From 51ed3f2ed08016532e7ded7449235d89a1ba78fe Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 9 Jul 2022 13:03:45 -0400
Subject: [PATCH] Reduce modal-related thread hopping.

---
 .../Mac/Clock Signal/ScanTarget/CSScanTarget.mm    | 14 ++++++++------
 Outputs/ScanTargets/BufferingScanTarget.cpp        | 12 +++++++++---
 Outputs/ScanTargets/BufferingScanTarget.hpp        |  7 ++++++-
 3 files changed, 23 insertions(+), 10 deletions(-)

diff --git a/OSBindings/Mac/Clock Signal/ScanTarget/CSScanTarget.mm b/OSBindings/Mac/Clock Signal/ScanTarget/CSScanTarget.mm
index 42548e778..a58a499b8 100644
--- a/OSBindings/Mac/Clock Signal/ScanTarget/CSScanTarget.mm	
+++ b/OSBindings/Mac/Clock Signal/ScanTarget/CSScanTarget.mm	
@@ -939,12 +939,14 @@ using BufferingScanTarget = Outputs::Display::BufferingScanTarget;
 
 - (void)updateFrameBuffer {
 	// TODO: rethink BufferingScanTarget::perform. Is it now really just for guarding the modals?
-	_scanTarget.perform([=] {
-		const Outputs::Display::ScanTarget::Modals *const newModals = _scanTarget.new_modals();
-		if(newModals) {
-			[self setModals:*newModals];
-		}
-	});
+	if(_scanTarget.has_new_modals()) {
+		_scanTarget.perform([=] {
+			const Outputs::Display::ScanTarget::Modals *const newModals = _scanTarget.new_modals();
+			if(newModals) {
+				[self setModals:*newModals];
+			}
+		});
+	}
 
 	@synchronized(self) {
 		if(!_frameBufferRenderPass) return;
diff --git a/Outputs/ScanTargets/BufferingScanTarget.cpp b/Outputs/ScanTargets/BufferingScanTarget.cpp
index c5bc95002..e65b9a136 100644
--- a/Outputs/ScanTargets/BufferingScanTarget.cpp
+++ b/Outputs/ScanTargets/BufferingScanTarget.cpp
@@ -302,7 +302,7 @@ size_t BufferingScanTarget::write_area_data_size() const {
 void BufferingScanTarget::set_modals(Modals modals) {
 	perform([=] {
 		modals_ = modals;
-		modals_are_dirty_ = true;
+		modals_are_dirty_.store(true, std::memory_order::memory_order_relaxed);
 	});
 }
 
@@ -374,10 +374,12 @@ void BufferingScanTarget::set_line_buffer(Line *line_buffer, LineMetadata *metad
 }
 
 const Outputs::Display::ScanTarget::Modals *BufferingScanTarget::new_modals() {
-	if(!modals_are_dirty_) {
+	const auto modals_are_dirty = modals_are_dirty_.load(std::memory_order::memory_order_relaxed);
+	if(!modals_are_dirty) {
 		return nullptr;
 	}
-	modals_are_dirty_ = false;
+
+	modals_are_dirty_.store(false, std::memory_order::memory_order_relaxed);
 
 	// MAJOR SHARP EDGE HERE: assume that because the new_modals have been fetched then the caller will
 	// now ensure their texture buffer is appropriate. They might provide a new pointer and might now.
@@ -392,3 +394,7 @@ const Outputs::Display::ScanTarget::Modals *BufferingScanTarget::new_modals() {
 const Outputs::Display::ScanTarget::Modals &BufferingScanTarget::modals() const {
 	return modals_;
 }
+
+bool BufferingScanTarget::has_new_modals() const {
+	return modals_are_dirty_.load(std::memory_order::memory_order_relaxed);
+}
diff --git a/Outputs/ScanTargets/BufferingScanTarget.hpp b/Outputs/ScanTargets/BufferingScanTarget.hpp
index 4b41d0a8b..83d8a3013 100644
--- a/Outputs/ScanTargets/BufferingScanTarget.hpp
+++ b/Outputs/ScanTargets/BufferingScanTarget.hpp
@@ -161,6 +161,11 @@ class BufferingScanTarget: public Outputs::Display::ScanTarget {
 		/// @returns the current @c Modals.
 		const Modals &modals() const;
 
+		/// @returns @c true if new modals are available; @c false otherwise.
+		///
+		/// Safe to call from any thread.
+		bool has_new_modals() const;
+
 	private:
 		// ScanTarget overrides.
 		void set_modals(Modals) final;
@@ -253,7 +258,7 @@ class BufferingScanTarget: public Outputs::Display::ScanTarget {
 		// Current modals and whether they've yet been returned
 		// from a call to @c get_new_modals.
 		Modals modals_;
-		bool modals_are_dirty_ = false;
+		std::atomic<bool> modals_are_dirty_ = false;
 
 		// Provides a per-data size implementation of end_data; a previous
 		// implementation used blind memcpy and that turned into something