diff --git a/src/library_gl.js b/src/library_gl.js
index 3d88e27172129..884e31f7e0dd1 100644
--- a/src/library_gl.js
+++ b/src/library_gl.js
@@ -251,8 +251,8 @@ var LibraryGL = {
     getSource: function(shader, count, string, length) {
       var source = '';
       for (var i = 0; i < count; ++i) {
-        var len = length ? {{{ makeGetValue('length', 'i*4', 'i32') }}} : undefined;
-        source += UTF8ToString({{{ makeGetValue('string', 'i*4', 'i32') }}}, len >= 0 ? len : undefined);
+        var len = length ? {{{ makeGetValue('length', 'i*4', 'i32') }}} : -1;
+        source += UTF8ToString({{{ makeGetValue('string', 'i*4', 'i32') }}}, len < 0 ? undefined : len);
       }
 #if LEGACY_GL_EMULATION
       // Let's see if we need to enable the standard derivatives extension
@@ -752,7 +752,7 @@ var LibraryGL = {
 
     registerContext: function(ctx, webGLContextAttributes) {
       var handle = _malloc(8); // Make space on the heap to store GL context attributes that need to be accessible as shared between threads.
-#if ASSERTIONS
+#if GL_ASSERTIONS
       assert(handle, 'malloc() failed in GL.registerContext!');
 #endif
 #if GL_SUPPORT_EXPLICIT_SWAP_CONTROL
@@ -833,25 +833,19 @@ var LibraryGL = {
     },
 
     makeContextCurrent: function(contextHandle) {
-      // Deactivating current context?
-      if (!contextHandle) {
-        GLctx = Module.ctx = GL.currentContext = null;
-        return true;
-      }
-      var context = GL.contexts[contextHandle];
-      if (!context) {
 #if GL_DEBUG
+      if (contextHandle && !GL.contexts[contextHandle]) {
 #if USE_PTHREADS
         console.error('GL.makeContextCurrent() failed! WebGL context ' + contextHandle + ' does not exist, or was created on another thread!');
 #else
         console.error('GL.makeContextCurrent() failed! WebGL context ' + contextHandle + ' does not exist!');
 #endif
-#endif
-        return false;
       }
-      GLctx = Module.ctx = context.GLctx; // Active WebGL context object.
-      GL.currentContext = context; // Active Emscripten GL layer context object.
-      return true;
+#endif
+
+      GL.currentContext = GL.contexts[contextHandle]; // Active Emscripten GL layer context object.
+      Module.ctx = GLctx = GL.currentContext && GL.currentContext.GLctx; // Active WebGL context object.
+      return !(contextHandle && !GLctx);
     },
 
     getContext: function(contextHandle) {
@@ -959,14 +953,13 @@ var LibraryGL = {
       GL.validateGLObjectID(GL.programs, program, 'populateUniformTable', 'program');
 #endif
       var p = GL.programs[program];
-      GL.programInfos[program] = {
+      var ptable = GL.programInfos[program] = {
         uniforms: {},
         maxUniformLength: 0, // This is eagerly computed below, since we already enumerate all uniforms anyway.
         maxAttributeLength: -1, // This is lazily computed and cached, computed when/if first asked, "-1" meaning not computed yet.
         maxUniformBlockNameLength: -1 // Lazily computed as well
       };
 
-      var ptable = GL.programInfos[program];
       var utable = ptable.uniforms;
       // A program's uniform table maps the string name of an uniform to an integer location of that uniform.
       // The global GL.uniforms map maps integer locations to WebGLUniformLocations.
@@ -978,8 +971,8 @@ var LibraryGL = {
         ptable.maxUniformLength = Math.max(ptable.maxUniformLength, name.length+1);
 
         // Strip off any trailing array specifier we might have got, e.g. "[0]".
-        if (name.indexOf(']', name.length-1) !== -1) {
-          var ls = name.lastIndexOf('[');
+        var ls = name.lastIndexOf('[');
+        if (ls > 0) {
           name = name.slice(0, ls);
         }
 
@@ -987,8 +980,7 @@ var LibraryGL = {
         // only store the string 'colors' in utable, and 'colors[0]', 'colors[1]' and 'colors[2]' will be parsed as 'colors'+i.
         // Note that for the GL.uniforms table, we still need to fetch the all WebGLUniformLocations for all the indices.
         var loc = GLctx.getUniformLocation(p, name);
-        if (loc != null)
-        {
+        if (loc) {
           var id = GL.getNewId(GL.uniforms);
           utable[name] = [u.size, id];
           GL.uniforms[id] = loc;
@@ -1416,7 +1408,10 @@ var LibraryGL = {
 
