std: Minimize size of panicking on wasm

This commit applies a few code size optimizations for the wasm target to
the standard library, namely around panics. We notably know that in most
configurations it's impossible for us to print anything in
wasm32-unknown-unknown so we can skip larger portions of panicking that
are otherwise simply informative. This allows us to get quite a nice
size reduction.

Finally we can also tweak where the allocation happens for the
`Box<Any>` that we panic with. By only allocating once unwinding starts
we can reduce the size of a panicking wasm module from 44k to 350 bytes.

Author: alexcrichton

src/ci/docker/wasm32-unknown/Dockerfile

@@ -25,6 +25,12 @@ ENV RUST_CONFIGURE_ARGS \
   --set build.nodejs=/node-v9.2.0-linux-x64/bin/node \
   --set rust.lld
 
+# Some run-make tests have assertions about code size, and enabling debug
+# assertions in libstd causes the binary to be much bigger than it would
+# otherwise normally be. We already test libstd with debug assertions in lots of
+# other contexts as well
+ENV NO_DEBUG_ASSERTIONS=1
+
 ENV SCRIPT python2.7 /checkout/x.py test --target $TARGETS \
   src/test/run-make \
   src/test/ui \

src/libcore/panic.rs

@@ -249,3 +249,13 @@ impl<'a> fmt::Display for Location<'a> {
         write!(formatter, "{}:{}:{}", self.file, self.line, self.col)
     }
 }
+
+/// An internal trait used by libstd to pass data from libstd to `panic_unwind`
+/// and other panic runtimes. Not intended to be stabilized any time soon, do
+/// not use.
+#[unstable(feature = "std_internals", issue = "0")]
+#[doc(hidden)]
+pub unsafe trait BoxMeUp {
+    fn box_me_up(&mut self) -> *mut (Any + Send);
+    fn get(&self) -> &(Any + Send);
+}

src/libpanic_abort/lib.rs

@@ -53,7 +53,7 @@ pub unsafe extern fn __rust_maybe_catch_panic(f: fn(*mut u8),
 // now hopefully.
 #[no_mangle]
 #[rustc_std_internal_symbol]
-pub unsafe extern fn __rust_start_panic(_data: usize, _vtable: usize) -> u32 {
+pub unsafe extern fn __rust_start_panic(_payload: usize) -> u32 {
     abort();
 
     #[cfg(any(unix, target_os = "cloudabi"))]

src/libpanic_unwind/lib.rs

@@ -30,13 +30,15 @@
        issue_tracker_base_url = "https://github.com/rust-lang/rust/issues/")]
 #![deny(warnings)]
 
+#![feature(allocator_api)]
 #![feature(alloc)]
 #![feature(core_intrinsics)]
 #![feature(lang_items)]
 #![feature(libc)]
 #![feature(panic_unwind)]
 #![feature(raw)]
 #![feature(staged_api)]
+#![feature(std_internals)]
 #![feature(unwind_attributes)]
 #![cfg_attr(target_env = "msvc", feature(raw))]
 
@@ -48,9 +50,11 @@ extern crate libc;
 #[cfg(not(any(target_env = "msvc", all(windows, target_arch = "x86_64", target_env = "gnu"))))]
 extern crate unwind;
 
+use alloc::boxed::Box;
 use core::intrinsics;
 use core::mem;
 use core::raw;
+use core::panic::BoxMeUp;
 
 // Rust runtime's startup objects depend on these symbols, so make them public.
 #[cfg(all(target_os="windows", target_arch = "x86", target_env="gnu"))]
@@ -114,9 +118,7 @@ pub unsafe extern "C" fn __rust_maybe_catch_panic(f: fn(*mut u8),
 #[no_mangle]
 #[cfg_attr(stage0, unwind)]
 #[cfg_attr(not(stage0), unwind(allowed))]
-pub unsafe extern "C" fn __rust_start_panic(data: usize, vtable: usize) -> u32 {
-    imp::panic(mem::transmute(raw::TraitObject {
-        data: data as *mut (),
-        vtable: vtable as *mut (),
-    }))
+pub unsafe extern "C" fn __rust_start_panic(payload: usize) -> u32 {
+    let payload = payload as *mut &mut BoxMeUp;
+    imp::panic(Box::from_raw((*payload).box_me_up()))
 }

src/libstd/lib.rs

@@ -297,6 +297,7 @@
 #![feature(rand)]
 #![feature(raw)]
 #![feature(rustc_attrs)]
+#![feature(std_internals)]
 #![feature(stdsimd)]
 #![feature(shrink_to)]
 #![feature(slice_bytes)]

src/libstd/panicking.rs

@@ -17,6 +17,8 @@
 //! * Executing a panic up to doing the actual implementation
 //! * Shims around "try"
 
