Track frame layout changes. (#1204)

* Track frame layout changes.

cranelift/codegen/src/ir/framelayout.rs

@@ -0,0 +1,70 @@
+//! Frame layout item changes.
+
+use crate::ir::entities::Inst;
+use crate::isa::RegUnit;
+use std::boxed::Box;
+
+use crate::HashMap;
+
+/// Change in the frame layout information.
+#[derive(Clone, Copy, Debug, Eq, PartialEq, Hash)]
+pub enum FrameLayoutChange {
+    /// Base CallFrameAddress (CFA) pointer moved to different register/offset.
+    CallFrameAddressAt {
+        /// CFA register.
+        reg: RegUnit,
+        /// CFA offset.
+        offset: isize,
+    },
+    /// Register saved at.
+    RegAt {
+        /// Saved register.
+        reg: RegUnit,
+        /// Offset in the frame (offset from CFA).
+        cfa_offset: isize,
+    },
+    /// Return address saved at.
+    ReturnAddressAt {
+        /// Offset in the frame (offset from CFA).
+        cfa_offset: isize,
+    },
+    /// The entire frame layout must be preserved somewhere to be restored at a corresponding
+    /// `Restore` change.
+    ///
+    /// This likely maps to the DWARF call frame instruction `.cfa_remember_state`.
+    Preserve,
+    /// Restore the entire frame layout from a corresponding prior `Preserve` frame change.
+    ///
+    /// This likely maps to the DWARF call frame instruction `.cfa_restore_state`.
+    Restore,
+}
+
+/// Set of frame layout changes.
+pub type FrameLayoutChanges = Box<[FrameLayoutChange]>;
+
+/// Frame items layout for (prologue/epilogue) instructions.
+#[derive(Debug, Clone)]
+pub struct FrameLayout {
+    /// Initial frame layout.
+    pub initial: FrameLayoutChanges,
+
+    /// Instruction frame layout (changes). Because the map will not be dense,
+    /// a HashMap is used instead of a SecondaryMap.
+    pub instructions: HashMap<Inst, FrameLayoutChanges>,
+}
+
+impl FrameLayout {
+    /// Create instance of FrameLayout.
+    pub fn new() -> Self {
+        FrameLayout {
+            initial: vec![].into_boxed_slice(),
+            instructions: HashMap::new(),
+        }
+    }
+
+    /// Clear the structure.
+    pub fn clear(&mut self) {
+        self.initial = vec![].into_boxed_slice();
+        self.instructions.clear();
+    }
+}