diff --git a/src/compiler.js b/src/compiler.js
index 3b0e8ecf3a697..424e9b3205885 100644
--- a/src/compiler.js
+++ b/src/compiler.js
@@ -169,10 +169,6 @@ EXPORTED_FUNCTIONS = set(EXPORTED_FUNCTIONS);
 EXCEPTION_CATCHING_WHITELIST = set(EXCEPTION_CATCHING_WHITELIST);
 IMPLEMENTED_FUNCTIONS = set(IMPLEMENTED_FUNCTIONS);
 
-// TODO: Implement support for proper preprocessing, e.g. "#if A || B" and "#if defined(A) || defined(B)" to
-// avoid needing this here.
-USES_GL_EMULATION = FULL_ES2 || LEGACY_GL_EMULATION;
-
 DEAD_FUNCTIONS.forEach(function(dead) {
   DEFAULT_LIBRARY_FUNCS_TO_INCLUDE.push(dead.substr(1));
 });
diff --git a/src/jsifier.js b/src/jsifier.js
index 9605cb063ff97..c11c8f3aa380d 100644
--- a/src/jsifier.js
+++ b/src/jsifier.js
@@ -42,9 +42,6 @@ var NEED_ALL_ASM2WASM_IMPORTS = BINARYEN_METHOD != 'native-wasm' || BINARYEN_TRA
 // the current compilation unit.
 var HAS_MAIN = ('_main' in IMPLEMENTED_FUNCTIONS) || MAIN_MODULE || SIDE_MODULE;
 
-var WASM_BACKEND_WITH_RESERVED_FUNCTION_POINTERS =
-  WASM_BACKEND && RESERVED_FUNCTION_POINTERS;
-
 // JSifier
 function JSify(data, functionsOnly) {
   var mainPass = !functionsOnly;
diff --git a/src/library_browser.js b/src/library_browser.js
index 1dd1fcfc104c3..4c2176763f3f5 100644
--- a/src/library_browser.js
+++ b/src/library_browser.js
@@ -1188,7 +1188,7 @@ var LibraryBrowser = {
 
       // Signal GL rendering layer that processing of a new frame is about to start. This helps it optimize
       // VBO double-buffering and reduce GPU stalls.
-#if USES_GL_EMULATION
+#if FULL_ES2 || LEGACY_GL_EMULATION
       GL.newRenderingFrameStarted();
 #endif
 
diff --git a/src/library_fs.js b/src/library_fs.js
index abcf326efb629..98b6d26a323ec 100644
--- a/src/library_fs.js
+++ b/src/library_fs.js
@@ -5,16 +5,16 @@
 
 mergeInto(LibraryManager.library, {
   $FS__deps: ['$ERRNO_CODES', '$ERRNO_MESSAGES', '__setErrNo', '$PATH', '$TTY', '$MEMFS',
-#if __EMSCRIPTEN_HAS_idbfs_js__
+#if LibraryManager.has('library_idbfs.js')
     '$IDBFS',
 #endif
-#if __EMSCRIPTEN_HAS_nodefs_js__
+#if LibraryManager.has('library_nodefs.js')
     '$NODEFS',
 #endif
-#if __EMSCRIPTEN_HAS_workerfs_js__
+#if LibraryManager.has('library_workerfs.js')
     '$WORKERFS',
 #endif
-#if __EMSCRIPTEN_HAS_noderawfs_js__
+#if LibraryManager.has('library_noderawfs.js')
     '$NODERAWFS',
 #endif
     'stdin', 'stdout', 'stderr'],
@@ -1418,13 +1418,13 @@ mergeInto(LibraryManager.library, {
 
       FS.filesystems = {
         'MEMFS': MEMFS,
-#if __EMSCRIPTEN_HAS_idbfs_js__
+#if LibraryManager.has('library_idbfs.js')
         'IDBFS': IDBFS,
 #endif
-#if __EMSCRIPTEN_HAS_nodefs_js__
+#if LibraryManager.has('library_nodefs.js')
         'NODEFS': NODEFS,
 #endif
-#if __EMSCRIPTEN_HAS_workerfs_js__
+#if LibraryManager.has('library_workerfs.js')
         'WORKERFS': WORKERFS,
 #endif
       };
diff --git a/src/library_gl.js b/src/library_gl.js
index dac225d941508..528f96db8ba19 100644
--- a/src/library_gl.js
+++ b/src/library_gl.js
@@ -40,7 +40,7 @@ var LibraryGL = {
     syncs: [],
 #endif
 
