blame: Reduce running time ~4.5% by skipping common subtrees

With this commit running blame on render_view_impl.cc[1] saves
about 644 ms over prior versions, reducing the time about 4.5%.

Large projects often contain strands of commits where no changes
are made to a particular subtree. Blame used to dive recursively
into these subtrees to look for the blob and check if its SHA-1
was changed. In chromium/src[1] only 20% of the commits modify
the content/renderer subtree relevant for the file.

The recursivePath is necessary to check for '/' and remember
if common subtree elimination should be attempted. When a file
lives within a subtree the extra cost to check for unmodified
subtrees saves time. However for files in the root tree the
extra work incurred by TreeWalk is not worthwhile and would
significantly increase overall running time.

Now typical running times from an otherwise idle desktop:

  real	0m13.387s  0m13.341s  0m13.443s
  user	0m15.410s  0m15.220s  0m15.350s

previously:

  real	0m14.085s  0m14.049s  0m13.968s
  user	0m15.730s  0m15.820s  0m15.770s

[1] 34d6e5c5b4/content/renderer/render_view_impl.cc

Change-Id: Ib16d684df7ffa034ee28def3fb22c797998d5b7b

org.eclipse.jgit/src/org/eclipse/jgit/blame/BlameGenerator.java

@@ -73,6 +73,7 @@
 import org.eclipse.jgit.revwalk.RevFlag;
 import org.eclipse.jgit.revwalk.RevWalk;
 import org.eclipse.jgit.treewalk.TreeWalk;
+import org.eclipse.jgit.treewalk.filter.AndTreeFilter;
 import org.eclipse.jgit.treewalk.filter.PathFilter;
 import org.eclipse.jgit.treewalk.filter.TreeFilter;
 
@@ -599,19 +600,19 @@ private boolean processOne(Candidate n) throws IOException {
 			return split(n.getNextCandidate(0), n);
 		revPool.parseHeaders(parent);
 
-		if (find(parent, n.sourcePath)) {
-			if (idBuf.equals(n.sourceBlob)) {
-				// The common case of the file not being modified in
-				// a simple string-of-pearls history. Blame parent.
-				n.sourceCommit = parent;
-				push(n);
-				return false;
+		if (n.sourceCommit != null && n.recursivePath) {
+			treeWalk.setFilter(AndTreeFilter.create(n.sourcePath, ID_DIFF));
+			treeWalk.reset(n.sourceCommit.getTree(), parent.getTree());
+			if (!treeWalk.next())
+				return blameEntireRegionOnParent(n, parent);
+			if (isFile(treeWalk.getRawMode(1))) {
+				treeWalk.getObjectId(idBuf, 1);
+				return splitBlameWithParent(n, parent);
 			}
-
-			Candidate next = n.create(parent, n.sourcePath);
-			next.sourceBlob = idBuf.toObjectId();
-			next.loadText(reader);
-			return split(next, n);
+		} else if (find(parent, n.sourcePath)) {
+			if (idBuf.equals(n.sourceBlob))
+				return blameEntireRegionOnParent(n, parent);
+			return splitBlameWithParent(n, parent);
 		}
 
 		if (n.sourceCommit == null)
@@ -625,7 +626,7 @@ private boolean processOne(Candidate n) throws IOException {
 			// A 100% rename without any content change can also
 			// skip directly to the parent.
 			n.sourceCommit = parent;
-			n.sourcePath = PathFilter.create(r.getOldPath());
+			n.setSourcePath(PathFilter.create(r.getOldPath()));
 			push(n);
 			return false;
 		}
@@ -637,6 +638,21 @@ private boolean processOne(Candidate n) throws IOException {
 		return split(next, n);
 	}
 
+	private boolean blameEntireRegionOnParent(Candidate n, RevCommit parent) {
+		// File was not modified, blame parent.
+		n.sourceCommit = parent;
+		push(n);
+		return false;
+	}
+
+	private boolean splitBlameWithParent(Candidate n, RevCommit parent)
+			throws IOException {
+		Candidate next = n.create(parent, n.sourcePath);
+		next.sourceBlob = idBuf.toObjectId();
+		next.loadText(reader);
+		return split(next, n);
+	}
+
 	private boolean split(Candidate parent, Candidate source)
 			throws IOException {
 		EditList editList = diffAlgorithm.diff(textComparator,
@@ -673,9 +689,7 @@ private boolean processMerge(Candidate n) throws IOException {
 			if (!find(parent, n.sourcePath))
 				continue;
 			if (!(n instanceof ReverseCandidate) && idBuf.equals(n.sourceBlob)) {
-				n.sourceCommit = parent;
-				push(n);
-				return false;
+				return blameEntireRegionOnParent(n, parent);
 			}
 			if (ids == null)
 				ids = new ObjectId[pCnt];
@@ -707,7 +721,7 @@ private boolean processMerge(Candidate n) throws IOException {
 					// we choose to follow the one parent over trying to do
 					// possibly both parents.
 					n.sourceCommit = parent;
-					n.sourcePath = PathFilter.create(r.getOldPath());
+					n.setSourcePath(PathFilter.create(r.getOldPath()));
 					push(n);
 					return false;
 				}
@@ -974,4 +988,26 @@ private static boolean isRename(DiffEntry ent) {
 		return ent.getChangeType() == ChangeType.RENAME
 				|| ent.getChangeType() == ChangeType.COPY;
 	}
+
+	private static final TreeFilter ID_DIFF = new TreeFilter() {
+		@Override
+		public boolean include(TreeWalk tw) {
+			return !tw.idEqual(0, 1);
+		}
+
+		@Override
+		public boolean shouldBeRecursive() {
+			return false;
+		}
+
+		@Override
+		public TreeFilter clone() {
+			return this;
+		}
+
+		@Override
+		public String toString() {
+			return "ID_DIFF"; //$NON-NLS-1$
+		}
+	};
 }

org.eclipse.jgit/src/org/eclipse/jgit/blame/Candidate.java

@@ -80,6 +80,7 @@ class Candidate {
 
 	/** Path of the candidate file in {@link #sourceCommit}. */
 	PathFilter sourcePath;
+	boolean recursivePath;
 
 	/** Unique name of the candidate blob in {@link #sourceCommit}. */
 	ObjectId sourceBlob;
@@ -110,6 +111,7 @@ class Candidate {
 	Candidate(RevCommit commit, PathFilter path) {
 		sourceCommit = commit;
 		sourcePath = path;
+		recursivePath = path.shouldBeRecursive();
 	}
 
 	int getParentCount() {
@@ -144,6 +146,11 @@ PersonIdent getAuthor() {
 		return sourceCommit.getAuthorIdent();
 	}
 
+	void setSourcePath(PathFilter path) {
+		sourcePath = path;
+		recursivePath = path.shouldBeRecursive();
+	}
+
 	Candidate create(RevCommit commit, PathFilter path) {
 		return new Candidate(commit, path);
 	}