vcacheoptimizer: Improve meshopt_optimizeVertexCache performance by ~15%

The core of this algorithm has a few branches in the inner loops that
are all difficult to predict. The triangle scoring was already optimized
to cmov by clang/gcc, but the cache copy wasn't - and the result
consumed a surprisingly large fraction of the total compute; making that
code branchless yielded ~7% speedup by itself.

The other ~7% speedup is achieved by a live_triangles check during
scoring update; there's no need to update anything if we aren't going to
use this vertex again. This is likely to be less effective on other
meshes, but it's rather effective on Buddha mesh.

Note: live_triangles is equivalent to adjacency.counts at the point of
cache score update, but checking adjacency.counts generates better code
on clang.

src/vcacheoptimizer.cpp

@@ -221,9 +221,9 @@ void meshopt_optimizeVertexCacheTable(unsigned int* destination, const unsigned
 		triangle_scores[i] = vertex_scores[a] + vertex_scores[b] + vertex_scores[c];
 	}
 
-	unsigned int cache_holder[2 * (kCacheSizeMax + 3)];
+	unsigned int cache_holder[2 * (kCacheSizeMax + 4)];
 	unsigned int* cache = cache_holder;
-	unsigned int* cache_new = cache_holder + kCacheSizeMax + 3;
+	unsigned int* cache_new = cache_holder + kCacheSizeMax + 4;
 	size_t cache_count = 0;
 
 	unsigned int current_triangle = 0;
@@ -260,10 +260,8 @@ void meshopt_optimizeVertexCacheTable(unsigned int* destination, const unsigned
 		{
 			unsigned int index = cache[i];
 
-			if (index != a && index != b && index != c)
-			{
-				cache_new[cache_write++] = index;
-			}
+			cache_new[cache_write] = index;
+			cache_write += (index != a && index != b && index != c);
 		}
 
 		unsigned int* cache_temp = cache;
@@ -305,6 +303,10 @@ void meshopt_optimizeVertexCacheTable(unsigned int* destination, const unsigned
 		{
 			unsigned int index = cache[i];
 
+			// no need to update scores if we are never going to use this vertex
+			if (adjacency.counts[index] == 0)
+				continue;
+
 			int cache_position = i >= cache_size ? -1 : int(i);
 
 			// update vertex score
@@ -325,11 +327,8 @@ void meshopt_optimizeVertexCacheTable(unsigned int* destination, const unsigned
 				float tri_score = triangle_scores[tri] + score_diff;
 				assert(tri_score > 0);
 
-				if (best_score < tri_score)
-				{
-					best_triangle = tri;
-					best_score = tri_score;
-				}
+				best_triangle = best_score < tri_score ? tri : best_triangle;
+				best_score = best_score < tri_score ? tri_score : best_score;
 
 				triangle_scores[tri] = tri_score;
 			}