GC target decay, break on convergence (#429)

Prior to this, since the original last updated time was the epoch (0),
the first GC, this loop would run over 25 million times. With this
change, that goes down to 57.

Author: schveiguy

sdlib/d/gc/collector.d

@@ -147,7 +147,7 @@ private:
 	/**
 	 * Track the targets to meet before collecting.
 	 */
-	ulong lastTargetAdjustement;
+	ulong lastTargetAdjustment;
 
 	size_t nextTarget = DefaultHeapSize;
 
@@ -191,13 +191,21 @@ private:
 		auto interval =
 			max(lastCollectionStop - lastCollectionStart, 100 * Millisecond);
 
-		while (now - lastTargetAdjustement >= interval) {
+		while (now - lastTargetAdjustment >= interval) {
 			auto delta = nextTarget - lastHeapSize;
 			delta -= delta >> lgTargetDecay;
 			delta += lastHeapSize >> (lgTargetDecay + lgMinOverhead);
-			nextTarget = lastHeapSize + delta;
 
-			lastTargetAdjustement += interval;
+			auto newTarget = lastHeapSize + delta;
+			if (newTarget == nextTarget) {
+				// Limit reached.
+				lastTargetAdjustment = now;
+				break;
+			}
+
+			nextTarget = newTarget;
+
+			lastTargetAdjustment += interval;
 		}
 
 		auto currentHeapSize = Arena.computeUsedPageCount();
@@ -240,7 +248,7 @@ private:
 		target = max(target, tbaseline);
 		target = min(target, tpeak);
 
-		lastTargetAdjustement = lastCollectionStop;
+		lastTargetAdjustment = lastCollectionStop;
 		nextTarget = max(target, minHeapSize);
 	}
 }