[opus] [PATCH] Make CELT FFT twiddle complex type aligned

[Zheng Lv]

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 at google.com>
---
 celt/kiss_fft.h | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/celt/kiss_fft.h b/celt/kiss_fft.h
index bffa2bfa..267f72f9 100644
--- a/celt/kiss_fft.h
+++ b/celt/kiss_fft.h
@@ -49,31 +49,41 @@ extern "C" {
 #ifdef FIXED_POINT
 #include "arch.h"
 
 #  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
 /*  default is float */
 #   define kiss_fft_scalar float
 #   define kiss_twiddle_scalar float
 #   define KF_SUFFIX _celt_single
 # 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;
 }kiss_fft_cpx;
 
 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
  as far as kissfft is concerned
  4*4*4*2
  */
-- 
2.37.2.789.g6183377224-goog