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ref: ab9082c00bf02763d47bab09698b37e4ef32d12f
parent: ea8f175fa4fda3ae8f6e1c0d07d4a831b288d16b
author: Siarhei Volkau <lis8215@gmail.com>
date: Sat Aug 23 13:26:29 EDT 2025 

MIPS: unroll fft_downshift loops for performance

Simple loop with shift by 1 unrolled 2 times.
More complex loop unrolled 4 times.

Signed-off-by: Siarhei Volkau <lis8215@gmail.com>
Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>


--- a/celt/kiss_fft.c
+++ b/celt/kiss_fft.c
@@ -535,6 +535,7 @@
 #endif /* CUSTOM_MODES */
 
 #ifdef FIXED_POINT
+#ifndef OVERRIDE_fft_downshift
 static void fft_downshift(kiss_fft_cpx *x, int N, int *total, int step) {
    int shift;
    shift = IMIN(step, *total);
@@ -553,6 +554,7 @@
       }
    }
 }
+#endif /* OVERRIDE_fft_downshift */
 #else
 #define fft_downshift(x, N, total, step)
 #endif
--- a/celt/mips/kiss_fft_mipsr1.h
+++ b/celt/mips/kiss_fft_mipsr1.h
@@ -213,4 +213,59 @@
 
 #endif /* defined(OVERRIDE_kf_bfly5) */
 
+#define OVERRIDE_fft_downshift
+/* Just unroll tight loop, should be ok for any mips */
+static void fft_downshift(kiss_fft_cpx *x, int N, int *total, int step) {
+    int shift;
+    shift = IMIN(step, *total);
+    *total -= shift;
+    if (shift == 1) {
+        int i;
+        for (i = 0; i < N - 1; i += 2) {
+            x[i].r   = SHR32(x[i].r,   1);
+            x[i].i   = SHR32(x[i].i,   1);
+            x[i+1].r = SHR32(x[i+1].r, 1);
+            x[i+1].i = SHR32(x[i+1].i, 1);
+        }
+        if (N & 1) {
+            x[i].r = SHR32(x[i].r, 1);
+            x[i].i = SHR32(x[i].i, 1);
+        }
+    } else if (shift > 0) {
+        int i;
+        for (i = 0; i < N - 3; i += 4) {
+            x[i].r   = PSHR32(x[i].r,   shift);
+            x[i].i   = PSHR32(x[i].i,   shift);
+            x[i+1].r = PSHR32(x[i+1].r, shift);
+            x[i+1].i = PSHR32(x[i+1].i, shift);
+            x[i+2].r = PSHR32(x[i+2].r, shift);
+            x[i+2].i = PSHR32(x[i+2].i, shift);
+            x[i+3].r = PSHR32(x[i+3].r, shift);
+            x[i+3].i = PSHR32(x[i+3].i, shift);
+        }
+        switch (N & 3) {
+        case 3:
+            x[i].r   = PSHR32(x[i].r,   shift);
+            x[i].i   = PSHR32(x[i].i,   shift);
+            x[i+1].r = PSHR32(x[i+1].r, shift);
+            x[i+1].i = PSHR32(x[i+1].i, shift);
+            x[i+2].r = PSHR32(x[i+2].r, shift);
+            x[i+2].i = PSHR32(x[i+2].i, shift);
+            break;
+        case 2:
+            x[i].r   = PSHR32(x[i].r,   shift);
+            x[i].i   = PSHR32(x[i].i,   shift);
+            x[i+1].r = PSHR32(x[i+1].r, shift);
+            x[i+1].i = PSHR32(x[i+1].i, shift);
+            break;
+        case 1:
+            x[i].r   = PSHR32(x[i].r,   shift);
+            x[i].i   = PSHR32(x[i].i,   shift);
+            break;
+        case 0:
+            break;
+        }
+    }
+}
+
 #endif /* KISS_FFT_MIPSR1_H */
-- 
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