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/context.rs

@@ -253,13 +253,16 @@ impl Context {
         &self,
         isa: &dyn TargetIsa,
     ) -> CodegenResult<Option<crate::isa::unwind::UnwindInfo>> {
-        if self.mach_compile_result.is_some() {
+        if let Some(backend) = isa.get_mach_backend() {
-            return Ok(self
+            use crate::isa::CallConv;
-                .mach_compile_result
+            use crate::machinst::UnwindInfoKind;
-                .as_ref()
+            let unwind_info_kind = match self.func.signature.call_conv {
-                .unwrap()
+                CallConv::Fast | CallConv::Cold | CallConv::SystemV => UnwindInfoKind::SystemV,
-                .unwind_info
+                CallConv::WindowsFastcall => UnwindInfoKind::Windows,
-                .clone());
+                _ => UnwindInfoKind::None,
+            };
+            let result = self.mach_compile_result.as_ref().unwrap();
+            return backend.emit_unwind_info(result, unwind_info_kind);
         }
         isa.create_unwind_info(&self.func)
     }

cranelift/codegen/src/isa/aarch64/inst/unwind.rs

@@ -1,5 +1,5 @@
 use super::*;
-use crate::isa::unwind::UnwindInfo;
+use crate::isa::unwind::input::UnwindInfo;
 use crate::result::CodegenResult;
 
 pub struct AArch64UnwindInfo;
@@ -7,8 +7,7 @@ pub struct AArch64UnwindInfo;
 impl UnwindInfoGenerator<Inst> for AArch64UnwindInfo {
     fn create_unwind_info(
         _context: UnwindInfoContext<Inst>,
-        _kind: UnwindInfoKind,
+    ) -> CodegenResult<Option<UnwindInfo<Reg>>> {
-    ) -> CodegenResult<Option<UnwindInfo>> {
         // TODO
         Ok(None)
     }

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

@@ -1,5 +1,5 @@
 use super::*;
-use crate::isa::unwind::UnwindInfo;
+use crate::isa::unwind::input::UnwindInfo;
 use crate::result::CodegenResult;
 
 pub struct Arm32UnwindInfo;
@@ -7,8 +7,7 @@ pub struct Arm32UnwindInfo;
 impl UnwindInfoGenerator<Inst> for Arm32UnwindInfo {
     fn create_unwind_info(
         _context: UnwindInfoContext<Inst>,
-        _kind: UnwindInfoKind,
+    ) -> CodegenResult<Option<UnwindInfo<Reg>>> {
-    ) -> CodegenResult<Option<UnwindInfo>> {
         // TODO
         Ok(None)
     }

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 {
+        /// Restores a frame pointer base to default register.
-            offset: CodeOffset,
+        RestoreFramePointer,
-        },
+        /// Saves the state.
-        RestoreState {
+        RememberState,
-            offset: CodeOffset,
+        /// 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,
+        /// Size of the prologue.
-        pub(crate) prologue_unwind_codes: Vec<UnwindCode<Reg>>,
+        pub prologue_size: CodeOffset,
-        pub(crate) epilogues_unwind_codes: Vec<Vec<UnwindCode<Reg>>>,
+        /// Unwind codes for prologue.
-        pub(crate) function_size: CodeOffset,
+        pub prologue_unwind_codes: Vec<(CodeOffset, UnwindCode<Reg>)>,
-        pub(crate) word_size: u8,
+        /// 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,
     }
 }

cranelift/codegen/src/isa/unwind/systemv.rs

@@ -98,8 +98,8 @@ impl Into<gimli::write::CallFrameInstruction> for CallFrameInstruction {
 pub(crate) trait RegisterMapper<Reg> {
     /// Maps Reg.
     fn map(&self, reg: Reg) -> Result<Register, RegisterMappingError>;
-    /// Gets RSP in gimli's index space.
+    /// Gets stack pointer register.
-    fn rsp(&self) -> Register;
+    fn sp(&self) -> Register;
 }
 
 /// Represents unwind information for a single System V ABI function.
@@ -118,9 +118,9 @@ impl UnwindInfo {
         map_reg: &'b dyn RegisterMapper<Reg>,
     ) -> CodegenResult<Self> {
         use input::UnwindCode;
-        let mut builder = InstructionBuilder::new(unwind.word_size, map_reg);
+        let mut builder = InstructionBuilder::new(unwind.initial_sp_offset, map_reg);
 
