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

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 +275,16 @@ 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 wasm-gc
+ // trampoline and JavaScript trampoline.
+ PyObject* (*emscripten_trampoline)(int* success,
+ PyCFunctionWithKeywords func,
+ PyObject* self,
+ PyObject* args,
+ PyObject* kw);
+#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

@@ -2,20 +2,59 @@
 
 #include <emscripten.h> // EM_JS, EM_JS_DEPS
 #include <Python.h>
+#include "pycore_runtime.h" // _PyRuntime
 
-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,
+ PyCFunctionWithKeywords 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 +66,32 @@ _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 +100,19 @@ _PyEM_TrampolineCall(PyCFunctionWithKeywords func,
  PyObject* args,
  PyObject* kw)
 {
+ TrampolineFunc trampoline = _PyRuntime.emscripten_trampoline;
+ if (trampoline == 0) {
+ return _PyEM_TrampolineCall_JS(func, self, args, kw);
+ }
  int success = 1;
- PyObject *result = _PyEM_TrampolineCall_inner(&success, 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.ac

@@ -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"])