Refactor unwind; add FDE support. (#1320)

* Refactor unwind

* add FDE support

* use sink directly in emit functions

* pref off all unwinding generation with feature

cranelift/codegen/src/binemit/mod.rs

@@ -155,6 +155,36 @@ pub trait CodeSink {
     fn add_stackmap(&mut self, _: &[Value], _: &Function, _: &dyn TargetIsa);
 }
 
+/// Type of the frame unwind information.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub enum FrameUnwindKind {
+    /// Windows fastcall unwinding (as in .pdata).
+    Fastcall,
+    /// FDE entry for libunwind (similar to .eh_frame format).
+    Libunwind,
+}
+
+/// Offset in frame unwind information buffer.
+pub type FrameUnwindOffset = usize;
+
+/// Sink for frame unwind information.
+pub trait FrameUnwindSink {
+    /// Get the current position.
+    fn len(&self) -> FrameUnwindOffset;
+
+    /// Add bytes to the code section.
+    fn bytes(&mut self, _: &[u8]);
+
+    /// Reserves bytes in the buffer.
+    fn reserve(&mut self, _len: usize) {}
+
+    /// Add a relocation entry.
+    fn reloc(&mut self, _: Reloc, _: FrameUnwindOffset);
+
+    /// Specified offset to main structure.
+    fn set_entry_offset(&mut self, _: FrameUnwindOffset);
+}
+
 /// Report a bad encoding error.
 #[cold]
 pub fn bad_encoding(func: &Function, inst: Inst) -> ! {