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Make CELT FFT twiddle complex type aligned

This makes kiss_twiddle_cpx 4-byte aligned (instead of 2-byte) for
fixed-point builds. Tested with an armv6j+nofp development board, CELT
encoding becomes 1.4x as fast, and decoding over 2x.

Performance gain is mostly attributed to the proper alignment of the
static const array mdct_twiddles960.

Co-authored-by: David Gao <davidgao@google.com>
Signed-off-by: Felicia Lim <flim@google.com>

--- a/celt/kiss_fft.h

+++ b/celt/kiss_fft.h

@@ -52,6 +52,10 @@

 #  define kiss_fft_scalar opus_int32

 #  define kiss_twiddle_scalar opus_int16

+/* Some 32-bit CPUs would load/store a kiss_twiddle_cpx with a single memory

+ * access, and could benefit from additional alignment.

+ */

+#  define KISS_TWIDDLE_CPX_ALIGNMENT (sizeof(opus_int32))

 #else

 # ifndef kiss_fft_scalar

@@ -62,6 +66,12 @@

 # endif

 #endif

+#if defined(__GNUC__) && defined(KISS_TWIDDLE_CPX_ALIGNMENT)

+#define KISS_TWIDDLE_CPX_ALIGNED __attribute__((aligned(KISS_TWIDDLE_CPX_ALIGNMENT)))

+#else

+#define KISS_TWIDDLE_CPX_ALIGNED

+#endif

+

 typedef struct {

     kiss_fft_scalar r;

     kiss_fft_scalar i;

@@ -70,7 +80,7 @@

 typedef struct {

    kiss_twiddle_scalar r;

    kiss_twiddle_scalar i;

-}kiss_twiddle_cpx;

+} KISS_TWIDDLE_CPX_ALIGNED kiss_twiddle_cpx;

 #define MAXFACTORS 8

 /* e.g. an fft of length 128 has 4 factors