From b3fda6236a7f62fa033d7c247a6d12d9f339061a Mon Sep 17 00:00:00 2001
From: Robin Gareus <robin@gareus.org>
Date: Wed, 6 Feb 2019 16:59:41 +0100
Subject: [PATCH] Optimize buffer zero-filling

---
 libs/ardour/disk_reader.cc     | 25 ++++++--------------
 libs/pbd/pbd/playback_buffer.h | 42 ++++++++++++++++++++++++++++++++--
 2 files changed, 47 insertions(+), 20 deletions(-)

diff --git a/libs/ardour/disk_reader.cc b/libs/ardour/disk_reader.cc
index 5b7b5bde52..c87f04daf2 100644
--- a/libs/ardour/disk_reader.cc
+++ b/libs/ardour/disk_reader.cc
@@ -692,11 +692,7 @@ DiskReader::audio_read (PBD::PlaybackBuffer<Sample>*rb,
 	Location *loc = 0;
 
 	if (!_playlists[DataType::AUDIO]) {
-		// TODO optimize use zero-buffer
-		for (uint32_t z = 0; z < cnt; ++z) {
-			Sample t = 0;
-			rb->write (&t, 1);
-		}
+		rb->write_zero (cnt);
 		return 0;
 	}
 
@@ -908,12 +904,7 @@ DiskReader::refill_audio (Sample* sum_buffer, Sample* mixdown_buffer, float* gai
 			/* at start: nothing to do but fill with silence */
 			for (chan_n = 0, i = c->begin(); i != c->end(); ++i, ++chan_n) {
 				ChannelInfo* chan (*i);
-				// TODO optimize use zero-buffer
-				samplecnt_t ws = chan->rbuf->write_space ();
-				for (uint32_t z = 0; z < ws; ++z) {
-					Sample t = 0;
-					chan->rbuf->write (&t, 1);
-				}
+				chan->rbuf->write_zero (chan->rbuf->write_space ());
 			}
 			return 0;
 		}
@@ -932,12 +923,7 @@ DiskReader::refill_audio (Sample* sum_buffer, Sample* mixdown_buffer, float* gai
 			/* at end: nothing to do but fill with silence */
 			for (chan_n = 0, i = c->begin(); i != c->end(); ++i, ++chan_n) {
 				ChannelInfo* chan (*i);
-				// TODO optimize use zero-buffer
-				samplecnt_t ws = chan->rbuf->write_space ();
-				for (uint32_t z = 0; z < ws; ++z) {
-					Sample t = 0;
-					chan->rbuf->write (&t, 1);
-				}
+				chan->rbuf->write_zero (chan->rbuf->write_space ());
 			}
 			return 0;
 		}
@@ -990,7 +976,10 @@ DiskReader::refill_audio (Sample* sum_buffer, Sample* mixdown_buffer, float* gai
 		}
 
 		if (zero_fill) {
-			/* XXX: do something */
+			/* not sure if action is needed,
+			 * we'll later hit the "to close to the end" case
+			 */
+			//chan->rbuf->write_zero (zero_fill);
 		}
 	}
 
diff --git a/libs/pbd/pbd/playback_buffer.h b/libs/pbd/pbd/playback_buffer.h
index fc9bd4d6e0..d9f2232144 100644
--- a/libs/pbd/pbd/playback_buffer.h
+++ b/libs/pbd/pbd/playback_buffer.h
@@ -106,7 +106,8 @@ public:
 	}
 
 	guint read  (T *dest, guint cnt, bool commit = true);
-	guint write (T const * src,  guint cnt);
+	guint write (T const * src, guint cnt);
+	guint write_zero (guint cnt);
 
 	T *buffer () { return buf; }
 	guint bufsize () const { return size; }
@@ -139,7 +140,6 @@ private:
 	Glib::Threads::Mutex _reset_lock;
 };
 
-
 template<class T> /*LIBPBD_API*/ guint
 PlaybackBuffer<T>::write (T const *src, guint cnt)
 {
@@ -178,6 +178,44 @@ PlaybackBuffer<T>::write (T const *src, guint cnt)
 	return to_write;
 }
 
+template<class T> /*LIBPBD_API*/ guint
+PlaybackBuffer<T>::write_zero (guint cnt)
+{
+	guint w = g_atomic_int_get (&write_idx);
+	const guint free_cnt = write_space ();
+
+	if (free_cnt == 0) {
+		return 0;
+	}
+
+	const guint to_write = cnt > free_cnt ? free_cnt : cnt;
+	const guint cnt2 = w + to_write;
+
+	guint n1, n2;
+	if (cnt2 > size) {
+		n1 = size - w;
+		n2 = cnt2 & size_mask;
+	} else {
+		n1 = to_write;
+		n2 = 0;
+	}
+
+	memset (&buf[w], 0, n1 * sizeof (T));
+	w = (w + n1) & size_mask;
+
+	if (n2) {
+		memset (buf, 0, n2 * sizeof (T));
+		w = n2;
+	}
+
+	{
+		SpinLock sl (_writepos_lock);
+		write_pos += to_write;
+		g_atomic_int_set (&write_idx, w);
+	}
+	return to_write;
+}
+
 template<class T> /*LIBPBD_API*/ guint
 PlaybackBuffer<T>::read (T *dest, guint cnt, bool commit)
 {
-- 
2.30.2