armv6: Accelerate ff_fft_calc for general case (nbits != 4)

The previous implementation targeted DTS Coherent Acoustics, which only
requires nbits == 4 (fft16()). This case was (and still is) linked directly
rather than being indirected through ff_fft_calc_vfp(), but now the full
range from radix-4 up to radix-65536 is available. This benefits other codecs
such as AAC and AC3.

The implementaion is based upon the C version, with each routine larger than
radix-16 calling a hierarchy of smaller FFT functions, then performing a
post-processing pass. This pass benefits a lot from loop unrolling to
counter the long pipelines in the VFP. A relaxed calling standard also
reduces the overhead of the call hierarchy, and avoiding the excessive
inlining performed by GCC probably helps with I-cache utilisation too.

I benchmarked the result by measuring the number of gperftools samples that
hit anywhere in the AAC decoder (starting from aac_decode_frame()) or
specifically in the FFT routines (fft4() to fft512() and pass()) for the
same sample AAC stream:

              Before          After
              Mean   StdDev   Mean   StdDev  Confidence  Change
Audio decode  2245.5 53.1     1599.6 43.8    100.0%      +40.4%
FFT routines  940.6  22.0     348.1  20.8    100.0%      +170.2%
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/arm/fft_init_arm.c

@@ -23,6 +23,8 @@
 #include "libavcodec/rdft.h"
 #include "libavcodec/synth_filter.h"
 
+void ff_fft_calc_vfp(FFTContext *s, FFTComplex *z);
+
 void ff_fft_permute_neon(FFTContext *s, FFTComplex *z);
 void ff_fft_calc_neon(FFTContext *s, FFTComplex *z);
 
@@ -38,10 +40,10 @@ av_cold void ff_fft_init_arm(FFTContext *s)
 {
     int cpu_flags = av_get_cpu_flags();
 
-    if (have_vfp(cpu_flags)) {
+    if (have_vfp(cpu_flags) && !have_vfpv3(cpu_flags)) {
+        s->fft_calc     = ff_fft_calc_vfp;
 #if CONFIG_MDCT
-        if (!have_vfpv3(cpu_flags))
-            s->imdct_half   = ff_imdct_half_vfp;
+        s->imdct_half   = ff_imdct_half_vfp;
 #endif
     }