+use core::panic::BoxMeUp;
+
 use io::prelude::*;
 
 use any::Any;
@@ -27,7 +29,7 @@ use intrinsics;
 use mem;
 use ptr;
 use raw;
-use sys::stdio::Stderr;
+use sys::stdio::{Stderr, stderr_prints_nothing};
 use sys_common::rwlock::RWLock;
 use sys_common::thread_info;
 use sys_common::util;
@@ -57,7 +59,7 @@ extern {
                                 vtable_ptr: *mut usize) -> u32;
     #[cfg_attr(stage0, unwind)]
     #[cfg_attr(not(stage0), unwind(allowed))]
-    fn __rust_start_panic(data: usize, vtable: usize) -> u32;
+    fn __rust_start_panic(payload: usize) -> u32;
 }
 
 #[derive(Copy, Clone)]
@@ -164,6 +166,12 @@ fn default_hook(info: &PanicInfo) {
     #[cfg(feature = "backtrace")]
     use sys_common::backtrace;
 
+    // Some platforms know that printing to stderr won't ever actually print
+    // anything, and if that's the case we can skip everything below.
+    if stderr_prints_nothing() {
+        return
+    }
+
     // If this is a double panic, make sure that we print a backtrace
     // for this panic. Otherwise only print it if logging is enabled.
     #[cfg(feature = "backtrace")]
@@ -213,15 +221,15 @@ fn default_hook(info: &PanicInfo) {
 
     let prev = LOCAL_STDERR.with(|s| s.borrow_mut().take());
     match (prev, err.as_mut()) {
-        (Some(mut stderr), _) => {
-            write(&mut *stderr);
-            let mut s = Some(stderr);
-            LOCAL_STDERR.with(|slot| {
-                *slot.borrow_mut() = s.take();
-            });
-        }
-        (None, Some(ref mut err)) => { write(err) }
-        _ => {}
+       (Some(mut stderr), _) => {
+           write(&mut *stderr);
+           let mut s = Some(stderr);
+           LOCAL_STDERR.with(|slot| {
+               *slot.borrow_mut() = s.take();
+           });
+       }
+       (None, Some(ref mut err)) => { write(err) }
+       _ => {}
     }
 }
 
@@ -346,7 +354,7 @@ pub fn begin_panic_fmt(msg: &fmt::Arguments,
 
     let mut s = String::new();
     let _ = s.write_fmt(*msg);
-    rust_panic_with_hook(Box::new(s), Some(msg), file_line_col)
+    rust_panic_with_hook(&mut PanicPayload::new(s), Some(msg), file_line_col)
 }
 
 /// This is the entry point of panicking for panic!() and assert!().
@@ -362,7 +370,34 @@ pub fn begin_panic<M: Any + Send>(msg: M, file_line_col: &(&'static str, u32, u3
     // be performed in the parent of this thread instead of the thread that's
     // panicking.
 
-    rust_panic_with_hook(Box::new(msg), None, file_line_col)
+    rust_panic_with_hook(&mut PanicPayload::new(msg), None, file_line_col)
+}
+
+struct PanicPayload<A> {
+    inner: Option<A>,
+}
+
+impl<A: Send + 'static> PanicPayload<A> {
+    fn new(inner: A) -> PanicPayload<A> {
+        PanicPayload { inner: Some(inner) }
+    }
+}
+
+unsafe impl<A: Send + 'static> BoxMeUp for PanicPayload<A> {
+    fn box_me_up(&mut self) -> *mut (Any + Send) {
+        let data = match self.inner.take() {
+            Some(a) => Box::new(a) as Box<Any + Send>,
+            None => Box::new(()),
+        };
+        Box::into_raw(data)
+    }
+
+    fn get(&self) -> &(Any + Send) {
+        match self.inner {
+            Some(ref a) => a,
+            None => &(),
+        }
+    }
 }
 