-#if USES_GL_EMULATION
+#if FULL_ES2 || LEGACY_GL_EMULATION
     currArrayBuffer: 0,
     currElementArrayBuffer: 0,
 #endif
@@ -77,7 +77,7 @@ var LibraryGL = {
     unpackAlignment: 4, // default alignment is 4 bytes
 
     init: function() {
-#if USES_GL_EMULATION
+#if FULL_ES2 || LEGACY_GL_EMULATION
       GL.createLog2ceilLookup(GL.MAX_TEMP_BUFFER_SIZE);
 #endif
       GL.miniTempBuffer = new Float32Array(GL.MINI_TEMP_BUFFER_SIZE);
@@ -113,7 +113,7 @@ var LibraryGL = {
     miniTempBuffer: null,
     miniTempBufferViews: [0], // index i has the view of size i+1
 
-#if USES_GL_EMULATION
+#if FULL_ES2 || LEGACY_GL_EMULATION
     // When user GL code wants to render from client-side memory, we need to upload the vertex data to a temp VBO
     // for rendering. Maintain a set of temp VBOs that are created-on-demand to appropriate sizes, and never destroyed.
     // Also, for best performance the VBOs are double-buffered, i.e. every second frame we switch the set of VBOs we
@@ -3642,7 +3642,7 @@ var LibraryGL = {
 #endif
     var bufferObj = buffer ? GL.buffers[buffer] : null;
 
-#if USES_GL_EMULATION
+#if FULL_ES2 || LEGACY_GL_EMULATION
     if (target == GLctx.ARRAY_BUFFER) {
       GL.currArrayBuffer = buffer;
 #if LEGACY_GL_EMULATION
@@ -4282,7 +4282,7 @@ var LibraryGL = {
 #endif
     GLctx['bindVertexArray'](GL.vaos[vao]);
 #endif
-#if USES_GL_EMULATION
+#if FULL_ES2 || LEGACY_GL_EMULATION
     var ibo = GLctx.getParameter(GLctx.ELEMENT_ARRAY_BUFFER_BINDING);
     GL.currElementArrayBuffer = ibo ? (ibo.name | 0) : 0;
 #endif
diff --git a/src/library_vr.js b/src/library_vr.js
index b1811ae651cab..1dd29cadbc2f9 100644
--- a/src/library_vr.js
+++ b/src/library_vr.js
@@ -195,7 +195,7 @@ var LibraryWebVR = {
         /* Prevent scheduler being called twice when loop is changed */
         display.mainLoop.running = true;
 
-#if USES_GL_EMULATION
+#if FULL_ES2 || LEGACY_GL_EMULATION
         GL.newRenderingFrameStarted();
 #endif
 
diff --git a/src/modules.js b/src/modules.js
index e8100efa42040..18c7bf9b5ce07 100644
--- a/src/modules.js
+++ b/src/modules.js
@@ -97,6 +97,11 @@ var LibraryManager = {
   library: null,
   structs: {},
   loaded: false,
+  libraries: [],
+
+  has: function(name) {
+    return this.libraries.indexOf(name) >= 0;
+  },
 
   load: function() {
     if (this.library) return;
@@ -163,18 +168,15 @@ var LibraryManager = {
 
     libraries = libraries.concat(additionalLibraries);
 
-    // For each JS library library_xxx.js, add a preprocessor token __EMSCRIPTEN_HAS_xxx_js__ so that code can conditionally dead code eliminate out
-    // if a particular feature is not being linked in.
-    for (var i = 0; i < libraries.length; ++i) {
-      global['__EMSCRIPTEN_HAS_' + libraries[i].replace('.', '_').replace('library_', '') + '__'] = 1
-    }
-
     if (BOOTSTRAPPING_STRUCT_INFO) libraries = ['library_bootstrap_structInfo.js', 'library_formatString.js'];
     if (ONLY_MY_CODE) {
-      libraries = [];
+      libraries.length = 0;
       LibraryManager.library = {};
     }
 
