From e2b328d6c36b12ce6b11e7d3c416cb9314ba809a Mon Sep 17 00:00:00 2001
From: Clement Courbet <courbet@google.com>
Date: Tue, 16 Jun 2020 08:34:21 +0200
Subject: [PATCH] Vectorize audio downsampling.
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Benchmarks:

```
name                                             old time/op             new time/op             delta
BM_Resample<false, 300, 22050, 8000>             36.9ms ± 1%             30.4ms ± 1%  -17.82%         (p=0.000 n=10+9)
BM_Resample<false, 300, 15000, 8000>             24.7ms ± 0%             23.4ms ± 1%   -5.27%          (p=0.000 n=9+9)
BM_Resample<false, 100, 4000, 400>               6.27ms ± 0%             3.88ms ± 1%  -38.10%         (p=0.000 n=9+10)
BM_Resample<true, 300, 22050, 8000>               124ms ± 1%               95ms ± 1%  -22.90%        (p=0.000 n=10+10)
BM_Resample<true, 300, 15000, 8000>              84.8ms ± 1%             75.2ms ± 1%  -11.26%         (p=0.000 n=9+10)
BM_Resample<true, 100, 4000, 400>                22.1ms ± 1%             19.2ms ± 2%  -13.31%         (p=0.000 n=9+10)
```
---
 src/filterkit.c | 43 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 41 insertions(+), 2 deletions(-)

diff --git a/src/filterkit.c b/src/filterkit.c
index d4360a4..953134b 100644
--- a/src/filterkit.c
+++ b/src/filterkit.c
@@ -20,6 +20,10 @@
 
 #include "filterkit.h"
 
+#ifdef __SSE4_1__
+#include <xmmintrin.h>
+#endif
+
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
@@ -202,7 +206,42 @@ float lrsFilterUD(float Imp[],  /* impulse response */
          Ho += dhb;		/* IR step */
          Xp += Inc;		/* Input signal step. NO CHECK ON BOUNDS */
       }
-   else 
+   else {
+#ifdef __SSE4_1__
+      __m128i vv = _mm_set1_ps(0.0f);
+      if (Inc == 1) {
+         while ((Hp = &Imp[(int)(Ho + 4*dhb)]) < End) {
+            /* Get IR sample */
+            __m128i vt = _mm_set_ps(Imp[(int)(Ho + 3 * dhb)],
+                                    Imp[(int)(Ho + 2*dhb)],
+                                    Imp[(int)(Ho + dhb)],
+                                    Imp[(int)(Ho)]);
+            vt = _mm_mul_ps(vt, _mm_loadu_ps(Xp));/* Mult coeff by input
+                                                     sample */
+            vv = _mm_add_ps(vv, vt);			/* The filter output */
+            Ho += 4*dhb;		/* IR step */
+            Xp += 4;		/* Input signal step. NO CHECK ON BOUNDS */
+         }
+      } else {
+         while ((Hp = &Imp[(int)(Ho + 4*dhb)]) < End) {
+            /* Get IR sample */
+            __m128i vt = _mm_set_ps(Imp[(int)(Ho)],
+                                    Imp[(int)(Ho + dhb)],
+                                    Imp[(int)(Ho + 2*dhb)],
+                                    Imp[(int)(Ho + 3*dhb)]);
+            vt = _mm_mul_ps(vt, _mm_loadu_ps(Xp - 3)); /* Mult coeff by input
+                                                          sample */
+            vv = _mm_add_ps(vv, vt);			/* The filter output */
+            Ho += 4*dhb;		/* IR step */
+            Xp -= 4;		/* Input signal step. NO CHECK ON BOUNDS */
+         }
+      }
+      float elems[4];
+      memcpy(elems, &vv, sizeof(vv));
+      v = elems[0] + elems[1] + elems[2] + elems[3];
+      // Fallthrough intended: the rest of the loop is done in a non-vectorized
+      // way.
+#endif
       while ((Hp = &Imp[(int)Ho]) < End) {
          t = *Hp;		/* Get IR sample */
          t *= *Xp;		/* Mult coeff by input sample */
@@ -210,6 +249,6 @@ float lrsFilterUD(float Imp[],  /* impulse response */
          Ho += dhb;		/* IR step */
          Xp += Inc;		/* Input signal step. NO CHECK ON BOUNDS */
       }
-
+   }
    return v;
 }