing the
'audio' array) instead of modifying it in place avoids the need
to save and restore the unscaled values.

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: declare a variable in the block it is used

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: do not bounce intermediate values via memory

Although a reasonable compiler will probably optimise out the
actual store and load, this operation still implies a truncation
to 16 bits which the compiler will probably not realise is not
necessary here.

Signed-off-by: Mans Rullgard <mans@mansr.com>
g723.1: do not pass large structs by value

Signed-off-by: Mans Rullgard <mans@mansr.com>