+    // Save the list for has() queries later.
+    this.libraries = libraries;
+
     for (var i = 0; i < libraries.length; i++) {
       var filename = libraries[i];
       var src = read(filename);
diff --git a/src/parseTools.js b/src/parseTools.js
index 5f5fd08e6e2a9..8700db430c3cd 100644
--- a/src/parseTools.js
+++ b/src/parseTools.js
@@ -42,47 +42,9 @@ function preprocess(text, filenameHint) {
         if (line[1] == 'i') {
           if (line[2] == 'f') { // if
             var parts = line.split(' ');
-            var ident = parts[1];
-            var op = parts[2];
-            var value = parts[3];
-            if (typeof value === 'string') {
-              // when writing
-              // #if option == 'stringValue'
-              // we need to get rid of the quotes
-              if (value[0] === '"' || value[0] === "'") {
-                assert(value[value.length - 1] == '"' || value[value.length - 1] == "'");
-                value = value.substring(1, value.length - 1);
-              }
-            }
-            if (op) {
-              if (op === '==') {
-                showStack.push(ident in this && this[ident] == value);
-              } else if (op === '!=') {
-                showStack.push(!(ident in this && this[ident] == value));
-              } else if (op === '<') {
-                showStack.push(ident in this && this[ident] < value);
-              } else if (op === '<=') {
-                showStack.push(ident in this && this[ident] <= value);
-              } else if (op === '>') {
-                showStack.push(ident in this && this[ident] > value);
-              } else if (op === '>=') {
-                showStack.push(ident in this && this[ident] >= value);
-              } else {
-                error('unsupported preprocessor op ' + op);
-              }
-            } else {
-              // Check if a value is truthy.
-              var short = ident[0] === '!' ? ident.substr(1) : ident;
-              var truthy = short in this;
-              if (truthy) {
-                truthy = !!this[short];
-              }
-              if (ident[0] === '!') {
-                showStack.push(!truthy);
-              } else {
-                showStack.push(truthy);
-              }
-            }
+            var after = parts.slice(1).join(' ');
+            var truthy = !!eval(after);
+            showStack.push(truthy);
           } else if (line[2] == 'n') { // include
             var filename = line.substr(line.indexOf(' ')+1);
             if (filename.indexOf('"') === 0) {
diff --git a/src/support.js b/src/support.js
index 0c857d0d11db7..532e2e1da24b2 100644
--- a/src/support.js
+++ b/src/support.js
@@ -603,15 +603,15 @@ Module['registerFunctions'] = registerFunctions;
 #endif // RELOCATABLE
 #endif // EMULATED_FUNCTION_POINTERS
 
-#if WASM_BACKEND_WITH_RESERVED_FUNCTION_POINTERS
+#if WASM_BACKEND && RESERVED_FUNCTION_POINTERS
 var jsCallStartIndex = {{{ JSCALL_START_INDEX }}};
 var jsCallSigOrder = {{{ JSON.stringify(JSCALL_SIG_ORDER) }}};
 var jsCallNumSigs = Object.keys(jsCallSigOrder).length;
 var functionPointers = new Array(jsCallNumSigs * {{{ RESERVED_FUNCTION_POINTERS }}});
-#else // WASM_BACKEND_WITH_RESERVED_FUNCTION_POINTERS == 0
+#else // WASM_BACKEND && RESERVED_FUNCTION_POINTERS == 0
 var jsCallStartIndex = 1;
 var functionPointers = new Array({{{ RESERVED_FUNCTION_POINTERS }}});
-#endif // WASM_BACKEND_WITH_RESERVED_FUNCTION_POINTERS
+#endif // WASM_BACKEND && RESERVED_FUNCTION_POINTERS
 
 // 'sig' parameter is only used on LLVM wasm backend
 function addFunction(func, sig) {
@@ -626,11 +626,11 @@ function addFunction(func, sig) {
   }
 #endif // ASSERTIONS
 #if EMULATED_FUNCTION_POINTERS == 0
-#if WASM_BACKEND_WITH_RESERVED_FUNCTION_POINTERS
+#if WASM_BACKEND && RESERVED_FUNCTION_POINTERS
   var base = jsCallSigOrder[sig] * {{{ RESERVED_FUNCTION_POINTERS }}};
-#else // WASM_BACKEND_WITH_RESERVED_FUNCTION_POINTERS == 0
+#else // WASM_BACKEND && RESERVED_FUNCTION_POINTERS == 0
   var base = 0;
-#endif // WASM_BACKEND_WITH_RESERVED_FUNCTION_POINTERS
+#endif // WASM_BACKEND && RESERVED_FUNCTION_POINTERS
   for (var i = base; i < base + {{{ RESERVED_FUNCTION_POINTERS }}}; i++) {
     if (!functionPointers[i]) {
       functionPointers[i] = func;