Optimize our GL extension checks to avoid mallocs.

We want to avoid any unnecessary mallocs when checking GL/GLX
extensions. The GL extension string can be quite long and contain
several hundred extensions. The old code forced one malloc for each
extension + 1 extra malloc for the extension string itself when
it was copied into the QByteArray.

Reviewed-by: Kim

1 files changed, 43 insertions, 0 deletions

diff --git a/src/opengl/qgl_p.h b/src/opengl/qgl_p.h
index 8a0b31f..11770d3 100644
--- a/src/opengl/qgl_p.h
+++ b/src/opengl/qgl_p.h
@@ -610,6 +610,49 @@ private:
     friend class QGLContextGroup;
 };
 
+
+// This class can be used to match GL extensions with doing any mallocs. The
+// class assumes that the GL extension string ends with a space character,
+// which it should do on all conformant platforms. Create the object and pass
+// in a pointer to the extension string, then call match() on each extension
+// that should be matched. The match() function takes the extension name
+// *without* the terminating space character as input.
+
+class QGLExtensionMatcher
+{
+public:
+    QGLExtensionMatcher(const char *str)
+        : gl_extensions(str), gl_extensions_length(qstrlen(str))
+    {}
+
+    bool match(const char *str) {
+        int str_length = qstrlen(str);
+        const char *extensions = gl_extensions;
+        int extensions_length = gl_extensions_length;
+
+        while (1) {
+            // the total length that needs to be matched is the str_length +
+            // the space character that terminates the extension name
+            if (extensions_length < str_length + 1)
+                return false;
+            if (qstrncmp(extensions, str, str_length) == 0 && extensions[str_length] == ' ')
+                return true;
+
+            int split_pos = 0;
+            while (split_pos < extensions_length && extensions[split_pos] != ' ')
+                ++split_pos;
+            ++split_pos; // added for the terminating space character
+            extensions += split_pos;
+            extensions_length -= split_pos;
+        }
+        return false;
+    }
+
+private:
+    const char *gl_extensions;
+    int gl_extensions_length;
+};
+
 QT_END_NAMESPACE
 
 #endif // QGL_P_H