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Rollup merge of rust-lang#121438 - coolreader18:wasm32-panic-unwind, …
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…r=cuviper

std support for wasm32 panic=unwind

Tracking issue: rust-lang#118168

This adds std support for `-Cpanic=unwind` on wasm, and with it slightly more fleshed out rustc support. Now, the stable default is still panic=abort without exception-handling, but if you `-Zbuild-std` with `RUSTFLAGS=-Cpanic=unwind`, you get wasm exception-handling try/catch blocks in the binary:

```rust
#[no_mangle]
pub fn foo_bar(x: bool) -> *mut u8 {
    let s = Box::<str>::from("hello");
    maybe_panic(x);
    Box::into_raw(s).cast()
}

#[inline(never)]
#[no_mangle]
fn maybe_panic(x: bool) {
    if x {
        panic!("AAAAA");
    }
}
```
```wat
;; snip...
(try $label$5
 (do
  (call $maybe_panic
   (local.get $0)
  )
  (br $label$1)
 )
 (catch_all
  (global.set $__stack_pointer
   (local.get $1)
  )
  (call $__rust_dealloc
   (local.get $2)
   (i32.const 5)
   (i32.const 1)
  )
  (rethrow $label$5)
 )
)
;; snip...
```
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workingjubilee authored Mar 11, 2024
2 parents a450339 + c7fcf43 commit 1279830
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Showing 9 changed files with 90 additions and 10 deletions.
9 changes: 9 additions & 0 deletions compiler/rustc_codegen_llvm/src/llvm_util.rs
Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,7 @@ use crate::errors::{
};
use crate::llvm;
use libc::c_int;
use rustc_codegen_ssa::base::wants_wasm_eh;
use rustc_codegen_ssa::traits::PrintBackendInfo;
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_data_structures::small_c_str::SmallCStr;
Expand Down Expand Up @@ -98,6 +99,10 @@ unsafe fn configure_llvm(sess: &Session) {
}
}

if wants_wasm_eh(sess) {
add("-wasm-enable-eh", false);
}

if sess.target.os == "emscripten" && sess.panic_strategy() == PanicStrategy::Unwind {
add("-enable-emscripten-cxx-exceptions", false);
}
Expand Down Expand Up @@ -523,6 +528,10 @@ pub(crate) fn global_llvm_features(sess: &Session, diagnostics: bool) -> Vec<Str
.map(String::from),
);

if wants_wasm_eh(sess) && sess.panic_strategy() == PanicStrategy::Unwind {
features.push("+exception-handling".into());
}

// -Ctarget-features
let supported_features = sess.target.supported_target_features();
let mut featsmap = FxHashMap::default();
Expand Down
2 changes: 1 addition & 1 deletion compiler/rustc_codegen_ssa/src/mir/block.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1597,7 +1597,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
let funclet;
let llbb;
let mut bx;
if base::wants_msvc_seh(self.cx.sess()) {
if base::wants_new_eh_instructions(self.cx.sess()) {
// This is a basic block that we're aborting the program for,
// notably in an `extern` function. These basic blocks are inserted
// so that we assert that `extern` functions do indeed not panic,
Expand Down
2 changes: 1 addition & 1 deletion library/panic_unwind/src/gcc.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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,
Expand Down
2 changes: 1 addition & 1 deletion library/panic_unwind/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -53,12 +53,12 @@ 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 {
// Targets that don't support unwinding.
// - family=wasm
// - os=none ("bare metal" targets)
// - os=uefi
// - os=espidf
Expand Down
8 changes: 4 additions & 4 deletions library/std/src/sys/personality/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -16,11 +16,12 @@ mod dwarf;
cfg_if::cfg_if! {
if #[cfg(target_os = "emscripten")] {
mod emcc;
} else if #[cfg(target_env = "msvc")] {
} else if #[cfg(any(target_env = "msvc", target_family = "wasm"))] {
// This is required by the compiler to exist (e.g., it's a lang item),
// but it's never actually called by the compiler because
// _CxxFrameHandler3 is the personality function that is always used.
// Hence this is just an aborting stub.
// __CxxFrameHandler3 (msvc) / __gxx_wasm_personality_v0 (wasm) is the
// personality function that is always used. Hence this is just an
// aborting stub.
#[lang = "eh_personality"]
fn rust_eh_personality() {
core::intrinsics::abort()
Expand All @@ -36,7 +37,6 @@ cfg_if::cfg_if! {
mod gcc;
} else {
// Targets that don't support unwinding.
// - family=wasm
// - os=none ("bare metal" targets)
// - os=uefi
// - os=espidf
Expand Down
8 changes: 7 additions & 1 deletion library/unwind/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,10 @@
#![cfg_attr(bootstrap, 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! {
Expand All @@ -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
Expand Down
2 changes: 1 addition & 1 deletion library/unwind/src/libunwind.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down
2 changes: 1 addition & 1 deletion library/unwind/src/unwinding.rs
Original file line number Diff line number Diff line change
Expand Up @@ -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 {
Expand Down
65 changes: 65 additions & 0 deletions library/unwind/src/wasm.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,65 @@
//! 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 {
#[cfg(panic = "unwind")]
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.
cfg_if::cfg_if! {
// for now, unless we're -Zbuild-std with panic=unwind, never codegen a throw.
if #[cfg(panic = "unwind")] {
wasm_throw(0, exception.cast())
} else {
let _ = exception;
core::arch::wasm32::unreachable()
}
}
}

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