   _computeUnpackAlignedImageSize: function(width, height, sizePerPixel, alignment) {
     function roundedToNextMultipleOf(x, y) {
-      return Math.ceil(x/y)*y;
+#if GL_ASSERTIONS
+      assert((y & (y-1)) === 0, 'Unpack alignment must be a power of 2! (Allowed values per WebGL spec are 1, 2, 4 or 8)');
+#endif
+      return (x + y - 1) & -y;
     }
     var plainRowSize = width * sizePerPixel;
     var alignedRowSize = roundedToNextMultipleOf(plainRowSize, alignment);
@@ -1620,10 +1615,7 @@ var LibraryGL = {
       return;
     }
 #endif
-
-    var pixelData = null;
-    if (pixels) pixelData = emscriptenWebGLGetTexPixelData(type, format, width, height, pixels, internalFormat);
-    GLctx.texImage2D(target, level, internalFormat, width, height, border, format, type, pixelData);
+    GLctx.texImage2D(target, level, internalFormat, width, height, border, format, type, pixels ? emscriptenWebGLGetTexPixelData(type, format, width, height, pixels, internalFormat) : null);
   },
 
   glTexSubImage2D__sig: 'viiiiiiiii',
@@ -1692,7 +1684,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.textures, texture, 'glBindTexture', 'texture');
 #endif
-    GLctx.bindTexture(target, texture ? GL.textures[texture] : null);
+    GLctx.bindTexture(target, GL.textures[texture]);
   },
 
   glGetTexParameterfv__sig: 'viii',
@@ -1754,17 +1746,16 @@ var LibraryGL = {
     ) {
     for (var i = 0; i < n; i++) {
       var buffer = GLctx[createFunction]();
-      if (!buffer) {
+      var id = buffer && GL.getNewId(objectTable);
+      if (buffer) {
+        buffer.name = id;
+        objectTable[id] = buffer;
+      } else {
         GL.recordError(0x0502 /* GL_INVALID_OPERATION */);
 #if GL_ASSERTIONS
         err('GL_INVALID_OPERATION in ' + functionName + ': GLctx.' + createFunction + ' returned null - most likely GL context is lost!');
 #endif
-        while(i < n) {{{ makeSetValue('buffers', 'i++*4', 0, 'i32') }}};
-        return;
       }
-      var id = GL.getNewId(objectTable);
-      buffer.name = id;
-      objectTable[id] = buffer;
       {{{ makeSetValue('buffers', 'i*4', 'id', 'i32') }}};
     }
   },
@@ -1860,17 +1851,21 @@ var LibraryGL = {
         break;
     }
 #endif
-    if (!data) {
-      GLctx.bufferData(target, size, usage);
-    } else {
 #if USE_WEBGL2
-      if (GL.currentContext.supportsWebGL2EntryPoints) { // WebGL 2 provides new garbage-free entry points to call to WebGL. Use those always when possible.
+    if (GL.currentContext.supportsWebGL2EntryPoints) { // WebGL 2 provides new garbage-free entry points to call to WebGL. Use those always when possible.
+      if (data) {
         GLctx.bufferData(target, HEAPU8, usage, data, size);
-        return;
+      } else {
+        GLctx.bufferData(target, size, usage);
       }
+    } else {
 #endif
-      GLctx.bufferData(target, HEAPU8.subarray(data, data+size), usage);
+      // N.b. here first form specifies a heap subarray, second form an integer size, so the ?: code here is polymorphic. It is advised to avoid
+      // randomly mixing both uses in calling code, to avoid any potential JS engine JIT issues.
+      GLctx.bufferData(target, data ? HEAPU8.subarray(data, data+size) : size, usage);
+#if USE_WEBGL2
     }
+#endif
   },
 
   glBufferSubData__sig: 'viiii',
@@ -1927,7 +1922,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.timerQueriesEXT, id, 'glBeginQueryEXT', 'id');
 #endif
-    GLctx.disjointTimerQueryExt['beginQueryEXT'](target, id ? GL.timerQueriesEXT[id] : null);
+    GLctx.disjointTimerQueryExt['beginQueryEXT'](target, GL.timerQueriesEXT[id]);
   },
 
