cranelift: refactor unwind logic to accommodate multiple backends (#2357)

*    Make cranelift_codegen::isa::unwind::input public
*    Move UnwindCode's common offset field out of the structure
*    Make MachCompileResult::unwind_info more generic
*    Record initial stack pointer offset

cranelift/codegen/src/isa/unwind.rs

@@ -21,51 +21,68 @@ pub enum UnwindInfo {
     SystemV(systemv::UnwindInfo),
 }
 
-pub(crate) mod input {
+/// Intermediate representation for the unwind information
+/// generated by a backend.
+pub mod input {
     use crate::binemit::CodeOffset;
     use alloc::vec::Vec;
     #[cfg(feature = "enable-serde")]
     use serde::{Deserialize, Serialize};
 
+    /// Elementary operation in the unwind operations.
     #[derive(Clone, Debug, PartialEq, Eq)]
     #[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
-    pub(crate) enum UnwindCode<Reg> {
+    pub enum UnwindCode<Reg> {
+        /// Defines that a register is saved at the specified offset.
         SaveRegister {
-            offset: CodeOffset,
+            /// The saved register.
             reg: Reg,
+            /// The specified offset relative to the stack pointer.
             stack_offset: u32,
         },
+        /// Defines that a register is as defined before call.
         RestoreRegister {
-            offset: CodeOffset,
+            /// The restored register.
             reg: Reg,
         },
+        /// The stack pointer was adjusted to allocate the stack.
         StackAlloc {
-            offset: CodeOffset,
+            /// Size to allocate.
             size: u32,
         },
+        /// The stack pointer was adjusted to free the stack.
         StackDealloc {
-            offset: CodeOffset,
+            /// Size to deallocate.
             size: u32,
         },
+        /// The alternative register was assigned as frame pointer base.
         SetFramePointer {
-            offset: CodeOffset,
+            /// The specified register.
             reg: Reg,
         },
-        RememberState {
-            offset: CodeOffset,
-        },
-        RestoreState {
-            offset: CodeOffset,
-        },
+        /// Restores a frame pointer base to default register.
+        RestoreFramePointer,
+        /// Saves the state.
+        RememberState,
+        /// Restores the state.
+        RestoreState,
     }
 
+    /// Unwind information as generated by a backend.
     #[derive(Clone, Debug, PartialEq, Eq)]
     #[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
     pub struct UnwindInfo<Reg> {
-        pub(crate) prologue_size: CodeOffset,
-        pub(crate) prologue_unwind_codes: Vec<UnwindCode<Reg>>,
-        pub(crate) epilogues_unwind_codes: Vec<Vec<UnwindCode<Reg>>>,
-        pub(crate) function_size: CodeOffset,
-        pub(crate) word_size: u8,
+        /// Size of the prologue.
+        pub prologue_size: CodeOffset,
+        /// Unwind codes for prologue.
+        pub prologue_unwind_codes: Vec<(CodeOffset, UnwindCode<Reg>)>,
+        /// Unwind codes for epilogues.
+        pub epilogues_unwind_codes: Vec<Vec<(CodeOffset, UnwindCode<Reg>)>>,
+        /// Entire function size.
+        pub function_size: CodeOffset,
+        /// Platform word size in bytes.
+        pub word_size: u8,
+        /// Initial stack pointer offset.
+        pub initial_sp_offset: u8,
     }
 }