diff --git a/src/backend/base-function-builder.js b/src/backend/base-function-builder.js
index 580e3810..6db5cd93 100644
--- a/src/backend/base-function-builder.js
+++ b/src/backend/base-function-builder.js
@@ -18,6 +18,7 @@ module.exports = class BaseFunctionBuilder {
 	constructor(gpu) {
 		this.nodeMap = {};
 		this.gpu = gpu;
+		this.rootKernel = null;
 	}
 	
 	///
@@ -46,6 +47,12 @@ module.exports = class BaseFunctionBuilder {
 	///
 	addFunctionNode(inNode) {
 		this.nodeMap[inNode.functionName] = inNode;
+		if (inNode.isRootKernel) {
+		  if (this.rootKernel) {
+		    throw new Error('root kernel already defined');
+      }
+		  this.rootKernel = inNode;
+    }
 	}
 
 	///
@@ -65,7 +72,7 @@ module.exports = class BaseFunctionBuilder {
   traceFunctionCalls(functionName, retList) {
 		functionName = functionName || 'kernel';
 		retList = retList || [];
-		
+
 		const fNode = this.nodeMap[functionName];
 		if(fNode) {
 			// Check if function already exists
@@ -145,6 +152,7 @@ module.exports = class BaseFunctionBuilder {
 	/// 	{String} The full webgl string, of all the various functions. Trace optimized if functionName given
 	///
   webGlPrototypeString(functionName) {
+    this.rootKernel.build();
 		if(functionName) {
 			return this.webGlPrototypeStringFromFunctionNames(this.traceFunctionCalls(functionName, []).reverse());
 		}
diff --git a/src/backend/base-function-node.js b/src/backend/base-function-node.js
index 88b618f0..3c7e553b 100644
--- a/src/backend/base-function-node.js
+++ b/src/backend/base-function-node.js
@@ -48,6 +48,7 @@ module.exports = class BaseFunctionNode {
     this.initVariables = [];
     this.readVariables = [];
     this.writeVariables = [];
+    this.addFunction = null;
 
     //
     // Missing jsFunction object exception
@@ -87,7 +88,7 @@ module.exports = class BaseFunctionNode {
     // Extract parameter name, and its argument types
     //
     this.paramNames = utils.getParamNamesFromString(this.jsFunctionString);
-    if(paramTypeArray != null) {
+    if(paramTypeArray) {
       if(paramTypeArray.length !== this.paramNames.length) {
         throw 'Invalid argument type array length, against function length -> ('+
         paramTypeArray.length+','+
@@ -108,6 +109,11 @@ module.exports = class BaseFunctionNode {
     this.returnType = returnType || 'float';
   }
 
+  setAddFunction(fn) {
+    this.addFunction = fn;
+    return this;
+  }
+
   //
   // Core function
   //----------------------------------------------------------------------------------------------------
@@ -153,7 +159,7 @@ module.exports = class BaseFunctionNode {
     }
 
     inParser = inParser || parser;
-    if(inParser == null) {
+    if(inParser === null) {
       throw 'Missing JS to AST parser';
     }
 
@@ -193,4 +199,8 @@ module.exports = class BaseFunctionNode {
   setFunctionString(functionString) {
     this.functionString = functionString;
   }
+
+  build(options) {
+    throw new Error('build not defined on BaseFunctionNode');
+  }
 };
diff --git a/src/backend/base-kernel.js b/src/backend/base-kernel.js
index 4e9f95d8..b6c9e88d 100644
--- a/src/backend/base-kernel.js
+++ b/src/backend/base-kernel.js
@@ -20,6 +20,7 @@ module.exports = class BaseKernel {
     this.floatTextures = null;
     this.floatOutput = null;
     this.floatOutputForce = null;
+    this.addFunction = null;
 
     for (let p in settings) {
       if (!settings.hasOwnProperty(p) || !this.hasOwnProperty(p)) continue;
@@ -31,6 +32,11 @@ module.exports = class BaseKernel {
     throw new Error('"build" not defined on Base');
   }
 
+  setAddFunction(cb) {
+    this.addFunction = cb;
+    return this;
+  }
+
   setDimensions(dimensions) {
     this.dimensions = dimensions;
     return this;
diff --git a/src/backend/base-runner.js b/src/backend/base-runner.js
index ded471c3..318ce069 100644
--- a/src/backend/base-runner.js
+++ b/src/backend/base-runner.js
@@ -66,6 +66,10 @@ module.exports = class BaseRunner {
       throw 'Unable to get body of kernel function';
     }
 
