Emit EM_JS/EM_ASM code after JS library code (#23878)

This change fixes an issue with trailing code in EM_JS macros. See
#23875 and #23884
#23875

The other advantage of this ordering is that it removes the needs for
the magic macro here.

Author: sbc100

src/jsifier.mjs

@@ -767,9 +767,11 @@ function(${args}) {
     const preFile = MINIMAL_RUNTIME ? 'preamble_minimal.js' : 'preamble.js';
     includeSystemFile(preFile);
 
+    writeOutput('// Begin JS library code\n');
     for (const item of libraryItems.concat(postSets)) {
       writeOutput(indentify(item || '', 2));
     }
+    writeOutput('// End JS library code\n');
 
     if (PTHREADS) {
       writeOutput(`

src/preamble.js

@@ -1084,5 +1084,3 @@ function getCompilerSetting(name) {
 // dynamic linker as symbols are loaded.
 var asyncifyStubs = {};
 #endif
-
-<<< EM_JS_CODE >>>

src/preamble_minimal.js

@@ -54,5 +54,3 @@ var runtimeExited = false;
 #if IMPORTED_MEMORY
 #include "runtime_init_memory.js"
 #endif // IMPORTED_MEMORY
-
-<<< EM_JS_CODE >>>

test/browser/test_small_js_flags.js.size

@@ -1 +1 @@
-4137
+4133

test/code_size/embind_hello_wasm.json

@@ -1,10 +1,10 @@
 {
   "a.html": 552,
   "a.html.gz": 380,
-  "a.js": 9746,
+  "a.js": 9750,
   "a.js.gz": 4262,
   "a.wasm": 7348,
   "a.wasm.gz": 3375,
-  "total": 17646,
+  "total": 17650,
   "total_gz": 8017
 }

test/other/test_unoptimized_code_size.js.size

@@ -1 +1 @@
-52624
+52673

test/other/test_unoptimized_code_size_no_asserts.js.size

@@ -1 +1 @@
-27506
+27555

test/other/test_unoptimized_code_size_strict.js.size

@@ -1 +1 @@
-50912
+50961

test/test_other.py

@@ -13505,6 +13505,29 @@ def test_em_js_external_usage(self):
     self.run_process([EMCC, 'em_js.c', '-c'])
     self.do_runf('main.c', 'js_func called\n', emcc_args=['em_js.o'])
 
+  def test_em_js_top_level(self):
+    # It turns out that EM_JS can be used to inject top level JS code.
+    # This test verifies that it works, despite it not being an officially
+    # supported feature.
+    # See https://github.com/emscripten-core/emscripten/issues/23884
+    create_file('main.c', r'''
+      #include <emscripten/em_js.h>
+
+      EM_JS_DEPS(deps, "$addOnPreRun");
+
+      EM_JS(void, js_func, (), {
+        out('js_func called');
+      }
+      console.log("Top level code");
+      addOnPreRun(() => console.log("hello from pre-run"));
+      );
+
+      int main() {
+        js_func();
+      }
+    ''')
+    self.do_runf('main.c', 'Top level code\nhello from pre-run\njs_func called\n')
+
   # On Windows maximum command line length is 32767 characters. Create such a long build line by linking together
   # several .o files to test that emcc internally uses response files properly when calling llvm-nm and wasm-ld.
   @is_slow_test

tools/emscripten.py

@@ -414,12 +414,10 @@ def emscript(in_wasm, out_wasm, outfile_js, js_syms, finalize=True, base_metadat
     pre = apply_static_code_hooks(forwarded_json, pre)
 
   asm_const_pairs = ['%s: %s' % (key, value) for key, value in asm_consts]
-  em_js_code = ''
   if asm_const_pairs or settings.MAIN_MODULE:
-    em_js_code += 'var ASM_CONSTS = {\n  ' + ',  \n '.join(asm_const_pairs) + '\n};\n'
+    pre += 'var ASM_CONSTS = {\n  ' + ',  \n '.join(asm_const_pairs) + '\n};\n'
   if em_js_funcs:
-    em_js_code += '\n'.join(em_js_funcs) + '\n'
-  pre = shared.do_replace(pre, '<<< EM_JS_CODE >>>', em_js_code)
+    pre += '\n'.join(em_js_funcs) + '\n'
 
   if base_metadata:
     function_exports = base_metadata.function_exports