A few small performance tweaks

Turns out the active voice calculation was kidna expensive
as were VUs on busses we weren't using, so do a couple of
little optimizations for each of those.

The bug ones (don't evaluate busses with no activity at all)
still isn't done.

Addresses #1068

src/engine/bus.cpp

@@ -289,7 +289,15 @@ void Bus::process()
     for (int c = 0; c < 2; ++c)
     {
         vuLevel[c] = std::min(2.f, a * vuLevel[c]);
-        vuLevel[c] = std::max((float)vuLevel[c], mech::blockAbsMax<BLOCK_SIZE>(output[c]));
+        /* we use this as a proxy for a silent block without having to scan the entire
+         * block. If the block isn't silent, we will get the next vlock if we just get
+         * unlucky with sample 0 except in a super pathological case of an exactly
+         * 16 sample sin wave or something.
+         */
+        if (std::fabs(output[c][0]) > 1e-15)
+        {
+            vuLevel[c] = std::max((float)vuLevel[c], mech::blockAbsMax<BLOCK_SIZE>(output[c]));
+        }
     }
 }
 

src/engine/engine.cpp

@@ -179,6 +179,7 @@ voice::Voice *Engine::initiateVoice(const pathToZone_t &path)
             voices[idx]->noteId = path.noteid;
             voices[idx]->setSampleRate(sampleRate, sampleRateInv);
             voices[idx]->endpoints = std::move(mp);
+            activeVoices++;
             return voices[idx];
         }
     }
@@ -248,7 +249,7 @@ bool Engine::processAudio()
 #endif
     messageController->engineProcessRuns++;
     messageController->isAudioRunning = true;
-    auto av = activeVoiceCount();
+    auto av = (uint32_t)activeVoices;
 
     bool tryToDrain{true};
     while (tryToDrain && !messageController->serializationToAudioQueue.empty())
@@ -316,7 +317,11 @@ bool Engine::processAudio()
             bl[idx++][c] = a.vuLevel[c];
     }
 
-    auto pav = activeVoiceCount();
+    auto pav = (uint32_t)activeVoices;
+#if BUILD_IS_DEBUG
+    if (pav != av)
+        assertActiveVoiceCount();
+#endif
 
     bool doUpdate = messageController->isClientConnected &&
                     (
@@ -372,15 +377,15 @@ bool Engine::processAudio()
     return true;
 }
 
-uint32_t Engine::activeVoiceCount()
+void Engine::assertActiveVoiceCount()
 {
     uint32_t res{0};
     for (const auto v : voices)
     {
         if (v)
             res += (v->isVoiceAssigned && v->isVoicePlaying);
     }
-    return res;
+    assert(res == activeVoices);
 }
 
 const std::optional<dsp::processor::ProcessorStorage>

src/engine/engine.h

@@ -229,7 +229,8 @@ struct Engine : MoveableOnly<Engine>, SampleRateSupport
     void stopAllSounds();
 
     // TODO: All this gets ripped out when voice management is fixed
-    uint32_t activeVoiceCount();
+    void assertActiveVoiceCount();
+    std::atomic<uint32_t> activeVoices{0};
 
     const std::unique_ptr<messaging::MessageController> &getMessageController() const
     {

src/voice/voice.cpp

@@ -68,6 +68,7 @@ void Voice::cleanupVoice()
     zone = nullptr;
     isVoiceAssigned = false;
     engine->voiceManagerResponder.doVoiceEndCallback(this);
+    engine->activeVoices--;
 
     // We cleanup processors here since they may have, say,
     // memory pool resources checked out that others could

src/voice/voice.h

@@ -178,6 +178,7 @@ struct alignas(16) Voice : MoveableOnly<Voice>,
 
         isVoicePlaying = true;
         isVoiceAssigned = true;
+
         voiceStarted();
     }
     void release() { isGated = false; }