From 3aac8e34835808b596540c5a721457685dd24e56 Mon Sep 17 00:00:00 2001
From: Robbert van der Helm <mail@robbertvanderhelm.nl>
Date: Tue, 27 Apr 2021 22:59:59 +0200
Subject: [PATCH] Add a RAII wrapper for enabling flush-to-zero

---
 src/common/utils.cpp | 10 ++++++++++
 src/common/utils.h   | 25 +++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/src/common/utils.cpp b/src/common/utils.cpp
index 0f1dac2e..25680d61 100644
--- a/src/common/utils.cpp
+++ b/src/common/utils.cpp
@@ -17,6 +17,7 @@
 #include "utils.h"
 
 #include <sched.h>
+#include <xmmintrin.h>
 #include <boost/process/environment.hpp>
 
 namespace bp = boost::process;
@@ -46,3 +47,12 @@ bool set_realtime_priority(bool sched_fifo, int priority) {
     return sched_setscheduler(0, sched_fifo ? SCHED_FIFO : SCHED_OTHER,
                               &params) == 0;
 }
+
+ScopedFlushToZero::ScopedFlushToZero() {
+    old_ftz_mode = _MM_GET_FLUSH_ZERO_MODE();
+    _MM_SET_FLUSH_ZERO_MODE(_MM_FLUSH_ZERO_ON);
+}
+
+ScopedFlushToZero::~ScopedFlushToZero() {
+    _MM_SET_FLUSH_ZERO_MODE(old_ftz_mode);
+}
diff --git a/src/common/utils.h b/src/common/utils.h
index 0df56d6c..cbac74d0 100644
--- a/src/common/utils.h
+++ b/src/common/utils.h
@@ -83,3 +83,28 @@ std::optional<int> get_realtime_priority();
  *   user does not have the privileges to set realtime priorities.
  */
 bool set_realtime_priority(bool sched_fifo, int priority = 5);
+
+/**
+ * A RAII wrapper that will temporarily enable the FTZ flag so that denormals
+ * are automatically flushed to zero, returning to whatever the flag was
+ * previously when it drops out of scope.
+ */
+class ScopedFlushToZero {
+    ScopedFlushToZero();
+    ~ScopedFlushToZero();
+
+    ScopedFlushToZero(const ScopedFlushToZero&) = delete;
+    ScopedFlushToZero& operator=(const ScopedFlushToZero&) = delete;
+
+    ScopedFlushToZero(ScopedFlushToZero&&) = delete;
+    ScopedFlushToZero& operator=(ScopedFlushToZero&&) = delete;
+
+   private:
+    /**
+     * The previous FTZ mode. When we use this on the Wine side, this should
+     * always be disabled. But, we'll make sure to do it correctly anyhow so we
+     * don't accidentally end up disabling FTZ somewhere where it should be
+     * enabled.
+     */
+    unsigned int old_ftz_mode;
+};