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gh-156780: Emscripten: drop the trampoline's dependence on exports #157238

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freakboy3742 merged 1 commit into from
Sep 11, 2026
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gh-156780: Emscripten: drop the trampoline's dependence on exports #157238

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gh-156780: Emscripten: drop the trampoline's dependence on exports 
The trampoline exports _PyRuntime's address itself and reuses a
placeholder's table slot, so libpython no longer needs
-sEXPORTED_FUNCTIONS=__PyRuntime or a growable function table.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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@clementperon @claude
clementperon and claude committed Sep 10, 2026
commit 0a2c8f45e8332e4ecad1e2736d6c244b221e45c2
21 changes: 18 additions & 3 deletions Python/emscripten_trampoline.c
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Original file line number Diff line number Diff line change
Expand Up @@ -47,6 +47,19 @@ typedef PyObject* (*TrampolineFunc)(int* success,
PyObject* args,
PyObject* kw);

// Lets JS reach _PyRuntime without it being in -sEXPORTED_FUNCTIONS.
EMSCRIPTEN_KEEPALIVE _PyRuntimeState *const _PyEM_runtime = &_PyRuntime;
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// Its table slot is taken over by the wasm-gc trampoline, so the table
// never grows.
static PyObject*
trampoline_placeholder(int* success, PyCFunctionWithKeywords func,
PyObject* self, PyObject* args, PyObject* kw)
{
Py_FatalError("Emscripten trampoline slot was not set up");
}
EMSCRIPTEN_KEEPALIVE const TrampolineFunc _PyEM_trampoline_slot = trampoline_placeholder;

/**
* Backwards compatible trampoline works with all JS runtimes
*/
Expand Down Expand Up @@ -79,7 +92,9 @@ function getPyEMTrampolinePtr() {
const trampolineInstance = new WebAssembly.Instance(trampolineModule, {
env: { __indirect_function_table: wasmTable, memory: wasmMemory },
});
return addFunction(trampolineInstance.exports.trampoline_call);
const slot = HEAPU32[__PyEM_trampoline_slot / 4];
wasmTable.set(slot, trampolineInstance.exports.trampoline_call);
return slot;
}
// We have to be careful to work correctly with memory snapshots -- the value of
// _PyRuntimeState.emscripten_trampoline needs to reflect whether wasm-gc is
Expand All @@ -90,12 +105,12 @@ function getPyEMTrampolinePtr() {
addOnPreRun(function setEmscriptenTrampoline() {
const ptr = getPyEMTrampolinePtr();
const offset = HEAP32[__PyEM_EMSCRIPTEN_TRAMPOLINE_OFFSET / 4];
HEAP32[(__PyRuntime + offset) / 4] = ptr;
HEAP32[(HEAPU32[__PyEM_runtime / 4] + offset) / 4] = ptr;
});
);

EM_JS_DEPS(_PyEM_TrampolineCall,
"$wasmTable,$wasmMemory,$addFunction,$addOnPreRun");
"$wasmTable,$wasmMemory,$addOnPreRun");

PyObject*
_PyEM_TrampolineCall(PyCFunctionWithKeywords func,
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2 changes: 1 addition & 1 deletion configure
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2 changes: 1 addition & 1 deletion configure.ac
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Original file line number Diff line number Diff line change
Expand Up @@ -2440,7 +2440,7 @@ 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,ERRNO_CODES"])
AS_VAR_APPEND([LINKFORSHARED], [" -sEXPORTED_FUNCTIONS=_main,_Py_Version,__PyRuntime,_PyGILState_GetThisThreadState,__PyEM_EMSCRIPTEN_TRAMPOLINE_OFFSET"])
AS_VAR_APPEND([LINKFORSHARED], [" -sEXPORTED_FUNCTIONS=_main,_Py_Version,_PyGILState_GetThisThreadState,__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"])
Expand Down
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