Reftypes part two: add support for stackmaps.

This commit adds support for generating stackmaps at safepoints to the
new backend framework and to the AArch64 backend in particular. It has
been tested to work with SpiderMonkey.

cranelift/codegen/src/machinst/mod.rs

@@ -96,7 +96,7 @@
 //!
 //! ```
 
-use crate::binemit::{CodeInfo, CodeOffset};
+use crate::binemit::{CodeInfo, CodeOffset, Stackmap};
 use crate::ir::condcodes::IntCC;
 use crate::ir::{Function, Type};
 use crate::result::CodegenResult;
@@ -191,6 +191,10 @@ pub trait MachInst: Clone + Debug {
     /// What is the worst-case instruction size emitted by this instruction type?
     fn worst_case_size() -> CodeOffset;
 
+    /// What is the register class used for reference types (GC-observable pointers)? Can
+    /// be dependent on compilation flags.
+    fn ref_type_rc(_flags: &Flags) -> RegClass;
+
     /// A label-use kind: a type that describes the types of label references that
     /// can occur in an instruction.
     type LabelUse: MachInstLabelUse;
@@ -256,9 +260,21 @@ pub enum MachTerminator<'a> {
 /// A trait describing the ability to encode a MachInst into binary machine code.
 pub trait MachInstEmit: MachInst {
     /// Persistent state carried across `emit` invocations.
-    type State: Default + Clone + Debug;
+    type State: MachInstEmitState<Self>;
     /// Emit the instruction.
     fn emit(&self, code: &mut MachBuffer<Self>, flags: &Flags, state: &mut Self::State);
+    /// Pretty-print the instruction.
+    fn pretty_print(&self, mb_rru: Option<&RealRegUniverse>, state: &mut Self::State) -> String;
+}
+
+/// A trait describing the emission state carried between MachInsts when
+/// emitting a function body.
+pub trait MachInstEmitState<I: MachInst>: Default + Clone + Debug {
+    /// Create a new emission state given the ABI object.
+    fn new(abi: &dyn ABIBody<I = I>) -> Self;
+    /// Update the emission state before emitting an instruction that is a
+    /// safepoint.
+    fn pre_safepoint(&mut self, _stackmap: Stackmap) {}
 }
 
 /// The result of a `MachBackend::compile_function()` call. Contains machine