Auto merge of #144225 - purplesyringa:unwinding-intrinsics, r=nikic

Don't special-case llvm.* as nounwind

Certain LLVM intrinsics, such as `llvm.wasm.throw`, can unwind. Marking them as nounwind causes us to skip cleanup of locals and optimize out `catch_unwind` under inlining or when `llvm.wasm.throw` is used directly by user code.

The motivation for forcibly marking llvm.* as nounwind is no longer present: most intrinsics are linked as `extern "C"` or other non-unwinding ABIs, so we won't codegen `invoke` for them anyway.

Closes rust-lang/rust#132416.

`@rustbot` label +T-compiler +A-panic

Author: bors

crates/core_arch/src/wasm32/mod.rs

@@ -191,6 +191,16 @@ unsafe extern "C-unwind" {
 // #[cfg_attr(test, assert_instr(throw, TAG = 0, ptr = core::ptr::null_mut()))]
 #[inline]
 #[unstable(feature = "wasm_exception_handling_intrinsics", issue = "122465")]
+// FIXME: Since this instruction unwinds, `core` built with `-C panic=unwind`
+//        cannot be linked with `-C panic=abort` programs. But that's not
+//        entirely supported anyway, because runtimes without EH support won't
+//        be able to handle `try` blocks in `-C panic=unwind` crates either.
+//        We ship `-C panic=abort` `core`, so this doesn't affect users
+//        directly. Resolving this will likely require patching out both `try`
+//        and `throw` instructions, at which point we can look into whitelisting
+//        this function in the compiler to allow linking.
+//        See https://github.com/rust-lang/rust/issues/118168.
+#[allow(ffi_unwind_calls)]
 pub unsafe fn throw<const TAG: i32>(ptr: *mut u8) -> ! {
     static_assert!(TAG == 0); // LLVM only supports tag 0 == C++ right now.
     wasm_throw(TAG, ptr)