Use inline asm rather than llvm intrinsic for panics on wasm

This way we don't have to support unwinding llvm intrinsics in the
codegen backends.

Author: bjorn3

library/unwind/src/lib.rs

@@ -7,7 +7,7 @@
 #![cfg_attr(not(target_env = "msvc"), feature(libc))]
 #![cfg_attr(
     all(target_family = "wasm", any(not(target_os = "emscripten"), emscripten_wasm_eh)),
-    feature(link_llvm_intrinsics, simd_wasm64)
+    feature(asm_experimental_arch, asm_unwind, simd_wasm64)
 )]
 #![allow(internal_features)]
 #![deny(unsafe_op_in_unsafe_fn)]

library/unwind/src/wasm.rs

@@ -47,33 +47,17 @@ pub unsafe fn _Unwind_RaiseException(exception: *mut _Unwind_Exception) -> _Unwi
     // enabled exceptions via `-Z build-std` with `-C panic=unwind`.
     cfg_select! {
         panic = "unwind" => {
-            // It's important that this intrinsic is defined here rather than in `core`. Since it
-            // unwinds, invoking it from Rust code compiled with `-C panic=unwind` immediately
-            // forces `panic_unwind` as the required panic runtime.
-            //
-            // We ship unwinding `core` on Emscripten, so making this intrinsic part of `core` would
-            // prevent linking precompiled `core` into `-C panic=abort` binaries. Unlike `core`,
-            // this particular module is never precompiled with `-C panic=unwind` because it's only
-            // used for bare-metal targets, so an error can only arise if the user both manually
-            // recompiles `std` with `-C panic=unwind` and manually compiles the binary crate with
-            // `-C panic=abort`, which we don't care to support.
-            //
-            // See https://github.com/rust-lang/rust/issues/148246.
-            unsafe extern "C-unwind" {
-                /// LLVM lowers this intrinsic to the `throw` instruction.
-                #[link_name = "llvm.wasm.throw"]
-                fn wasm_throw(tag: i32, ptr: *mut u8) -> !;
-            }
-
-            // The wasm `throw` instruction takes a "tag", which differentiates certain types of
-            // exceptions from others. LLVM currently just identifies these via integers, with 0
-            // corresponding to C++ exceptions and 1 to C setjmp()/longjmp(). Ideally, we'd be able
-            // to choose something unique for Rust, but for now, we pretend to be C++ and implement
-            // the Itanium exception-handling ABI.
-            // corresponds with llvm::WebAssembly::Tag::CPP_EXCEPTION
-            //     in llvm-project/llvm/include/llvm/CodeGen/WasmEHFuncInfo.h
-            const CPP_EXCEPTION_TAG: i32 = 0;
-            wasm_throw(CPP_EXCEPTION_TAG, exception.cast())
+            // LLVM currently only runs cleanup code for exception using the C++ exception tag and
+            // not those for any other exception tag like the longjmp exception tag. Ideally, we'd
+            // be able to choose something unique for Rust, but for now, we pretend to be C++ and
+            // implement the Itanium exception-handling ABI.
+            // This is using inline asm rather than the llvm.wasm.throw llvm intrinsic as supporting
+            // unwinding for llvm intrinsics complicates things in the backend.
+            core::arch::asm!("
+                .tagtype __cpp_exception i32
+                local.get {exc}
+                throw __cpp_exception
+            ", exc = in(local) exception, options(may_unwind, noreturn, nostack));
         }
         _ => {
             let _ = exception;

tests/codegen-llvm/wasm_exceptions.rs

@@ -3,7 +3,7 @@
 //@ [WASMEXN] compile-flags: -C panic=unwind -Z emscripten-wasm-eh
 
 #![crate_type = "lib"]
-#![feature(core_intrinsics, link_llvm_intrinsics)]
+#![feature(asm_experimental_arch, asm_unwind, core_intrinsics)]
 
 extern "C-unwind" {
     fn may_panic();
@@ -82,15 +82,15 @@ pub fn test_rtry() {
 pub fn test_intrinsic() {
     let _log_on_drop = LogOnDrop;
 
-    unsafe extern "C-unwind" {
-        #[link_name = "llvm.wasm.throw"]
-        fn wasm_throw(tag: i32, ptr: *mut u8) -> !;
-    }
     unsafe {
-        wasm_throw(0, core::ptr::null_mut());
+        std::arch::asm!("
+            .tagtype __cpp_exception i32
+            local.get {exc}
+            throw __cpp_exception
+        ", exc = in(local) core::ptr::null_mut::<()>(), options(may_unwind, noreturn, nostack));
     }
 
     // WASMEXN-NOT: call
-    // WASMEXN: invoke void @llvm.wasm.throw(i32 noundef 0, ptr noundef null)
+    // WASMEXN: invoke void asm sideeffect unwind
     // WASMEXN: %cleanuppad = cleanuppad within none []
 }