Delay loading of embedded file data until static constructors are run (#17209)

This code was relying on static data relocations being applied during
module instantiation, but a recent change to llvm reverts to the
previous behaviour of applying relocation during `__wasm_call_ctors`:
https://reviews.llvm.org/D127333

This change delays the loading of embedded files until static ctor time
after the relocations have been applied.  As a followup I think we can
look into completely removing the pre-js in this case.

Author: sbc100

src/library.js

@@ -3625,6 +3625,33 @@ mergeInto(LibraryManager.library, {
   __c_longjmp_import: true,
 #endif
 #endif
+
+  _emscripten_fs_load_embedded_files__deps: ['$FS'],
+  _emscripten_fs_load_embedded_files__sig: 'vp',
+  _emscripten_fs_load_embedded_files: function(ptr) {
+#if MEMORY64
+    var start64 = ptr >> 3;
+    do {
+      var name_addr = Number(HEAPU64[start64++]);
+      var len = HEAPU32[start64 << 1];
+      start64++;
+      var content = Number(HEAPU64[start64++]);
+      var name = UTF8ToString(name_addr)
+      // canOwn this data in the filesystem, it is a slice of wasm memory that will never change
+      FS.createDataFile(name, null, HEAP8.subarray(content, content + len), true, true, true);
+    } while (HEAPU64[start64]);
+#else
+    var start32 = ptr >> 2;
+    do {
+      var name_addr = HEAPU32[start32++];
+      var len = HEAPU32[start32++];
+      var content = HEAPU32[start32++];
+      var name = UTF8ToString(name_addr)
+      // canOwn this data in the filesystem, it is a slice of wasm memory that will never change
+      FS.createDataFile(name, null, HEAP8.subarray(content, content + len), true, true, true);
+    } while (HEAPU32[start32]);
+#endif
+  },
 });
 
 function autoAddDeps(object, name) {

system/lib/README.md

@@ -19,6 +19,7 @@ These current set of static constructors in system libraries and their prioritie
 - 1: `emscripten_stack_init` (stack_limits.S)
 - 47: `initialize_emmalloc_heap` (emmalloc.c)
 - 48: `__emscripten_init_main_thread` (pthread/library_pthread.c)
+- 49: `init_file_data` (generated by file_packager.py)
 - 50: asan init (??)
 - 100: `WasmFS wasmFS` (wasmfs/wasmfs.cpp)
 

tools/file_packager.py

@@ -290,18 +290,36 @@ def generate_object_file(data_files):
 
     if options.wasm64:
       align = 3
+      ptr_type = 'i64'
+      bits = 64
     else:
       align = 2
-    out.write(dedent('''
+      ptr_type = 'i32'
+      bits = 32
+    out.write(dedent(f'''
+      .functype _emscripten_fs_load_embedded_files ({ptr_type}) -> ()
+      .section .text,"",@
+      init_file_data:
+        .functype init_file_data () -> ()
+        global.get __emscripten_embedded_file_data@GOT
+        call _emscripten_fs_load_embedded_files
+        end_function
+
+      # Run init_file_data on startup.
+      # See system/lib/README.md for ordering of system constructors.
+      .section .init_array.49,"",@
+      .p2align {align}
+      .int{bits} init_file_data
+
       # A list of triples of:
       # (file_name_ptr, file_data_size, file_data_ptr)
       # The list in null terminate with a single 0
       .globl __emscripten_embedded_file_data
       .export_name __emscripten_embedded_file_data, __emscripten_embedded_file_data
       .section .rodata.__emscripten_embedded_file_data,"",@
       __emscripten_embedded_file_data:
-      .p2align %s
-      ''' % align))
+      .p2align {align}
+      '''))
 
     for f in embed_files:
       # The `.dc.a` directive gives us a pointer (address) sized entry.
@@ -654,35 +672,10 @@ def generate_js(data_target, data_files, metadata):
         new DataRequest(files[i]['start'], files[i]['end'], files[i]['audio'] || 0).open('GET', files[i]['filename']);
       }\n''' % (create_preloaded if options.use_preload_plugins else create_data)
 
-  if options.has_embedded:
-    if options.obj_output:
-      if options.wasm64:
-        code += '''\
-          var start64 = Module['___emscripten_embedded_file_data'] >> 3;
-          do {
-            var name_addr = Number(HEAPU64[start64++]);
-            var len = HEAPU32[start64 << 1];
-            start64++;
-            var content = Number(HEAPU64[start64++]);
-            var name = UTF8ToString(name_addr)
-            // canOwn this data in the filesystem, it is a slice of wasm memory that will never change
-            Module['FS_createDataFile'](name, null, HEAP8.subarray(content, content + len), true, true, true);
-          } while (HEAPU64[start64]);'''
-      else:
-        code += '''\
-          var start32 = Module['___emscripten_embedded_file_data'] >> 2;
-          do {
-            var name_addr = HEAPU32[start32++];
-            var len = HEAPU32[start32++];
-            var content = HEAPU32[start32++];
-            var name = UTF8ToString(name_addr)
-            // canOwn this data in the filesystem, it is a slice of wasm memory that will never change
-            Module['FS_createDataFile'](name, null, HEAP8.subarray(content, content + len), true, true, true);
-          } while (HEAPU32[start32]);'''
-    else:
-      err('--obj-output is recommended when using --embed.  This outputs an object file for linking directly into your application is more effecient than JS encoding')
+  if options.has_embedded and not options.obj_output:
+    err('--obj-output is recommended when using --embed.  This outputs an object file for linking directly into your application is more effecient than JS encoding')
 
-  for (counter, file_) in enumerate(data_files):
+  for counter, file_ in enumerate(data_files):
     filename = file_.dstpath
     dirname = os.path.dirname(filename)
     basename = os.path.basename(filename)