WIP unwinding support

Author: Amanieu

src/arch/x86_64.rs

@@ -49,6 +49,7 @@
 use core::mem;
 use stack::Stack;
 use arch::StackPointer;
+use unwind;
 
 pub const STACK_ALIGNMENT: usize = 16;
 
@@ -127,8 +128,9 @@ pub unsafe fn init(stack: &Stack, f: unsafe extern "C" fn(usize, StackPointer))
         # trampoline_2.
         nop
 
-        # Call the provided function.
-        callq   *16(%rsp)
+        # Call unwind_wrapper with the provided function.
+        movq    16(%rsp), %rdx
+        call    ${0:c}
 
         # Clear the stack pointer. We can't call into this context any more once
         # the function has returned.
@@ -150,7 +152,7 @@ pub unsafe fn init(stack: &Stack, f: unsafe extern "C" fn(usize, StackPointer))
         .cfi_register %rip, %rax
         jmpq    *%rax
       "#
-      : : : : "volatile")
+      : : "s" (unwind::unwind_wrapper as usize) : : "volatile")
   }
 
   // We set up the stack in a somewhat special way so that to the unwinder it
@@ -315,3 +317,66 @@ pub unsafe fn swap(arg: usize, new_sp: StackPointer) -> (usize, StackPointer) {
     : "volatile", "alignstack");
   (ret, mem::transmute(ret_sp))
 }
+
+
+#[inline(always)]
+pub unsafe fn unwind(new_sp: StackPointer, new_stack: &Stack) {
+  // Address of the topmost CFA stack slot.
+  let new_cfa = (new_stack.base() as *mut usize).offset(-4);
+
+  #[naked]
+  unsafe extern "C" fn trampoline() {
+    asm!(
+      r#"
+        # Save frame pointer explicitly; the unwinder uses it to find CFA of
+        # the caller, and so it has to have the correct value immediately after
+        # the call instruction that invoked the trampoline.
+        pushq   %rbp
+        .cfi_adjust_cfa_offset 8
+        .cfi_rel_offset %rbp, 0
+
+        # Link the call stacks together by writing the current stack bottom
+        # address to the CFA slot in the new stack.
+        movq    %rsp, (%rcx)
+
+        # Load stack pointer of the new context.
+        movq    %rdx, %rsp
+
+        # Restore frame pointer of the new context.
+        popq    %rbp
+        .cfi_adjust_cfa_offset -8
+        .cfi_restore %rbp
+
+        # Jump to the unwind function, which will force a stack unwind in the
+        # target context. This will eventually return to our caller through the
+        # stack link.
+        jmp     ${0:c}
+      "#
+      : : "s" (unwind::force_unwind as usize) : : "volatile")
+  }
+
+  asm!(
+    r#"
+      # Push instruction pointer of the old context and switch to
+      # the new context.
+      call    ${0:c}
+    "#
+    :
+    : "s" (trampoline as usize)
+      "{rdx}" (new_sp.0)
+      "{rcx}" (new_cfa)
+    : "rax",   "rbx",   "rcx",   "rdx",   "rsi",   "rdi", /*"rbp",   "rsp",*/
+      "r8",    "r9",    "r10",   "r11",   "r12",   "r13",   "r14",   "r15",
+      "mm0",   "mm1",   "mm2",   "mm3",   "mm4",   "mm5",   "mm6",   "mm7",
+      "xmm0",  "xmm1",  "xmm2",  "xmm3",  "xmm4",  "xmm5",  "xmm6",  "xmm7",
+      "xmm8",  "xmm9",  "xmm10", "xmm11", "xmm12", "xmm13", "xmm14", "xmm15",
+      "xmm16", "xmm17", "xmm18", "xmm19", "xmm20", "xmm21", "xmm22", "xmm23",
+      "xmm24", "xmm25", "xmm26", "xmm27", "xmm28", "xmm29", "xmm30", "xmm31",
+      "cc", "dirflag", "fpsr", "flags", "memory"
+      // Ideally, we would set the LLVM "noredzone" attribute on this function
+      // (and it would be propagated to the call site). Unfortunately, rustc
+      // provides no such functionality. Fortunately, by a lucky coincidence,
+      // the "alignstack" LLVM inline assembly option does exactly the same
+      // thing on x86_64.
+    : "volatile", "alignstack");
+}

