machinst x64: New backend unwind (#2266)

Addresses unwind for experimental x64 backend. The preliminary code enables backtrace on SystemV call convension.

cranelift/codegen/src/isa/arm32/inst/emit.rs

@@ -274,6 +274,7 @@ impl MachInstEmitInfo for EmitInfo {
 impl MachInstEmit for Inst {
     type Info = EmitInfo;
     type State = EmitState;
+    type UnwindInfo = super::unwind::Arm32UnwindInfo;
 
     fn emit(&self, sink: &mut MachBuffer<Inst>, emit_info: &Self::Info, state: &mut EmitState) {
         let start_off = sink.cur_offset();

cranelift/codegen/src/isa/arm32/inst/mod.rs

@@ -22,6 +22,7 @@ mod emit;
 pub use self::emit::*;
 mod regs;
 pub use self::regs::*;
+pub mod unwind;
 
 #[cfg(test)]
 mod emit_tests;

cranelift/codegen/src/isa/arm32/inst/unwind.rs

@@ -0,0 +1,15 @@
+use super::*;
+use crate::isa::unwind::UnwindInfo;
+use crate::result::CodegenResult;
+
+pub struct Arm32UnwindInfo;
+
+impl UnwindInfoGenerator<Inst> for Arm32UnwindInfo {
+    fn create_unwind_info(
+        _context: UnwindInfoContext<Inst>,
+        _kind: UnwindInfoKind,
+    ) -> CodegenResult<Option<UnwindInfo>> {
+        // TODO
+        Ok(None)
+    }
+}

cranelift/codegen/src/isa/arm32/mod.rs

@@ -73,6 +73,7 @@ impl MachBackend for Arm32Backend {
             buffer,
             frame_size,
             disasm,
+            unwind_info: None,
         })
     }