Avoid mangling + demangling in jsifier.js (#13230)

We had a bug that only showed up with symbols that start with `$_`.
These happen to exist in `library_fetch.js` e.g:
`$__emscripten_fetch_xhr`.  In this case, when we mangle, we remove the
leading `$` giving `__emscripten_fetch_xhr`, and then if we demangle we
do that wrong thing and end up removing the `_` instead of re-adding the
  `$`.

This change avoids the round trip by always storing both the mangled
and demangled version of a given symbol.

See #12268

src/jsifier.js

@@ -28,11 +28,6 @@ function mangleCSymbolName(f) {
   return f[0] == '$' ? f.substr(1) : '_' + f;
 }
 
-// Reverses C/JS name mangling: _foo -> foo, and foo -> $foo.
-function demangleCSymbolName(f) {
-  return f[0] == '_' ? f.substr(1) : '$' + f;
-}
-
 // Splits out items that pass filter. Returns also the original sans the filtered
 function splitter(array, filter) {
   var splitOut = array.filter(filter);
@@ -87,7 +82,8 @@ function JSify(data, functionsOnly) {
     }
     libFuncsToInclude.forEach(function(ident) {
       data.functionStubs.push({
-        ident: mangleCSymbolName(ident)
+        identOrig: ident,
+        identMangled: mangleCSymbolName(ident)
       });
     });
   }
@@ -118,31 +114,32 @@ function JSify(data, functionsOnly) {
     // In LLVM, exceptions generate a set of functions of form __cxa_find_matching_catch_1(), __cxa_find_matching_catch_2(), etc.
     // where the number specifies the number of arguments. In Emscripten, route all these to a single function '__cxa_find_matching_catch'
     // that variadically processes all of these functions using JS 'arguments' object.
-    if (item.ident.startsWith('___cxa_find_matching_catch_')) {
+    if (item.identMangled.startsWith('___cxa_find_matching_catch_')) {
       if (DISABLE_EXCEPTION_THROWING) {
         error('DISABLE_EXCEPTION_THROWING was set (likely due to -fno-exceptions), which means no C++ exception throwing support code is linked in, but exception catching code appears. Either do not set DISABLE_EXCEPTION_THROWING (if you do want exception throwing) or compile all source files with -fno-except (so that no exceptions support code is required); also make sure DISABLE_EXCEPTION_CATCHING is set to the right value - if you want exceptions, it should be off, and vice versa.');
         return;
       }
-      var num = +item.ident.split('_').slice(-1)[0];
+      var num = +item.identMangled.split('_').slice(-1)[0];
       addCxaCatch(num);
       // Continue, with the code below emitting the proper JavaScript based on
       // what we just added to the library.
     }
 
-    function addFromLibrary(ident, dependent) {
+    function addFromLibrary(item, dependent) {
+      // dependencies can be JS functions, which we just run
+      if (typeof item == 'function') return item();
+
+      var ident = item.identOrig;
+      var finalName = item.identMangled;
+
       if (ident in addedLibraryItems) return '';
       addedLibraryItems[ident] = true;
 
-      // dependencies can be JS functions, which we just run
-      if (typeof ident == 'function') return ident();
-
       // don't process any special identifiers. These are looked up when processing the base name of the identifier.
       if (isJsLibraryConfigIdentifier(ident)) {
         return '';
       }
 
-      var finalName = mangleCSymbolName(ident);
-
       // if the function was implemented in compiled code, we just need to export it so we can reach it from JS
       if (finalName in IMPLEMENTED_FUNCTIONS) {
         EXPORTED_FUNCTIONS[finalName] = 1;
@@ -182,13 +179,11 @@ function JSify(data, functionsOnly) {
           // (not useful to warn/error multiple times)
           LibraryManager.library[ident + '__docs'] = '/** @type {function(...*):?} */';
         } else {
-          var realIdent = ident;
-
-          var target = "Module['" + mangleCSymbolName(realIdent) + "']";
+          var target = "Module['" + finalName + "']";
           var assertion = '';
           if (ASSERTIONS) {
             var what = 'function';
-            assertion += 'if (!' + target + ') abort("external symbol \'' + realIdent + '\' is missing. perhaps a side module was not linked in? if this function was expected to arrive from a system library, try to build the MAIN_MODULE with EMCC_FORCE_STDLIBS=1 in the environment");\n';
+            assertion += 'if (!' + target + ') abort("external symbol \'' + ident + '\' is missing. perhaps a side module was not linked in? if this function was expected to arrive from a system library, try to build the MAIN_MODULE with EMCC_FORCE_STDLIBS=1 in the environment");\n';
 
           }
           var functionBody = assertion + "return " + target + ".apply(null, arguments);";
@@ -274,9 +269,14 @@ function JSify(data, functionsOnly) {
           }
         });
       });
-      if (VERBOSE) printErr('adding ' + finalName + ' and deps ' + deps + ' : ' + (snippet + '').substr(0, 40));
+      if (VERBOSE) {
+        printErr('adding ' + finalName + ' and deps ' + deps + ' : ' + (snippet + '').substr(0, 40));
+      }
       var identDependents = ident + "__deps: ['" + deps.join("','")+"']";
       function addDependency(dep) {
+        if (typeof dep !== 'function') {
+          dep = {identOrig: dep, identMangled: mangleCSymbolName(dep)};
+        }
         return addFromLibrary(dep, identDependents + ', referenced by ' + dependent);
       }
       var depsText = (deps ? deps.map(addDependency).filter(function(x) { return x != '' }).join('\n') + '\n' : '');
@@ -332,8 +332,7 @@ function JSify(data, functionsOnly) {
     }
 
     itemsDict.functionStub.push(item);
-    var shortident = demangleCSymbolName(item.ident);
-    item.JS = addFromLibrary(shortident, 'top-level compiled C/C++ code');
+    item.JS = addFromLibrary(item, 'top-level compiled C/C++ code');
   }
 
   // Final combiner

tests/test_other.py

@@ -9713,6 +9713,14 @@ exec "$@"
     proc = self.run_process([EMCC, path_from_root('tests', 'hello_world.c'), '--js-library=lib.js'], stderr=PIPE)
     self.assertContained('warning: use of #ifdef in js library.  Use #if instead.', proc.stderr)
 
+  def test_jslib_mangling(self):
+    create_test_file('lib.js', '''
+      mergeInto(LibraryManager.library, {
+       $__foo: function() { return 43; },
+      });
+      ''')
+    self.run_process([EMCC, path_from_root('tests', 'hello_world.c'), '--js-library=lib.js', '-s', 'DEFAULT_LIBRARY_FUNCS_TO_INCLUDE=[$__foo]'])
+
   def test_wasm2js_no_dynamic_linking(self):
     for arg in ['-sMAIN_MODULE', '-sSIDE_MODULE', '-sRELOCATABLE']:
       err = self.expect_fail([EMCC, path_from_root('tests', 'hello_world.c'), '-sMAIN_MODULE', '-sWASM=0'])