From 6823fa2de211e57bca9fb73b90ae04b7edc5f484 Mon Sep 17 00:00:00 2001
From: Chris Robinson <chris.kcat@gmail.com>
Date: Sat, 2 May 2020 16:42:24 -0700
Subject: Improve the leftover and realignment mixing loops

Using a mask helps the compiler recognize that the leftover (any remaining non-
multiple-of-4) and realignment loops will only have 3 iterations at most, which
it can unroll or otherwise make more meaningful optimizations for. Previously
it would try to vectorize and partially unroll the loops, which is wasteful
when there would never be enough to vectorize.
---
 alc/mixer/mixer_neon.cpp | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

(limited to 'alc/mixer/mixer_neon.cpp')

diff --git a/alc/mixer/mixer_neon.cpp b/alc/mixer/mixer_neon.cpp
index 6f58a72b..610ea25c 100644
--- a/alc/mixer/mixer_neon.cpp
+++ b/alc/mixer/mixer_neon.cpp
@@ -219,7 +219,7 @@ void Mix_<NEONTag>(const al::span<const float> InSamples, const al::span<FloatBu
 {
     const float delta{(Counter > 0) ? 1.0f / static_cast<float>(Counter) : 0.0f};
     const auto min_len = minz(Counter, InSamples.size());
-    const auto aligned_len = minz((min_len+3) & ~size_t{3}, InSamples.size());
+    const auto aligned_len = minz((min_len+3) & ~size_t{3}, InSamples.size()) - min_len;
     for(FloatBufferLine &output : OutBuffer)
     {
         float *RESTRICT dst{al::assume_aligned<16>(output.data()+OutPos)};
@@ -258,7 +258,7 @@ void Mix_<NEONTag>(const al::span<const float> InSamples, const al::span<FloatBu
                 step_count = vgetq_lane_f32(step_count4, 0);
             }
             /* Mix with applying left over gain steps that aren't aligned multiples of 4. */
-            for(;pos != min_len;++pos)
+            for(uint_fast32_t leftover{min_len&3};leftover;++pos,--leftover)
             {
                 dst[pos] += InSamples[pos] * (gain + step*step_count);
                 step_count += 1.0f;
@@ -269,7 +269,7 @@ void Mix_<NEONTag>(const al::span<const float> InSamples, const al::span<FloatBu
                 gain += step*step_count;
 
             /* Mix until pos is aligned with 4 or the mix is done. */
-            for(;pos != aligned_len;++pos)
+            for(uint_fast32_t leftover{aligned_len&3};leftover;++pos,--leftover)
                 dst[pos] += InSamples[pos] * gain;
         }
         *CurrentGains = gain;
@@ -289,7 +289,7 @@ void Mix_<NEONTag>(const al::span<const float> InSamples, const al::span<FloatBu
                 pos += 4;
             } while(--todo);
         }
-        for(;pos != InSamples.size();++pos)
+        for(uint_fast32_t leftover{InSamples.size()&3};leftover;++pos,--leftover)
             dst[pos] += InSamples[pos] * gain;
     }
 }
-- 
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