gh-145037: Fix Emscripten trampoline with emcc >= 4.0.19

This undoes a change made as a part of PR 137470. We add an `emscripten_trampoline`
field in `pycore_runtime_structs.h` and initialize it from JS initialization code with
the wasm-gc based trampoline if possible. Otherwise we fall back to the JS trampoline.

Author: hoodmane

Include/internal/pycore_emscripten_trampoline.h

@@ -27,9 +27,6 @@
 
 #if defined(__EMSCRIPTEN__) && defined(PY_CALL_TRAMPOLINE)
 
-void
-_Py_EmscriptenTrampoline_Init(_PyRuntimeState *runtime);
-
 PyObject*
 _PyEM_TrampolineCall(PyCFunctionWithKeywords func,
                      PyObject* self,

Include/internal/pycore_runtime_structs.h

@@ -275,6 +275,12 @@ struct pyruntimestate {
     struct _types_runtime_state types;
     struct _Py_time_runtime_state time;
 
+#if defined(__EMSCRIPTEN__) && defined(PY_CALL_TRAMPOLINE)
+    // Used in "Python/emscripten_trampoline.c" to choose between type
+    // reflection trampoline and EM_JS trampoline.
+    int (*emscripten_trampoline)(PyCFunctionWithKeywords func);
+#endif
+
     /* All the objects that are shared by the runtime's interpreters. */
     struct _Py_cached_objects cached_objects;
     struct _Py_static_objects static_objects;

Python/emscripten_trampoline.c

@@ -3,19 +3,57 @@
 #include <emscripten.h>             // EM_JS, EM_JS_DEPS
 #include <Python.h>
 
-EM_JS(
-PyObject*,
-_PyEM_TrampolineCall_inner, (int* success,
-                             PyCFunctionWithKeywords func,
-                             PyObject *arg1,
-                             PyObject *arg2,
-                             PyObject *arg3), {
-    // JavaScript fallback trampoline
+// We use the _PyRuntime.emscripten_trampoline field to store a function pointer
+// for a wasm-gc based trampoline if it works. Otherwise fall back to JS
+// trampoline. The JS trampoline breaks stack switching but every runtime that
+// supports stack switching also supports wasm-gc.
+//
+// We'd like to make the trampoline call into a direct call but currently we
+// need to import the wasmTable to compile trampolineModule. emcc >= 4.0.19
+// defines the table in WebAssembly and exports it so we won't have access to it
+// until after the main module is compiled.
+//
+// To fix this, one natural solution would be to pass a funcref to the
+// trampoline instead of a table index. Several PRs would be needed to fix
+// things in llvm and emscripten in order to make this possible.
+//
+// The performance costs of an extra call_indirect aren't that large anyways.
+// The JIT should notice that the target is always the same and turn into a
+// check
+//
+// if (call_target != expected) deoptimize;
+// direct_call(call_target, args);
+
+// Offset of emscripten_trampoline in _PyRuntimeState. There's a couple of
+// alternatives:
+//
+// 1. Just make emscripten_trampoline a real C global variable instead of a
+//    field of _PyRuntimeState. This would violate our rule against mutable
+//    globals.
+//
+// 2. #define a preprocessor constant equal to a hard coded number and make a
+//    _Static_assert(offsetof(_PyRuntimeState, emscripten_trampoline) == OURCONSTANT)
+//    This has the disadvantage that we have to update the hard coded constant
+//    when _PyRuntimeState changes
+//
+// So putting the mutable constant in _PyRuntime and using a immutable global to
+// record the offset so we can access it from JS is probably the best way.
+EMSCRIPTEN_KEEPALIVE const int _PyEM_EMSCRIPTEN_TRAMPOLINE_OFFSET = offsetof(_PyRuntimeState, emscripten_trampoline);
+
+typedef PyObject* (*TrampolineFunc)(int* success,
+                                    void* func,
+                                    PyObject* self,
+                                    PyObject* args,
+                                    PyObject* kw);
+
+/**
+ * Backwards compatible trampoline works with all JS runtimes
+ */
+EM_JS(PyObject*, _PyEM_TrampolineCall_JS, (PyCFunctionWithKeywords func, PyObject *arg1, PyObject *arg2, PyObject *arg3), {
     return wasmTable.get(func)(arg1, arg2, arg3);
 }
-// Try to replace the JS definition of _PyEM_TrampolineCall_inner with a wasm
-// version.
-(function () {
+// Try to compile wasm-gc trampoline if possible.
+function getPyEMTrampolinePtr() {
     // Starting with iOS 18.3.1, WebKit on iOS has an issue with the garbage
     // collector that breaks the call trampoline. See #130418 and
     // https://bugs.webkit.org/show_bug.cgi?id=293113 for details.
@@ -27,19 +65,32 @@ _PyEM_TrampolineCall_inner, (int* success,
         (navigator.platform === 'MacIntel' && typeof navigator.maxTouchPoints !== 'undefined' && navigator.maxTouchPoints > 1)
     );
     if (isIOS) {
-        return;
+        return 0;
     }
+    let trampolineModule;
     try {
-        const trampolineModule = getWasmTrampolineModule();
-        const trampolineInstance = new WebAssembly.Instance(trampolineModule, {
-            env: { __indirect_function_table: wasmTable, memory: wasmMemory },
-        });
-        _PyEM_TrampolineCall_inner = trampolineInstance.exports.trampoline_call;
+        trampolineModule = getWasmTrampolineModule();
     } catch (e) {
         // Compilation error due to missing wasm-gc support, fall back to JS
         // trampoline
+        return 0;
     }
-})();
+    const trampolineInstance = new WebAssembly.Instance(trampolineModule, {
+        env: { __indirect_function_table: wasmTable, memory: wasmMemory },
+    });
+    return addFunction(trampolineInstance.exports.trampoline_call);
+}
+// We have to be careful to work correctly with memory snapshots -- the value of
+// _PyRuntimeState.emscripten_trampoline needs to reflect whether wasm-gc is
+// available in the current runtime, not in the runtime the snapshot was taken
+// in. This writes the appropriate value to
+// _PyRuntimeState.emscripten_trampoline from JS startup code that runs every
+// time, whether we are restoring a snapshot or not.
+addOnPreRun(function setEmscriptenTrampoline() {
+    const ptr = getPyEMTrampolinePtr();
+    const offset = HEAP32[__PyEM_EMSCRIPTEN_TRAMPOLINE_OFFSET / 4];
+    HEAP32[(__PyRuntime + offset) / 4] = ptr;
+});
 );
 
