Simplify the try intrinsic by using a callback in the catch block

Author: Amanieu

src/libcore/intrinsics.rs

@@ -1390,14 +1390,16 @@ extern "rust-intrinsic" {
     /// cast to a `u64`; if `T` has no discriminant, returns 0.
     pub fn discriminant_value<T>(v: &T) -> u64;
 
-    /// Rust's "try catch" construct which invokes the function pointer `f` with
-    /// the data pointer `data`.
+    /// Rust's "try catch" construct which invokes the function pointer `try_fn`
+    /// with the data pointer `data`.
     ///
-    /// The third pointer is a target-specific data pointer which is filled in
-    /// with the specifics of the exception that occurred. For examples on Unix
-    /// platforms this is a `*mut *mut T` which is filled in by the compiler and
-    /// on MSVC it's `*mut [usize; 2]`. For more information see the compiler's
+    /// The third argument is a function called if a panic occurs. This function
+    /// takes the data pointer and a pointer to the target-specific exception
+    /// object that was caught. For more information see the compiler's
     /// source as well as std's catch implementation.
+    #[cfg(not(bootstrap))]
+    pub fn r#try(try_fn: fn(*mut u8), data: *mut u8, catch_fn: fn(*mut u8, *mut u8)) -> i32;
+    #[cfg(bootstrap)]
     pub fn r#try(f: fn(*mut u8), data: *mut u8, local_ptr: *mut u8) -> i32;
 
     /// Emits a `!nontemporal` store according to LLVM (see their docs).

src/libpanic_abort/lib.rs

@@ -20,11 +20,8 @@
 
 use core::any::Any;
 
-// We need the definition of TryPayload for __rust_panic_cleanup.
-include!("../libpanic_unwind/payload.rs");
-
 #[rustc_std_internal_symbol]
-pub unsafe extern "C" fn __rust_panic_cleanup(_: TryPayload) -> *mut (dyn Any + Send + 'static) {
+pub unsafe extern "C" fn __rust_panic_cleanup(_: *mut u8) -> *mut (dyn Any + Send + 'static) {
     unreachable!()
 }
 

src/libpanic_unwind/emcc.rs

@@ -52,7 +52,7 @@ struct Exception {
     // This needs to be an Option because the object's lifetime follows C++
     // semantics: when catch_unwind moves the Box out of the exception it must
     // still leave the exception object in a valid state because its destructor
-    // is still going to be called by __cxa_end_catch..
+    // is still going to be called by __cxa_end_catch.
     data: Option<Box<dyn Any + Send>>,
 }
 

src/libpanic_unwind/lib.rs

@@ -35,8 +35,6 @@ use alloc::boxed::Box;
 use core::any::Any;
 use core::panic::BoxMeUp;
 
-// If adding to this list, you should also look at the list of TryPayload types
-// defined in payload.rs and likely add to there as well.
 cfg_if::cfg_if! {
     if #[cfg(target_os = "emscripten")] {
         #[path = "emcc.rs"]
@@ -62,8 +60,6 @@ cfg_if::cfg_if! {
     }
 }
 
-include!("payload.rs");
-
 extern "C" {
     /// Handler in libstd called when a panic object is dropped outside of
     /// `catch_unwind`.
@@ -73,9 +69,7 @@ extern "C" {
 mod dwarf;
 
 #[rustc_std_internal_symbol]
-pub unsafe extern "C" fn __rust_panic_cleanup(
-    payload: TryPayload,
-) -> *mut (dyn Any + Send + 'static) {
+pub unsafe extern "C" fn __rust_panic_cleanup(payload: *mut u8) -> *mut (dyn Any + Send + 'static) {
     Box::into_raw(imp::cleanup(payload))
 }
 

src/libpanic_unwind/payload.rs

@@ -49,10 +49,17 @@
 
 use alloc::boxed::Box;
 use core::any::Any;
-use core::mem;
-use core::raw;
+use core::mem::{self, ManuallyDrop};
 use libc::{c_int, c_uint, c_void};
 
