Prepare for fastcomp update to fix cross-module function pointers (em…

…scripten-core#9393)

There is upstream fastcomp change coming which addresses function
pointer equality between wasm modules (i.e. SIDE_MODULE/MAIN_MODULE):
emscripten-core/emscripten-fastcomp#259

This change prepares for that change by handling the new
externFunctions metadata attribute.  This new key represents a list
of all non-static functions in a relocatable module which are address
taken.  The module will call fp$<name>() in order to find out the
runtime address (table entry) for such functions.

Once both these changes land they they will fix emscripten-core#8268.

emscripten.py

@@ -149,6 +149,10 @@ def parse_fastcomp_output(backend_output, DEBUG):
     logger.error('emscript: failure to parse metadata output from compiler backend. raw output is: \n' + metadata_raw)
     raise
 
+  # This key is being added to fastcomp but doesn't exist in the current
+  # version.
+  metadata.setdefault('externFunctions', [])
+
   if 'externUses' not in metadata:
     exit_with_error('Your fastcomp compiler is out of date, please update! (need >= 1.38.26)')
 
@@ -354,6 +358,7 @@ def move_preasm(m):
   global_funcs = sorted(global_funcs.difference(set(global_vars)).difference(implemented_functions))
   if shared.Settings.RELOCATABLE:
     global_funcs += ['g$' + extern for extern in metadata['externs']]
+    global_funcs += ['fp$' + extern for extern in metadata['externFunctions']]
 
   # Tracks the set of used (minified) function names in
   # JS symbols imported to asm.js module.
@@ -663,6 +668,7 @@ def update_settings_glue(metadata, DEBUG):
   all_funcs = shared.Settings.DEFAULT_LIBRARY_FUNCS_TO_INCLUDE + [shared.JS.to_nice_ident(d) for d in metadata['declares']]
   implemented_funcs = [x[1:] for x in metadata['implementedFunctions']]
   shared.Settings.DEFAULT_LIBRARY_FUNCS_TO_INCLUDE = sorted(set(all_funcs).difference(implemented_funcs))
+
   shared.Settings.DEFAULT_LIBRARY_FUNCS_TO_INCLUDE += [x[1:] for x in metadata['externs']]
 
   if metadata['simd']:
@@ -1503,14 +1509,27 @@ def create_asm_setup(debug_tables, function_table_data, invoke_function_names, m
 
     def check(extern):
       if shared.Settings.ASSERTIONS:
-        return ('\n  assert(%sModule["%s"], "external global `%s` is missing.' % (side, extern, extern) +
+        return ('\n  assert(%sModule["%s"] || %s, "external symbol `%s` is missing.' % (side, extern, extern, extern) +
                 'perhaps a side module was not linked in? if this symbol was expected to arrive '
                 'from a system library, try to build the MAIN_MODULE with '
                 'EMCC_FORCE_STDLIBS=1 in the environment");')
       return ''
 
     for extern in metadata['externs']:
       asm_setup += 'var g$' + extern + ' = function() {' + check(extern) + '\n  return ' + side + 'Module["' + extern + '"];\n}\n'
+    for extern in metadata['externFunctions']:
+      barename, sig = extern.split('$')
+      fullname = "fp$" + extern
+      key = '%sModule["%s"]' % (side, fullname)
+      asm_setup += '''\
+    var %s = function() {
+      if (!%s) { %s
+        var fid = addFunction(%sModule["%s"] || %s, "%s");
+        %s = fid;
+      }
+      return %s;
+    }
+    ''' % (fullname, key, check(barename), side, barename, barename, sig, key, key)
 
   asm_setup += create_invoke_wrappers(invoke_function_names)
   asm_setup += setup_function_pointers(function_table_sigs)
@@ -1778,7 +1797,7 @@ def create_fp_accessors(metadata):
   var func = Module['%(mangled)s'];
   if (!func)
     func = %(mangled)s;
-  var fp = addFunctionWasm(func, '%(sig)s');
+  var fp = addFunction(func, '%(sig)s');
   Module['%(full)s'] = function() { return fp };
   return fp;
 }

src/support.js

@@ -500,7 +500,7 @@ function loadWebAssemblyModule(binary, flags) {
           return obj[prop] = function() {
             if (!fp) {
               var f = resolveSymbol(name, 'function');
-              fp = addFunctionWasm(f, sig);
+              fp = addFunction(f, sig);
             }
             return fp;
           };
@@ -763,10 +763,13 @@ function removeFunctionWasm(index) {
 // 'sig' parameter is required for the llvm backend but only when func is not
 // already a WebAssembly function.
 function addFunction(func, sig) {
+#if ASSERTIONS
+  assert(typeof func !== 'undefined');
 #if ASSERTIONS == 2
   if (typeof sig === 'undefined') {
     err('warning: addFunction(): You should provide a wasm function signature string as a second argument. This is not necessary for asm.js and asm2wasm, but can be required for the LLVM wasm backend, so it is recommended for full portability.');
   }
+#endif // ASSERTIONS == 2
 #endif // ASSERTIONS
 
 #if WASM_BACKEND

tests/test_core.py

@@ -3638,9 +3638,12 @@ def test_dylink_asmjs_funcpointers(self):
         printf("main\n");
         EM_ASM({
           // make the function table sizes a non-power-of-two
-          alignFunctionTables();
-          Module['FUNCTION_TABLE_v'].push(0, 0, 0, 0, 0);
           var newSize = alignFunctionTables();
+          //out('old size of function tables: ' + newSize);
+          while ((newSize & 3) !== 3) {
+            Module['FUNCTION_TABLE_v'].push(0);
+            newSize = alignFunctionTables();
+          }
           //out('new size of function tables: ' + newSize);
           // when masked, the two function pointers 1 and 2 should not happen to fall back to the right place
           assert(((newSize+1) & 3) !== 1 || ((newSize+2) & 3) !== 2);