   glEndQueryEXT__sig: 'vi',
@@ -1940,7 +1935,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.timerQueriesEXT, id, 'glQueryCounterEXT', 'id');
 #endif
-    GLctx.disjointTimerQueryExt['queryCounterEXT'](id ? GL.timerQueriesEXT[id] : null, target);
+    GLctx.disjointTimerQueryExt['queryCounterEXT'](GL.timerQueriesEXT[id], target);
   },
 
   glGetQueryivEXT__sig: 'viii',
@@ -2235,7 +2230,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.queries, id, 'glBeginQuery', 'id');
 #endif
-    GLctx['beginQuery'](target, id ? GL.queries[id] : null);
+    GLctx['beginQuery'](target, GL.queries[id]);
   },
 
   glGetQueryiv__sig: 'viii',
@@ -2312,7 +2307,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.samplers, sampler, 'glBindSampler', 'sampler');
 #endif
-    GLctx['bindSampler'](unit, sampler ? GL.samplers[sampler] : null);
+    GLctx['bindSampler'](unit, GL.samplers[sampler]);
   },
 
   glSamplerParameterf__sig: 'viif',
@@ -2320,7 +2315,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.samplers, sampler, 'glBindSampler', 'sampler');
 #endif
-    GLctx['samplerParameterf'](sampler ? GL.samplers[sampler] : null, pname, param);
+    GLctx['samplerParameterf'](GL.samplers[sampler], pname, param);
   },
 
   glSamplerParameteri__sig: 'viii',
@@ -2328,7 +2323,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.samplers, sampler, 'glBindSampler', 'sampler');
 #endif
-    GLctx['samplerParameteri'](sampler ? GL.samplers[sampler] : null, pname, param);
+    GLctx['samplerParameteri'](GL.samplers[sampler], pname, param);
   },
 
   glSamplerParameterfv__sig: 'viii',
@@ -2337,7 +2332,7 @@ var LibraryGL = {
     GL.validateGLObjectID(GL.samplers, sampler, 'glBindSampler', 'sampler');
 #endif
     var param = {{{ makeGetValue('params', '0', 'float') }}};
-    GLctx['samplerParameterf'](sampler ? GL.samplers[sampler] : null, pname, param);
+    GLctx['samplerParameterf'](GL.samplers[sampler], pname, param);
   },
 
   glSamplerParameteriv__sig: 'viii',
@@ -2346,7 +2341,7 @@ var LibraryGL = {
     GL.validateGLObjectID(GL.samplers, sampler, 'glBindSampler', 'sampler');
 #endif
     var param = {{{ makeGetValue('params', '0', 'i32') }}};
-    GLctx['samplerParameteri'](sampler ? GL.samplers[sampler] : null, pname, param);
+    GLctx['samplerParameteri'](GL.samplers[sampler], pname, param);
   },
 
   glGetSamplerParameterfv__sig: 'viii',
@@ -2403,10 +2398,8 @@ var LibraryGL = {
   },
 
   glIsTransformFeedback__sig: 'ii',
-  glIsTransformFeedback: function(transformFeedback) {
-    var transformFeedback = GL.transformFeedbacks[transformFeedback];
-    if (!transformFeedback) return 0;
-    return GLctx['isTransformFeedback'](transformFeedback);
+  glIsTransformFeedback: function(id) {
+    return GLctx['isTransformFeedback'](GL.transformFeedbacks[id]);
   },
 
   glBindTransformFeedback__sig: 'vii',
@@ -2414,12 +2407,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.transformFeedbacks, id, 'glBindTransformFeedback', 'id');
 #endif
-    var transformFeedback = id ? GL.transformFeedbacks[id] : null;
-    if (id && !transformFeedback) { // Passing an nonexisting or an already deleted id is an error.
-      GL.recordError(0x0502 /* GL_INVALID_OPERATION */);
-      return;
-    }
-    GLctx['bindTransformFeedback'](target, transformFeedback);
+    GLctx['bindTransformFeedback'](target, GL.transformFeedbacks[id]);
   },
 
   glTransformFeedbackVaryings__sig: 'viiii',
@@ -2534,8 +2522,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.buffers, buffer, 'glBindBufferBase', 'buffer');
 #endif
-    var bufferObj = buffer ? GL.buffers[buffer] : null;
-    GLctx['bindBufferBase'](target, index, bufferObj);
+    GLctx['bindBufferBase'](target, index, GL.buffers[buffer]);
   },
 
