Source code of MARS Assembler

First commit of the 4.5 version (latest version available)

mars/tools/BHTEntry.java

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+/*
+Copyright (c) 2009,  Ingo Kofler, ITEC, Klagenfurt University, Austria
+
+Developed by Ingo Kofler (ingo.kofler@itec.uni-klu.ac.at)
+
+Permission is hereby granted, free of charge, to any person obtaining 
+a copy of this software and associated documentation files (the 
+"Software"), to deal in the Software without restriction, including 
+without limitation the rights to use, copy, modify, merge, publish, 
+distribute, sublicense, and/or sell copies of the Software, and to 
+permit persons to whom the Software is furnished to do so, subject 
+to the following conditions:
+
+The above copyright notice and this permission notice shall be 
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 
+IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR 
+ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF 
+CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION 
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+(MIT license, http://www.opensource.org/licenses/mit-license.html)
+ */
+
+package mars.tools;//.bhtsim;
+
+/**
+ * Represents a single entry of the Branch History Table.
+ * <p>
+ * The entry holds the information about former branch predictions and outcomes.
+ * The number of past branch outcomes can be configured and is called the history.
+ * The semantics of the history of size <i>n</i> is as follows. 
+ * The entry will change its prediction, if it mispredicts the branch <i>n</i> times in series.
+ * The prediction of the entry can be obtained by the {@link BHTEntry#getPrediction()} method.
+ * Feedback of taken or not taken branches is provided to the entry via the {@link BHTEntry#updatePrediction(boolean)} method.
+ * This causes the history and the prediction to be updated.  
+ * <p>
+ * Additionally the entry keeps track about how many times the prediction was correct or incorrect.
+ * The statistics can be obtained by the methods {@link BHTEntry#getStatsPredCorrect()}, {@link BHTEntry#getStatsPredIncorrect()} and {@link BHTEntry#getStatsPredPrecision()}.
+ * 
+ * @author ingo.kofler@itec.uni-klu.ac.at
+ */
+
+public class BHTEntry {
+	
+	/** the history of the BHT entry. Each boolean value signals if the branch was taken or not. The value at index n-1 represents the most recent branch outcome. */
+	private boolean m_history[];
+
+	/** the current prediction */
+	private boolean m_prediction;
+	
+	/** absolute number of incorrect predictions */
+	private int m_incorrect;
+	
+	/** absolute number of correct predictions */
+	private int m_correct;
+	
+	
+	/**
+	 * Constructs a BHT entry with a given history size.  
+	 * 
+	 * The size of the history can only be set via the constructor and cannot be changed afterwards. 
+	 * 
+	 * @param historySize number of past branch outcomes to remember
+	 * @param initVal the initial value of the entry (take or do not take) 
+	 */
+	public BHTEntry(int historySize, boolean initVal) {
+		m_prediction = initVal;	
+		m_history = new boolean[historySize];
+		
+		for (int i=0; i < historySize; i++) {
+			m_history[i] = initVal;
+		}		
+		m_correct = m_incorrect = 0;				
+	}
+	
+	
+	/**
+	 * Returns the branch prediction based on the history.
+	 * 
+	 * @return true if prediction is to take the branch, false otherwise
+	 */
+	public boolean getPrediction() {
+		return m_prediction;
+	}
+	
+	
+	/**
+	 * Updates the entry's history and prediction. 
+	 * This method provides feedback for a prediction. 
+	 * The history and the statistics are updated accordingly.
+	 * Based on the updated history a new prediction is calculated 
+	 * 
+	 * @param branchTaken signals if the branch was taken (true) or not (false)
+	 */	
+	public void updatePrediction(boolean branchTaken) {
+		
+		// update history
+		for (int i=0; i < m_history.length-1; i++) {
+			m_history[i] = m_history[i+1];
+		}
+		m_history[m_history.length-1] = branchTaken;
+		
+		
+		// if the prediction was correct, update stats and keep prediction
+		if (branchTaken == m_prediction) {
+			m_correct ++;
+		}
+		else {
+			m_incorrect ++;
+			
+			// check if the prediction should change
+			boolean changePrediction = true;
+			
+			for (int i=0; i < m_history.length; i++) {				
+				if (m_history[i] != branchTaken) 
+					changePrediction = false;				
+			}
+			
+			if (changePrediction) 
+				m_prediction = !m_prediction;
+			
+		}
+	}	
+	
+	
+	/**
+	 * Get the absolute number of mispredictions.
+	 * 	
+	 * @return number of incorrect predictions (mispredictions)
+	 */
+	public int getStatsPredIncorrect() {
+		return m_incorrect;
+	}
+	
+	
+	/**
+	 * Get the absolute number of correct predictions.
+	 * 
+	 * @return number of correct predictions
+	 */
+	public int getStatsPredCorrect() {
+		return m_correct;
+	}
+	
+	
+	/**
+	 * Get the percentage of correct predictions.
+	 * 
+	 * @return the percentage of correct predictions
+	 */
+	public double getStatsPredPrecision() {
+		int sum = m_incorrect + m_correct;		
+		return (sum==0) ? 0 : m_correct * 100.0 / sum;
+	}
+	
+	
+	/***
+	 * Builds a string representation of the BHT entry's history.
+	 * The history is a sequence of flags that signal if the branch was taken (T) or not taken (NT). 
+	 * 
+	 * @return a string representation of the BHT entry's history
+	 */
+	public String getHistoryAsStr() {
+		String result = "";
+		
+		for (int i=0; i<m_history.length; i++) {
+			if (i>0) result = result + ", "; 
+			result += m_history[i] ? "T" : "NT";
+		}
+		return result;
+	}	
+	
+	
+	/***
+	 * Returns a string representation of the BHT entry's current prediction.
+	 * The prediction can be either to TAKE or do NOT TAKE the branch.
+	 *  
+	 * @return a string representation of the BHT entry's current prediction
+	 */
+	public String getPredictionAsStr() {
+		return m_prediction ? "TAKE" : "NOT TAKE";
+	}
+}