+    if (!settings.functionBuilder) {
+      settings.functionBuilder = this.functionBuilder;
+    }
+
     return kernelRunShortcut(new this.Kernel(fnString, settings));
   }
 };
diff --git a/src/backend/cpu/cpu-function-builder.js b/src/backend/cpu/cpu-function-builder.js
index a2006b8f..fcb2bc47 100644
--- a/src/backend/cpu/cpu-function-builder.js
+++ b/src/backend/cpu/cpu-function-builder.js
@@ -3,6 +3,9 @@ const CPUFunctionNode = require('./cpu-function-node');
 
 module.exports = class CPUFunctionBuilder extends BaseFunctionBuilder {
   addFunction(functionName, jsFunction, paramTypeArray, returnType) {
-    this.addFunctionNode(new CPUFunctionNode(functionName, jsFunction, paramTypeArray, returnType));
+    this.addFunctionNode(
+      new CPUFunctionNode(functionName, jsFunction, paramTypeArray, returnType)
+        .setAddFunction(this.addFunction.bind(this))
+    );
   }
 };
\ No newline at end of file
diff --git a/src/backend/gpu/gpu-function-builder.js b/src/backend/gpu/gpu-function-builder.js
index 7616cb22..7682c2a7 100644
--- a/src/backend/gpu/gpu-function-builder.js
+++ b/src/backend/gpu/gpu-function-builder.js
@@ -3,6 +3,9 @@ const GPUFunctionNode = require('./gpu-function-node');
 
 module.exports = class GPUFunctionBuilder extends BaseFunctionBuilder {
   addFunction(functionName, jsFunction, paramTypeArray, returnType) {
-    this.addFunctionNode(new GPUFunctionNode(functionName, jsFunction, paramTypeArray, returnType));
+    this.addFunctionNode(
+      new GPUFunctionNode(functionName, jsFunction, paramTypeArray, returnType)
+        .setAddFunction(this.addFunction.bind(this))
+    );
   }
 };
\ No newline at end of file
diff --git a/src/backend/gpu/gpu-function-node.js b/src/backend/gpu/gpu-function-node.js
index 41c6e845..24cea5d7 100644
--- a/src/backend/gpu/gpu-function-node.js
+++ b/src/backend/gpu/gpu-function-node.js
@@ -190,7 +190,9 @@ module.exports = class GPUFunctionNode extends BaseFunctionNode {
     funcArr.push(lines[end.line-1].slice(0, end.column));
 
     const funcStr = funcArr.join('\n');
-    this.gpu.addFunction(funcStr);
+    if (this.addFunction) {
+      this.addFunction(funcStr);
+    }
 
     return retArr;
   }
@@ -857,4 +859,8 @@ module.exports = class GPUFunctionNode extends BaseFunctionNode {
     }
     return this.webGlFunctionPrototypeString = this.generate(options);
   }
+
+  build(options) {
+    return this.getFunctionPrototypeString(options).length > 0;
+  }
 };
\ No newline at end of file
diff --git a/src/backend/gpu/gpu-kernel.js b/src/backend/gpu/gpu-kernel.js
index d2e7f8b0..be34a44c 100644
--- a/src/backend/gpu/gpu-kernel.js
+++ b/src/backend/gpu/gpu-kernel.js
@@ -14,7 +14,7 @@ module.exports = class GPUKernel extends BaseKernel {
     this.framebufferCache = {};
     this.buffer = null;
     this.program = null;
-    this.functionBuilder = new GPUFunctionBuilder();
+    this.functionBuilder = settings.functionBuilder;
     this.endianness = utils.systemEndianness;
   }
 
@@ -125,6 +125,7 @@ module.exports = class GPUKernel extends BaseKernel {
     }
 
     const kernelNode = new GPUFunctionNode('kernel', this.fnString);
+    kernelNode.setAddFunction(builder.addFunction.bind(builder));
     kernelNode.paramNames = paramNames;
     kernelNode.paramType = paramType;
     kernelNode.isRootKernel = true;
@@ -320,7 +321,6 @@ void main(void) {
       }
   }
 }`;
-    console.log(builder.webGlPrototypeString('kernel'));
     const fragShader = gl.createShader(gl.FRAGMENT_SHADER);
 
     gl.shaderSource(vertShader, vertShaderSrc);
diff --git a/test/src/features/mult_AB.js b/test/src/features/mult_AB.js
index ef50ae7c..0b6010ba 100644
--- a/test/src/features/mult_AB.js
+++ b/test/src/features/mult_AB.js
@@ -1,5 +1,5 @@
 function mult_AB_test( assert, mode ) {
-	var gpu = new GPU();
+	var gpu = new GPU({ mode });
 	var f = gpu.createKernel(function(a, b) {
 		var sum = 0;
 		sum += a[this.thread.y][0] * b[0][this.thread.x];
@@ -7,8 +7,7 @@ function mult_AB_test( assert, mode ) {
 		sum += a[this.thread.y][2] * b[2][this.thread.x];
 		return sum;
 	}, {
-		dimensions : [3, 3],
-		mode: mode
+		dimensions : [3, 3]
 	});
 	
 	assert.ok( f !== null, "function generated test");
@@ -27,7 +26,7 @@ function mult_AB_test( assert, mode ) {
 			[30, 36, 42],
 			[66, 81, 96],
 			[102, 126, 150]
-		], 
+		],
 		"basic mult function test"
 	);
 }