 PyObject*
@@ -48,12 +99,18 @@ _PyEM_TrampolineCall(PyCFunctionWithKeywords func,
                      PyObject* args,
                      PyObject* kw)
 {
-    int success = 1;
-    PyObject *result = _PyEM_TrampolineCall_inner(&success, func, self, args, kw);
+    TrampolineFunc trampoline = _PyRuntime.emscripten_trampoline;
+    if (trampoline == 0) {
+        return _PyEM_TrampolineCall_JS(func, self, args, kw);
+    }
+    PyObject *result = trampoline(&success, func, self, args, kw);
     if (!success) {
         PyErr_SetString(PyExc_SystemError, "Handler takes too many arguments");
     }
     return result;
 }
 
+#else
+// This is exported so we need to define it even when it isn't used
+__attribute__((used)) const int _PyEM_EMSCRIPTEN_TRAMPOLINE_OFFSET = 0;
 #endif

configure

@@ -2357,7 +2357,7 @@ AS_CASE([$ac_sys_system],
     dnl Include file system support
     AS_VAR_APPEND([LINKFORSHARED], [" -sFORCE_FILESYSTEM -lidbfs.js -lnodefs.js -lproxyfs.js -lworkerfs.js"])
     AS_VAR_APPEND([LINKFORSHARED], [" -sEXPORTED_RUNTIME_METHODS=FS,callMain,ENV,HEAPU32,TTY"])
-    AS_VAR_APPEND([LINKFORSHARED], [" -sEXPORTED_FUNCTIONS=_main,_Py_Version,__PyRuntime,_PyGILState_GetThisThreadState,__Py_DumpTraceback"])
+    AS_VAR_APPEND([LINKFORSHARED], [" -sEXPORTED_FUNCTIONS=_main,_Py_Version,__PyRuntime,_PyGILState_GetThisThreadState,__Py_DumpTraceback,__PyEM_EMSCRIPTEN_TRAMPOLINE_OFFSET"])
     AS_VAR_APPEND([LINKFORSHARED], [" -sSTACK_SIZE=5MB"])
     dnl Avoid bugs in JS fallback string decoding path
     AS_VAR_APPEND([LINKFORSHARED], [" -sTEXTDECODER=2"])