From 5671fde1aeef8da1e5e36380f242fda149cbe518 Mon Sep 17 00:00:00 2001
From: Connor McLaughlin <stenzek@gmail.com>
Date: Sun, 26 Apr 2020 18:32:23 +1000
Subject: [PATCH] BitUtils: Make Count{Leading,Trailing}Zeros UB for value==0

---
 README.md                                  |  1 +
 src/common/bitutils.h                      | 36 ++++++++++------------
 src/duckstation-qt/inputbindingwidgets.cpp |  3 +-
 3 files changed, 19 insertions(+), 21 deletions(-)

diff --git a/README.md b/README.md
index c178ef069..3046dc6b7 100644
--- a/README.md
+++ b/README.md
@@ -27,6 +27,7 @@ Other features include:
  - Direct booting of homebrew executables
  - Direct loading of Portable Sound Format (psf) files
  - Digital and analog controllers for input (rumble is forwarded to host)
+ - Namco GunCon lightgun support (simulated with mouse)
  - Qt and SDL frontends for desktop
  - Automatic content scanning - game titles/regions are provided by redump.org
 
diff --git a/src/common/bitutils.h b/src/common/bitutils.h
index e4a182422..12c1d211b 100644
--- a/src/common/bitutils.h
+++ b/src/common/bitutils.h
@@ -5,6 +5,7 @@
 #include <intrin.h>
 #endif
 
+/// Returns the number of zero bits before the first set bit, going MSB->LSB.
 template<typename T>
 ALWAYS_INLINE unsigned CountLeadingZeros(T value)
 {
@@ -12,27 +13,26 @@ ALWAYS_INLINE unsigned CountLeadingZeros(T value)
   if constexpr (sizeof(value) >= sizeof(u64))
   {
     unsigned long index;
-    return _BitScanReverse64(&index, ZeroExtend64(value)) ? static_cast<unsigned>(index) : 0;
+    _BitScanReverse64(&index, ZeroExtend64(value));
+    return static_cast<unsigned>(index) ^ static_cast<unsigned>((sizeof(value) * 8u) - 1u);
   }
   else
   {
     unsigned long index;
-    return _BitScanReverse(&index, ZeroExtend32(value)) ? static_cast<unsigned>(index) : 0;
+    _BitScanReverse(&index, ZeroExtend32(value));
+    return static_cast<unsigned>(index) ^ static_cast<unsigned>((sizeof(value) * 8u) - 1u);
   }
 #else
   if constexpr (sizeof(value) >= sizeof(u64))
-  {
-    const unsigned bits = static_cast<unsigned>(__builtin_clzl(ZeroExtend64(value)));
-    return (value != 0) ? static_cast<unsigned>(bits) : 0;
-  }
+    return static_cast<unsigned>(__builtin_clzl(ZeroExtend64(value)));
+  else if constexpr (sizeof(value) == sizeof(u32))
+    return static_cast<unsigned>(__builtin_clz(ZeroExtend32(value)));
   else
-  {
-    const unsigned bits = static_cast<unsigned>(__builtin_clz(ZeroExtend32(value)));
-    return (value != 0) ? static_cast<unsigned>(bits) : 0;
-  }
+    return static_cast<unsigned>(__builtin_clz(ZeroExtend32(value))) & static_cast<unsigned>((sizeof(value) * 8u) - 1u);
 #endif
 }
 
+/// Returns the number of zero bits before the first set bit, going LSB->MSB.
 template<typename T>
 ALWAYS_INLINE unsigned CountTrailingZeros(T value)
 {
@@ -40,23 +40,19 @@ ALWAYS_INLINE unsigned CountTrailingZeros(T value)
   if constexpr (sizeof(value) >= sizeof(u64))
   {
     unsigned long index;
-    return _BitScanForward64(&index, ZeroExtend64(value)) ? static_cast<unsigned>(index) : 0;
+    _BitScanForward64(&index, ZeroExtend64(value));
+    return index;
   }
   else
   {
     unsigned long index;
-    return _BitScanForward(&index, ZeroExtend32(value)) ? static_cast<unsigned>(index) : 0;
+    _BitScanForward(&index, ZeroExtend32(value));
+    return index;
   }
 #else
   if constexpr (sizeof(value) >= sizeof(u64))
-  {
-    const unsigned bits = static_cast<unsigned>(__builtin_ctzl(ZeroExtend64(value)));
-    return (value != 0) ? static_cast<unsigned>(bits) : 0;
-  }
+    return static_cast<unsigned>(__builtin_ctzl(ZeroExtend64(value)));
   else
-  {
-    const unsigned bits = static_cast<unsigned>(__builtin_ctz(ZeroExtend32(value)));
-    return (value != 0) ? static_cast<unsigned>(bits) : 0;
-  }
+    return static_cast<unsigned>(__builtin_ctz(ZeroExtend32(value)));
 #endif
 }
diff --git a/src/duckstation-qt/inputbindingwidgets.cpp b/src/duckstation-qt/inputbindingwidgets.cpp
index f1357dd74..67ad82097 100644
--- a/src/duckstation-qt/inputbindingwidgets.cpp
+++ b/src/duckstation-qt/inputbindingwidgets.cpp
@@ -168,7 +168,8 @@ bool InputButtonBindingWidget::eventFilter(QObject* watched, QEvent* event)
   }
   else if (event_type == QEvent::MouseButtonRelease)
   {
-    const u32 button_index = CountTrailingZeros(static_cast<u32>(static_cast<const QMouseEvent*>(event)->button()));
+    const u32 button_mask = static_cast<u32>(static_cast<const QMouseEvent*>(event)->button());
+    const u32 button_index = (button_mask == 0u) ? 0 : CountTrailingZeros(button_mask);
     m_new_binding_value = QStringLiteral("Mouse/Button%1").arg(button_index + 1);
     setNewBinding();
     stopListeningForInput();