Use Itanium ABI for thrown exceptions

rust-lang · matthiaskrgr · Feb 18, 2024 · Feb 21, 2024 · Feb 22, 2024 · Feb 22, 2024
commit 125b26acf6f3127ecdd344c372691cefe0e9243e
diff --git a/library/panic_unwind/src/gcc.rs b/library/panic_unwind/src/gcc.rs
@@ -62,7 +62,7 @@ pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
     let exception = Box::new(Exception {
         _uwe: uw::_Unwind_Exception {
             exception_class: rust_exception_class(),
-            exception_cleanup,
+            exception_cleanup: Some(exception_cleanup),
             private: [core::ptr::null(); uw::unwinder_private_data_size],
         },
         canary: &CANARY,

diff --git a/library/panic_unwind/src/lib.rs b/library/panic_unwind/src/lib.rs
@@ -16,10 +16,6 @@
 #![doc(issue_tracker_base_url = "https://github.com/rust-lang/rust/issues/")]
 #![feature(core_intrinsics)]
 #![feature(lang_items)]
-#![cfg_attr(
-    all(target_family = "wasm", not(target_os = "emscripten")),
-    feature(link_llvm_intrinsics)
-)]
 #![feature(panic_unwind)]
 #![feature(staged_api)]
 #![feature(std_internals)]
@@ -57,12 +53,10 @@ cfg_if::cfg_if! {
         target_os = "solid_asp3",
         all(target_family = "unix", not(target_os = "espidf")),
         all(target_vendor = "fortanix", target_env = "sgx"),
+        target_family = "wasm",
     ))] {
         #[path = "gcc.rs"]
         mod real_imp;
-    } else if #[cfg(target_family = "wasm")] {
-        #[path = "wasm.rs"]
-        mod real_imp;
     } else {
         // Targets that don't support unwinding.
         // - os=none ("bare metal" targets)

diff --git a/library/panic_unwind/src/wasm.rs b/library/panic_unwind/src/wasm.rs
diff --git a/library/unwind/src/lib.rs b/library/unwind/src/lib.rs
@@ -6,6 +6,10 @@
 #![feature(cfg_target_abi)]
 #![feature(strict_provenance)]
 #![cfg_attr(not(target_env = "msvc"), feature(libc))]
+#![cfg_attr(
+    all(target_family = "wasm", not(target_os = "emscripten")),
+    feature(link_llvm_intrinsics)
+)]
 #![allow(internal_features)]
 
 cfg_if::cfg_if! {
@@ -29,9 +33,11 @@ cfg_if::cfg_if! {
     } else if #[cfg(target_os = "xous")] {
         mod unwinding;
         pub use unwinding::*;
+    } else if #[cfg(target_family = "wasm")] {
+        mod wasm;
+        pub use wasm::*;
     } else {
         // no unwinder on the system!
-        // - wasm32 (not emscripten, which is "unix" family)
         // - os=none ("bare metal" targets)
         // - os=hermit
         // - os=uefi

diff --git a/library/unwind/src/libunwind.rs b/library/unwind/src/libunwind.rs
@@ -91,7 +91,7 @@ pub struct _Unwind_Exception {
 pub enum _Unwind_Context {}
 
 pub type _Unwind_Exception_Cleanup_Fn =
-    extern "C" fn(unwind_code: _Unwind_Reason_Code, exception: *mut _Unwind_Exception);
+    Option<extern "C" fn(unwind_code: _Unwind_Reason_Code, exception: *mut _Unwind_Exception)>;
 
 // FIXME: The `#[link]` attributes on `extern "C"` block marks those symbols declared in
 // the block are reexported in dylib build of std. This is needed when build rustc with

diff --git a/library/unwind/src/unwinding.rs b/library/unwind/src/unwinding.rs
@@ -46,7 +46,7 @@ pub const unwinder_private_data_size: usize = core::mem::size_of::<UnwindExcepti
     - core::mem::size_of::<_Unwind_Exception_Cleanup_Fn>();
 
 pub type _Unwind_Exception_Cleanup_Fn =
-    extern "C" fn(unwind_code: _Unwind_Reason_Code, exception: *mut _Unwind_Exception);
+    Option<extern "C" fn(unwind_code: _Unwind_Reason_Code, exception: *mut _Unwind_Exception)>;
 
 #[repr(C)]
 pub struct _Unwind_Exception {

diff --git a/library/unwind/src/wasm.rs b/library/unwind/src/wasm.rs
@@ -0,0 +1,56 @@
+//! A shim for libunwind implemented in terms of the native wasm `throw` instruction.
+
+#![allow(nonstandard_style)]
+
+#[repr(C)]
+#[derive(Debug, Copy, Clone, PartialEq)]
+pub enum _Unwind_Reason_Code {
+    _URC_NO_REASON = 0,
+    _URC_FOREIGN_EXCEPTION_CAUGHT = 1,
+    _URC_FATAL_PHASE2_ERROR = 2,
+    _URC_FATAL_PHASE1_ERROR = 3,
+    _URC_NORMAL_STOP = 4,
+    _URC_END_OF_STACK = 5,
+    _URC_HANDLER_FOUND = 6,
+    _URC_INSTALL_CONTEXT = 7,
+    _URC_CONTINUE_UNWIND = 8,
+    _URC_FAILURE = 9, // used only by ARM EHABI
+}
+pub use _Unwind_Reason_Code::*;
+
+pub type _Unwind_Exception_Class = u64;
+pub type _Unwind_Word = *const u8;
+
+pub const unwinder_private_data_size: usize = 2;
+
+#[repr(C)]
+pub struct _Unwind_Exception {
+    pub exception_class: _Unwind_Exception_Class,
+    pub exception_cleanup: _Unwind_Exception_Cleanup_Fn,
+    pub private: [_Unwind_Word; unwinder_private_data_size],
+}
+
+pub type _Unwind_Exception_Cleanup_Fn =
+    Option<extern "C" fn(unwind_code: _Unwind_Reason_Code, exception: *mut _Unwind_Exception)>;
+
+pub unsafe fn _Unwind_DeleteException(exception: *mut _Unwind_Exception) {
+    if let Some(exception_cleanup) = unsafe { (*exception).exception_cleanup } {
+        exception_cleanup(_URC_FOREIGN_EXCEPTION_CAUGHT, exception);
+    }
+}
+
+pub unsafe fn _Unwind_RaiseException(exception: *mut _Unwind_Exception) -> _Unwind_Reason_Code {
+    extern "C" {
+        /// LLVM lowers this intrinsic to the `throw` instruction.
+        // FIXME(coolreader18): move to stdarch
+        #[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.
+    wasm_throw(0, exception.cast())
+}