[performance] merge WebGL BufferAttribute update ranges before issuin…

…g updates to the GPU (mrdoob#29189)

* sort update ranges before applying

* self review

src/renderers/webgl/WebGLAttributes.js

@@ -90,6 +90,50 @@ function WebGLAttributes( gl ) {
 
 		if ( updateRanges.length !== 0 ) {
 
+			// Before applying update ranges, we merge any adjacent / overlapping
+			// ranges to reduce load on `gl.bufferSubData`. Empirically, this has led
+			// to performance improvements for applications which make heavy use of
+			// update ranges. Likely due to GPU command overhead.
+			//
+			// Note that to reduce garbage collection between frames, we merge the
+			// update ranges in-place. This is safe because this method will clear the
+			// update ranges once updated.
+
+			updateRanges.sort( ( a, b ) => a.start - b.start );
+
+			// To merge the update ranges in-place, we work from left to right in the
+			// existing updateRanges array, merging ranges. This may result in a final
+			// array which is smaller than the original. This index tracks the last
+			// index representing a merged range, any data after this index can be
+			// trimmed once the merge algorithm is completed.
+			let mergeIndex = 0;
+
+			for ( let i = 1; i < updateRanges.length; i ++ ) {
+
+				const previousRange = updateRanges[ mergeIndex ];
+				const range = updateRanges[ i ];
+
+				// We add one here to merge adjacent ranges. This is safe because ranges
+				// operate over positive integers.
+				if ( range.start <= previousRange.start + previousRange.count + 1 ) {
+
+					previousRange.count = Math.max(
+						previousRange.count,
+						range.start + range.count - previousRange.start
+					);
+
+				} else {
+
+					++ mergeIndex;
+					updateRanges[ mergeIndex ] = range;
+
+				}
+
+			}
+
+			// Trim the array to only contain the merged ranges.
+			updateRanges.length = mergeIndex + 1;
+
 			for ( let i = 0, l = updateRanges.length; i < l; i ++ ) {
 
 				const range = updateRanges[ i ];