+struct Exception {
+    // This needs to be an Option because we catch the exception by reference
+    // and its destructor is executed by the C++ runtime. When we take the Box
+    // out of the exception, we need to leave the exception in a valid state
+    // for its destructor to run without double-dropping the Box.
+    data: Option<Box<dyn Any + Send>>,
+}
+
 // First up, a whole bunch of type definitions. There's a few platform-specific
 // oddities here, and a lot that's just blatantly copied from LLVM. The purpose
 // of all this is to implement the `panic` function below through a call to
@@ -186,7 +193,7 @@ static mut CATCHABLE_TYPE: _CatchableType = _CatchableType {
     properties: 0,
     pType: ptr!(0),
     thisDisplacement: _PMD { mdisp: 0, pdisp: -1, vdisp: 0 },
-    sizeOrOffset: mem::size_of::<[u64; 2]>() as c_int,
+    sizeOrOffset: mem::size_of::<Exception>() as c_int,
     copyFunction: ptr!(0),
 };
 
@@ -229,16 +236,16 @@ static mut TYPE_DESCRIPTOR: _TypeDescriptor = _TypeDescriptor {
 // because Box<dyn Any> isn't clonable.
 macro_rules! define_cleanup {
     ($abi:tt) => {
-        unsafe extern $abi fn exception_cleanup(e: *mut [u64; 2]) {
-            if (*e)[0] != 0 {
-                cleanup(*e);
+        unsafe extern $abi fn exception_cleanup(e: *mut Exception) {
+            if let Some(b) = e.read().data {
+                drop(b);
                 super::__rust_drop_panic();
             }
         }
         #[unwind(allowed)]
-        unsafe extern $abi fn exception_copy(_dest: *mut [u64; 2],
-                                             _src: *mut [u64; 2])
-                                             -> *mut [u64; 2] {
+        unsafe extern $abi fn exception_copy(_dest: *mut Exception,
+                                             _src: *mut Exception)
+                                             -> *mut Exception {
             panic!("Rust panics cannot be copied");
         }
     }
@@ -258,12 +265,11 @@ pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
     // need to otherwise transfer `data` to the heap. We just pass a stack
     // pointer to this function.
     //
-    // The first argument is the payload being thrown (our two pointers), and
-    // the second argument is the type information object describing the
-    // exception (constructed above).
-    let ptrs = mem::transmute::<_, raw::TraitObject>(data);
-    let mut ptrs = [ptrs.data as u64, ptrs.vtable as u64];
-    let throw_ptr = ptrs.as_mut_ptr() as *mut _;
+    // The ManuallyDrop is needed here since we don't want Exception to be
+    // dropped when unwinding. Instead it will be dropped by exception_cleanup
+    // which is invoked by the C++ runtime.
+    let mut exception = ManuallyDrop::new(Exception { data: Some(data) });
+    let throw_ptr = &mut exception as *mut _ as *mut _;
 
     // This... may seems surprising, and justifiably so. On 32-bit MSVC the
     // pointers between these structure are just that, pointers. On 64-bit MSVC,
@@ -311,8 +317,9 @@ pub unsafe fn panic(data: Box<dyn Any + Send>) -> u32 {
     _CxxThrowException(throw_ptr, &mut THROW_INFO as *mut _ as *mut _);
 }
 
-pub unsafe fn cleanup(payload: [u64; 2]) -> Box<dyn Any + Send> {
-    mem::transmute(raw::TraitObject { data: payload[0] as *mut _, vtable: payload[1] as *mut _ })
+pub unsafe fn cleanup(payload: *mut u8) -> Box<dyn Any + Send> {
+    let exception = &mut *(payload as *mut Exception);
+    exception.data.take().unwrap()
 }
 
 // This is required by the compiler to exist (e.g., it's a lang item), but