   glBindBufferRange__sig: 'viiiii',
@@ -2543,8 +2530,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.buffers, buffer, 'glBindBufferRange', 'buffer');
 #endif
-    var bufferObj = buffer ? GL.buffers[buffer] : null;
-    GLctx['bindBufferRange'](target, index, bufferObj, offset, ptrsize);
+    GLctx['bindBufferRange'](target, index, GL.buffers[buffer], offset, ptrsize);
   },
 
   glGetUniformIndices__sig: 'viiii',
@@ -2617,9 +2603,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.programs, program, 'glGetUniformBlockIndex', 'program');
 #endif
-    program = GL.programs[program];
-    uniformBlockName = UTF8ToString(uniformBlockName);
-    return GLctx['getUniformBlockIndex'](program, uniformBlockName);
+    return GLctx['getUniformBlockIndex'](GL.programs[program], UTF8ToString(uniformBlockName));
   },
 
   glGetActiveUniformBlockiv__sig: 'viiii',
@@ -2851,7 +2835,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.renderbuffers, renderbuffer, 'glBindRenderbuffer', 'renderbuffer');
 #endif
-    GLctx.bindRenderbuffer(target, renderbuffer ? GL.renderbuffers[renderbuffer] : null);
+    GLctx.bindRenderbuffer(target, GL.renderbuffers[renderbuffer]);
   },
 
   glGetRenderbufferParameteriv__sig: 'viii',
@@ -2932,28 +2916,17 @@ var LibraryGL = {
 #endif
     name = UTF8ToString(name);
 
-    var arrayOffset = 0;
+    var arrayIndex = 0;
     // If user passed an array accessor "[index]", parse the array index off the accessor.
-    if (name.indexOf(']', name.length-1) !== -1) {
-      var ls = name.lastIndexOf('[');
-      var arrayIndex = name.slice(ls+1, -1);
-      if (arrayIndex.length > 0) {
-        arrayOffset = parseInt(arrayIndex);
-        if (arrayOffset < 0) {
-          return -1;
-        }
-      }
-      name = name.slice(0, ls);
+    if (name[name.length - 1] == ']') {
+      var leftBrace = name.lastIndexOf('[');
+      arrayIndex = name[leftBrace+1] != ']' ? parseInt(name.slice(leftBrace + 1)) : 0; // "index]", parseInt will ignore the ']' at the end; but treat "foo[]" as "foo[0]"
+      name = name.slice(0, leftBrace);
     }
 
-    var ptable = GL.programInfos[program];
-    if (!ptable) {
-      return -1;
-    }
-    var utable = ptable.uniforms;
-    var uniformInfo = utable[name]; // returns pair [ dimension_of_uniform_array, uniform_location ]
-    if (uniformInfo && arrayOffset < uniformInfo[0]) { // Check if user asked for an out-of-bounds element, i.e. for 'vec4 colors[3];' user could ask for 'colors[10]' which should return -1.
-      return uniformInfo[1]+arrayOffset;
+    var uniformInfo = GL.programInfos[program] && GL.programInfos[program].uniforms[name]; // returns pair [ dimension_of_uniform_array, uniform_location ]
+    if (uniformInfo && arrayIndex >= 0 && arrayIndex < uniformInfo[0]) { // Check if user asked for an out-of-bounds element, i.e. for 'vec4 colors[3];' user could ask for 'colors[10]' which should return -1.
+      return uniformInfo[1] + arrayIndex;
     } else {
       return -1;
     }
@@ -3636,8 +3609,6 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.buffers, buffer, 'glBindBuffer', 'buffer');
 #endif
-    var bufferObj = buffer ? GL.buffers[buffer] : null;
-
 #if FULL_ES2 || LEGACY_GL_EMULATION
     if (target == GLctx.ARRAY_BUFFER) {
       GL.currArrayBuffer = buffer;
@@ -3664,7 +3635,7 @@ var LibraryGL = {
       GLctx.currentPixelUnpackBufferBinding = buffer;
     }
 #endif
-    GLctx.bindBuffer(target, bufferObj);
+    GLctx.bindBuffer(target, GL.buffers[buffer]);
   },
 
   glVertexAttrib1fv__sig: 'vii',
@@ -3784,7 +3755,7 @@ var LibraryGL = {
     {{{ makeSetValue('count', '0', 'len', 'i32') }}};
     for (var i = 0; i < len; ++i) {
       var id = GL.shaders.indexOf(result[i]);
-#if ASSERTIONS
+#if GL_ASSERTIONS
       assert(id !== -1, 'shader not bound to local id');
 #endif
       {{{ makeSetValue('shaders', 'i*4', 'id', 'i32') }}};
@@ -4045,7 +4016,6 @@ var LibraryGL = {
     var log = (GLctx.getProgramInfoLog(GL.programs[program]) || '').trim();
     if (log) console.error('glLinkProgram: ' + log);
 #endif
-    GL.programInfos[program] = null; // uniforms no longer keep the same names after linking
     GL.populateUniformTable(program);
   },
 