 /// Executes the primary logic for a panic, including checking for recursive
@@ -371,9 +406,7 @@ pub fn begin_panic<M: Any + Send>(msg: M, file_line_col: &(&'static str, u32, u3
 /// This is the entry point or panics from libcore, formatted panics, and
 /// `Box<Any>` panics. Here we'll verify that we're not panicking recursively,
 /// run panic hooks, and then delegate to the actual implementation of panics.
-#[inline(never)]
-#[cold]
-fn rust_panic_with_hook(payload: Box<Any + Send>,
+fn rust_panic_with_hook(payload: &mut BoxMeUp,
                         message: Option<&fmt::Arguments>,
                         file_line_col: &(&'static str, u32, u32)) -> ! {
     let (file, line, col) = *file_line_col;
@@ -393,7 +426,7 @@ fn rust_panic_with_hook(payload: Box<Any + Send>,
 
     unsafe {
         let info = PanicInfo::internal_constructor(
-            &*payload,
+            payload.get(),
             message,
             Location::internal_constructor(file, line, col),
         );
@@ -421,16 +454,29 @@ fn rust_panic_with_hook(payload: Box<Any + Send>,
 /// Shim around rust_panic. Called by resume_unwind.
 pub fn update_count_then_panic(msg: Box<Any + Send>) -> ! {
     update_panic_count(1);
-    rust_panic(msg)
+
+    struct RewrapBox(Box<Any + Send>);
+
+    unsafe impl BoxMeUp for RewrapBox {
+        fn box_me_up(&mut self) -> *mut (Any + Send) {
+            Box::into_raw(mem::replace(&mut self.0, Box::new(())))
+        }
+
+        fn get(&self) -> &(Any + Send) {
+            &*self.0
+        }
+    }
+
+    rust_panic(&mut RewrapBox(msg))
 }
 
 /// A private no-mangle function on which to slap yer breakpoints.
 #[no_mangle]
 #[allow(private_no_mangle_fns)] // yes we get it, but we like breakpoints
-pub fn rust_panic(msg: Box<Any + Send>) -> ! {
+pub fn rust_panic(mut msg: &mut BoxMeUp) -> ! {
     let code = unsafe {
-        let obj = mem::transmute::<_, raw::TraitObject>(msg);
-        __rust_start_panic(obj.data as usize, obj.vtable as usize)
+        let obj = &mut msg as *mut &mut BoxMeUp;
+        __rust_start_panic(obj as usize)
     };
     rtabort!("failed to initiate panic, error {}", code)
 }

src/libstd/sys/cloudabi/stdio.rs

@@ -77,3 +77,7 @@ pub fn is_ebadf(err: &io::Error) -> bool {
 }
 
 pub const STDIN_BUF_SIZE: usize = ::sys_common::io::DEFAULT_BUF_SIZE;
+
+pub fn stderr_prints_nothing() -> bool {
+    false
+}

src/libstd/sys/redox/stdio.rs

@@ -75,3 +75,7 @@ pub fn is_ebadf(err: &io::Error) -> bool {
 }
 
 pub const STDIN_BUF_SIZE: usize = ::sys_common::io::DEFAULT_BUF_SIZE;
+
+pub fn stderr_prints_nothing() -> bool {
+    false
+}

src/libstd/sys/unix/stdio.rs

@@ -75,3 +75,7 @@ pub fn is_ebadf(err: &io::Error) -> bool {
 }
 
 pub const STDIN_BUF_SIZE: usize = ::sys_common::io::DEFAULT_BUF_SIZE;
+
+pub fn stderr_prints_nothing() -> bool {
+    false
+}

src/libstd/sys/wasm/rwlock.rs

@@ -30,7 +30,7 @@ impl RWLock {
         if *mode >= 0 {
             *mode += 1;
         } else {
-            panic!("rwlock locked for writing");
+            rtabort!("rwlock locked for writing");
         }
     }
 
@@ -51,7 +51,7 @@ impl RWLock {
         if *mode == 0 {
             *mode = -1;
         } else {
-            panic!("rwlock locked for reading")
+            rtabort!("rwlock locked for reading")
         }
     }