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This makes it so that in wasi builds if `-DPY_CALL_TRAMPOLINE` is passed, a call trampoline adaptor is used to prevent traps when there is a call signature mismatch between a C slot handler and the signature that the interpreter uses to call it. This requires Clang >= 22. PEP 11 specifies that builds of Python 3.15 use WASI SDK version 33 which includes clang 23.1.0, so this could be used with Python 3.15 WASI builds but not earlier ones. It requires a WebAssembly runtime that supports wasm-gc. Wasmtime has supported wasm-gc since version 27.0 released November 25, 2024. Some other runtimes still don't support it, but for those runtimes people can use builds with the trampoline disabled. I also removed the Emscripten trampoline support for JS runtimes with no wasm-gc support. This removes a lot of implementation complexity and allows WASI and Emscripten to share code. The main concern is that it drops support for some very old iPhones, but every iPhone model released since 2018 is compatible with versions of Safari that support wasm-gc. I also added tests that define handlers with various wrong numbers of arguments. The tests also run on non-webassembly platforms since they should work there as well.
hoodmane
requested review from
a team,
brettcannon,
emmatyping and
savannahostrowski
as code owners
October 2, 2026 19:23
hoodmane
requested review from
ericsnowcurrently and
erlend-aasland
as code owners
October 2, 2026 19:23
hoodmane
added
the
needs backport to 3.15
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This makes it so that in wasi builds if
-DPY_CALL_TRAMPOLINEis passed, a call trampoline adaptor is used to prevent traps when there is a call signature mismatch between a C slot handler and the signature that the interpreter uses to call it.This requires Clang >= 22. PEP 11 specifies that builds of Python 3.15 use WASI SDK version 33 which includes clang 23.1.0, so this could be used with Python 3.15 WASI builds but not earlier ones.
It requires a WebAssembly runtime that supports wasm-gc. Wasmtime has supported wasm-gc since version 27.0 released November 25, 2024. Some other runtimes still don't support it, but for those runtimes people can use builds with the trampoline disabled.
I also removed the Emscripten trampoline support for JS runtimes with no wasm-gc support. This removes a lot of implementation complexity and allows WASI and Emscripten to share code. The main concern is that it drops support for some very old iPhones, but every iPhone model released since 2018 is compatible with versions of Safari that support wasm-gc.
I also added tests that define handlers with various wrong numbers of arguments. The tests also run on non-webassembly platforms since they should work there as well.
cc @brettcannon