From 46bba78feb15eed9c2a304bf7a04e816d283f6e8 Mon Sep 17 00:00:00 2001
From: "Unknown W. Brackets" <checkins@unknownbrackets.org>
Date: Wed, 6 Nov 2013 07:47:05 -0800
Subject: [PATCH] Avoid a mutex lock on a small parallel loop.

---
 thread/threadpool.cpp | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/thread/threadpool.cpp b/thread/threadpool.cpp
index d79f2cd6c9..4524082e58 100644
--- a/thread/threadpool.cpp
+++ b/thread/threadpool.cpp
@@ -57,25 +57,26 @@ void ThreadPool::StartWorkers() {
 }
 
 void ThreadPool::ParallelLoop(std::function<void(int,int)> loop, int lower, int upper) {
-	mutex.lock();
-	StartWorkers();
-	int range = upper-lower;
-	if(range >= numThreads*2) { // don't parallelize tiny loops (this could be better, maybe add optional parameter that estimates work per iteration)
+	int range = upper - lower;
+	if (range >= numThreads * 2) { // don't parallelize tiny loops (this could be better, maybe add optional parameter that estimates work per iteration)
+		lock_guard guard(mutex);
+		StartWorkers();
+
 		// could do slightly better load balancing for the generic case, 
 		// but doesn't matter since all our loops are power of 2
-		int chunk = range/numThreads; 
+		int chunk = range / numThreads;
 		int s = lower;
-		for(int i=0; i<numThreads-1; ++i) {
+		for (int i = 0; i < numThreads - 1; ++i) {
 			workers[i]->Process(std::bind(loop, s, s+chunk));
 			s+=chunk;
 		}
+		// This is the final chunk.
 		loop(s, upper);
-		for(int i=0; i<numThreads-1; ++i) {
+		for (int i = 0; i < numThreads - 1; ++i) {
 			workers[i]->WaitForCompletion();
 		}
 	} else {
 		loop(lower, upper);
 	}
-	mutex.unlock();
 }