@@ -4070,7 +4040,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.programs, program, 'glUseProgram', 'program');
 #endif
-    GLctx.useProgram(program ? GL.programs[program] : null);
+    GLctx.useProgram(GL.programs[program]);
   },
 
   glValidateProgram__sig: 'vi',
@@ -4119,8 +4089,7 @@ var LibraryGL = {
 #if GL_ASSERTIONS
     GL.validateGLObjectID(GL.programs, program, 'glBindAttribLocation', 'program');
 #endif
-    name = UTF8ToString(name);
-    GLctx.bindAttribLocation(GL.programs[program], index, name);
+    GLctx.bindAttribLocation(GL.programs[program], index, UTF8ToString(name));
   },
 
   glBindFramebuffer__sig: 'vii',
@@ -4135,7 +4104,7 @@ var LibraryGL = {
     // is being used, but that is ultimately decided at context creation time.
     GLctx.bindFramebuffer(target, framebuffer ? GL.framebuffers[framebuffer] : GL.currentContext.defaultFbo);
 #else
-    GLctx.bindFramebuffer(target, framebuffer ? GL.framebuffers[framebuffer] : null);
+    GLctx.bindFramebuffer(target, GL.framebuffers[framebuffer]);
 #endif
 
   },
@@ -4375,7 +4344,7 @@ var LibraryGL = {
   glVertexAttribPointer: function(index, size, type, normalized, stride, ptr) {
 #if FULL_ES2
     var cb = GL.currentContext.clientBuffers[index];
-#if ASSERTIONS
+#if GL_ASSERTIONS
     assert(cb, index);
 #endif
     if (!GL.currArrayBuffer) {
@@ -4403,7 +4372,7 @@ var LibraryGL = {
   glVertexAttribIPointer: function(index, size, type, stride, ptr) {
 #if FULL_ES3
     var cb = GL.currentContext.clientBuffers[index];
-#if ASSERTIONS
+#if GL_ASSERTIONS
     assert(cb, index);
 #endif
     if (!GL.currArrayBuffer) {
@@ -4432,7 +4401,7 @@ var LibraryGL = {
   glEnableVertexAttribArray: function(index) {
 #if FULL_ES2
     var cb = GL.currentContext.clientBuffers[index];
-#if ASSERTIONS
+#if GL_ASSERTIONS
     assert(cb, index);
 #endif
     cb.enabled = true;
@@ -4444,7 +4413,7 @@ var LibraryGL = {
   glDisableVertexAttribArray: function(index) {
 #if FULL_ES2
     var cb = GL.currentContext.clientBuffers[index];
-#if ASSERTIONS
+#if GL_ASSERTIONS
     assert(cb, index);
 #endif
     cb.enabled = false;
diff --git a/tests/test_other.py b/tests/test_other.py
index 098e3f7b00984..93906cfe5c167 100644
--- a/tests/test_other.py
+++ b/tests/test_other.py
@@ -8890,8 +8890,8 @@ def test_minimal_runtime_code_size(self):
     test_cases = [
       (asmjs + opts, hello_world_sources, {'a.html': 665, 'a.js': 518,  'a.asm.js': 741, 'a.mem': 6}),
       (opts, hello_world_sources, {'a.html': 623, 'a.js': 624, 'a.wasm': 86}),
-      (asmjs + opts, hello_webgl_sources, {'a.html': 665, 'a.js': 5605, 'a.asm.js': 11361, 'a.mem': 321}),
-      (opts, hello_webgl_sources, {'a.html': 623, 'a.js': 5568, 'a.wasm': 8978})
+      (asmjs + opts, hello_webgl_sources, {'a.html': 665, 'a.js': 5403, 'a.asm.js': 11361, 'a.mem': 321}),
+      (opts, hello_webgl_sources, {'a.html': 623, 'a.js': 5380, 'a.wasm': 8978})
     ]
 
     success = True