Add branch prediction macros

This adds support for Q_LIKELY and Q_UNLIKELY macros which instruct the
compiler on which branch of an "if" statement is more likely to happen.

The current patch only supports GCC's __builtin_expect().

Merge-request: 2580
Reviewed-by: ossi

3 files changed, 69 insertions, 0 deletions

diff --git a/doc/src/snippets/code/src_corelib_global_qglobal.cpp b/doc/src/snippets/code/src_corelib_global_qglobal.cpp
index 0b54cef..c79a714 100644
--- a/doc/src/snippets/code/src_corelib_global_qglobal.cpp
+++ b/doc/src/snippets/code/src_corelib_global_qglobal.cpp
@@ -531,3 +531,27 @@ class MyClass : public QObject
 //! [47]
 CApaApplication *myApplicationFactory();
 //! [47]
+
+//! [qlikely]
+    // the condition inside the "if" will be successful most of the times
+    for (int i = 1; i <= 365; i++) {
+        if (Q_LIKELY(isWorkingDay(i))) {
+            ...
+        }
+        ...
+    }
+//! [qlikely]
+
+//! [qunlikely]
+bool readConfiguration(const QFile &file)
+{
+    // We expect to be asked to read an existing file
+    if (Q_UNLIKELY(!file.exists())) {
+        qWarning() << "File not found";
+        return false;
+    }
+
+    ...
+    return true;
+}
+//! [qunlikely]
diff --git a/src/corelib/global/qglobal.cpp b/src/corelib/global/qglobal.cpp
index 54b9d89..b9b875f 100644
--- a/src/corelib/global/qglobal.cpp
+++ b/src/corelib/global/qglobal.cpp
@@ -2870,6 +2870,40 @@ int qrand()
 */
 
 /*!
+    \macro Q_LIKELY(expr)
+    \relates <QtGlobal>
+    \since 4.8
+
+    \brief Hints the compiler that the enclosed condition is likely to evaluate
+    to \c true.
+
+    Use of this macro can help the compiler to optimize the code.
+
+    Example:
+
+    \snippet doc/src/snippets/code/src_corelib_global_qglobal.cpp qlikely
+
+    \sa Q_UNLIKELY()
+*/
+
+/*!
+    \macro Q_UNLIKELY(expr)
+    \relates <QtGlobal>
+    \since 4.8
+
+    \brief Hints the compiler that the enclosed condition is likely to evaluate
+    to \c false.
+
+    Use of this macro can help the compiler to optimize the code.
+
+    Example:
+
+    \snippet doc/src/snippets/code/src_corelib_global_qglobal.cpp qunlikely
+
+    \sa Q_LIKELY()
+*/
+
+/*!
     \macro QT_POINTER_SIZE
     \relates <QtGlobal>
 
diff --git a/src/corelib/global/qglobal.h b/src/corelib/global/qglobal.h
index 8dd8850..07e0d4c 100644
--- a/src/corelib/global/qglobal.h
+++ b/src/corelib/global/qglobal.h
@@ -497,6 +497,10 @@ namespace QT_NAMESPACE {}
 #    define Q_TYPEOF(expr)    __typeof__(expr)
 #    define Q_DECL_ALIGN(n)   __attribute__((__aligned__(n)))
 #  endif
+#  if __GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 96)
+#    define Q_LIKELY(expr)    __builtin_expect(!!(expr), true)
+#    define Q_UNLIKELY(expr)  __builtin_expect(!!(expr), false)
+#  endif
 /* GCC 3.1 and GCC 3.2 wrongly define _SB_CTYPE_MACROS on HP-UX */
 #  if defined(Q_OS_HPUX) && __GNUC__ == 3 && __GNUC_MINOR__ >= 1
 #    define Q_WRONG_SB_CTYPE_MACROS
@@ -801,6 +805,13 @@ namespace QT_NAMESPACE {}
 #  undef Q_NO_PACKED_REFERENCE
 #endif
 
+#ifndef Q_LIKELY
+#  define Q_LIKELY(x) (x)
+#endif
+#ifndef Q_UNLIKELY
+#  define Q_UNLIKELY(x) (x)
+#endif
+
 #ifndef Q_CONSTRUCTOR_FUNCTION
 # define Q_CONSTRUCTOR_FUNCTION0(AFUNC) \
    static const int AFUNC ## __init_variable__ = AFUNC();