src/generator.rs

@@ -176,14 +176,26 @@ impl<'a, Input, Output, Stack> Generator<'a, Input, Output, Stack>
   /// Extracts the stack from a generator when the generator function has returned.
   /// If the generator function has not returned
   /// (i.e. `self.state() == State::Runnable`), panics.
-  pub fn unwrap(self) -> Stack {
-    match self.state() {
-      State::Runnable    => panic!("Argh! Bastard! Don't touch that!"),
-      State::Unavailable => self.stack
+  pub fn unwrap(mut self) -> Stack {
+    unsafe {
+      self.stack_ptr.map(|stack_ptr| arch::unwind(stack_ptr, &self.stack));
+
+      // We can't just return self.stack since Generator has a Drop impl
+      let stack = ptr::read(&self.stack);
+      ptr::drop_in_place(&mut self.stack_id);
+      mem::forget(self);
+      stack
     }
   }
 }
 
+impl<Input, Output, Stack> Drop for Generator<Input, Output, Stack>
+    where Input: Send, Output: Send, Stack: stack::Stack {
+  fn drop(&mut self) {
+    self.stack_ptr.map(|stack_ptr| unsafe { arch::unwind(stack_ptr, &self.stack) });
+  }
+}
+
 /// Yielder is an interface provided to every generator through which it
 /// returns a value.
 #[derive(Debug)]

src/lib.rs

@@ -29,7 +29,7 @@
 //!   * a stack allocator based on anonymous memory mappings with guard pages,
 //!     [OsStack](struct.OsStack.html).
 
-#[cfg(test)]
+//#[cfg(test)]
 #[macro_use]
 extern crate std;
 
@@ -51,6 +51,8 @@ pub const STACK_ALIGNMENT: usize = arch::STACK_ALIGNMENT;
 
 mod debug;
 
+mod unwind;
+
 mod stack;
 mod slice_stack;
 pub mod generator;

src/os/mod.rs

@@ -28,8 +28,8 @@ impl Stack {
   pub fn new(size: usize) -> Result<Stack, IoError> {
     let page_size = sys::page_size();
 
-    // Stacks have to be at least one page long.
-    let len = if size == 0 { page_size } else { size };
+    // Stacks have to be at least 16KB to support unwinding.
+    let len = if size == 0 { 16384 } else { size };
 
     // Round the length one page size up, using the fact that the page size
     // is a power of two.

src/unwind.rs

@@ -0,0 +1,30 @@
+// This file is part of libfringe, a low-level green threading library.
+// Copyright (c) Amanieu d'Antras <[email protected]>,
+// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
+// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
+// http://opensource.org/licenses/MIT>, at your option. This file may not be
+// copied, modified, or distributed except according to those terms.
+
+use std::panic;
+use arch::StackPointer;
+use std::boxed::Box;
+
+struct UnwindMarker;
+
+pub unsafe extern "C" fn unwind_wrapper(arg: usize, sp: StackPointer,
+                                        f: unsafe extern "C" fn(usize, StackPointer)) {
+  match panic::catch_unwind(move || f(arg, sp)) {
+    Ok(_) => (),
+    Err(err) => {
+      if !err.is::<UnwindMarker>() {
+        // Propagate the panic to the parent context if it wasn't generated by us
+        panic::resume_unwind(err)
+      }
+    }
+  }
+}
+
+pub unsafe extern "C" fn force_unwind() -> ! {
+  // Use resume_unwind instead of panic! to avoid printing a message
+  panic::resume_unwind(Box::new(UnwindMarker));
+}

tests/generator.rs

@@ -68,7 +68,7 @@ fn panic_safety() {
 
 #[test]
 fn with_slice_stack() {
-  let mut memory = [0; 1024];
+  let mut memory = [0; 16384];
   let stack = SliceStack(&mut memory);
   let mut add_one = unsafe { Generator::unsafe_new(stack, add_one_fn) };
   assert_eq!(add_one.resume(1), Some(2));
@@ -77,7 +77,7 @@ fn with_slice_stack() {
 
 #[test]
 fn with_owned_stack() {
-  let stack = OwnedStack::new(1024);
+  let stack = OwnedStack::new(16384);
   let mut add_one = unsafe { Generator::unsafe_new(stack, add_one_fn) };
   assert_eq!(add_one.resume(1), Some(2));
   assert_eq!(add_one.resume(2), Some(3));