-        for c in unwind.prologue_unwind_codes.iter().chain(
+        for (offset, c) in unwind.prologue_unwind_codes.iter().chain(
             unwind
                 .epilogues_unwind_codes
                 .iter()
@@ -128,38 +128,36 @@ impl UnwindInfo {
                 .flatten(),
         ) {
             match c {
-                UnwindCode::SaveRegister {
+                UnwindCode::SaveRegister { reg, stack_offset } => {
-                    offset,
-                    reg,
-                    stack_offset: 0,
-                } => {
                     builder
-                        .save_reg(*offset, *reg)
+                        .save_reg(*offset, *reg, *stack_offset)
                         .map_err(CodegenError::RegisterMappingError)?;
                 }
-                UnwindCode::StackAlloc { offset, size } => {
+                UnwindCode::StackAlloc { size } => {
                     builder.adjust_sp_down_imm(*offset, *size as i64);
                 }
-                UnwindCode::StackDealloc { offset, size } => {
+                UnwindCode::StackDealloc { size } => {
                     builder.adjust_sp_up_imm(*offset, *size as i64);
                 }
-                UnwindCode::RestoreRegister { offset, reg } => {
+                UnwindCode::RestoreRegister { reg } => {
                     builder
                         .restore_reg(*offset, *reg)
                         .map_err(CodegenError::RegisterMappingError)?;
                 }
-                UnwindCode::SetFramePointer { offset, reg } => {
+                UnwindCode::SetFramePointer { reg } => {
                     builder
                         .set_cfa_reg(*offset, *reg)
                         .map_err(CodegenError::RegisterMappingError)?;
                 }
-                UnwindCode::RememberState { offset } => {
+                UnwindCode::RestoreFramePointer => {
+                    builder.restore_cfa(*offset);
+                }
+                UnwindCode::RememberState => {
                     builder.remember_state(*offset);
                 }
-                UnwindCode::RestoreState { offset } => {
+                UnwindCode::RestoreState => {
                     builder.restore_state(*offset);
                 }
-                _ => {}
             }
         }
 
@@ -190,9 +188,9 @@ struct InstructionBuilder<'a, Reg: PartialEq + Copy> {
 }
 
 impl<'a, Reg: PartialEq + Copy> InstructionBuilder<'a, Reg> {
-    fn new(word_size: u8, map_reg: &'a (dyn RegisterMapper<Reg> + 'a)) -> Self {
+    fn new(sp_offset: u8, map_reg: &'a (dyn RegisterMapper<Reg> + 'a)) -> Self {
         Self {
-            sp_offset: word_size as i32, // CFA offset starts at word size offset to account for the return address on stack
+            sp_offset: sp_offset as i32, // CFA offset starts at the specified offset to account for the return address on stack
             saved_state: None,
             frame_register: None,
             map_reg,
@@ -200,11 +198,19 @@ impl<'a, Reg: PartialEq + Copy> InstructionBuilder<'a, Reg> {
         }
     }
 
-    fn save_reg(&mut self, offset: u32, reg: Reg) -> Result<(), RegisterMappingError> {
+    fn save_reg(
+        &mut self,
+        offset: u32,
+        reg: Reg,
+        stack_offset: u32,
+    ) -> Result<(), RegisterMappingError> {
         // Pushes in the prologue are register saves, so record an offset of the save
         self.instructions.push((
             offset,
-            CallFrameInstruction::Offset(self.map_reg.map(reg)?, -self.sp_offset),
+            CallFrameInstruction::Offset(
+                self.map_reg.map(reg)?,
+                stack_offset as i32 - self.sp_offset,
+            ),
         ));
 
         Ok(())
@@ -270,15 +276,16 @@ impl<'a, Reg: PartialEq + Copy> InstructionBuilder<'a, Reg> {
         Ok(())
     }
 
-    fn restore_reg(&mut self, offset: u32, reg: Reg) -> Result<(), RegisterMappingError> {
+    fn restore_cfa(&mut self, offset: u32) {
-        // Update the CFA if this is the restore of the frame pointer register.
+        // Restore SP and its offset.
-        if Some(reg) == self.frame_register {
-            self.frame_register = None;
         self.instructions.push((
             offset,
-                CallFrameInstruction::Cfa(self.map_reg.rsp(), self.sp_offset),
+            CallFrameInstruction::Cfa(self.map_reg.sp(), self.sp_offset),
         ));
+        self.frame_register = None;
     }
+
+    fn restore_reg(&mut self, offset: u32, reg: Reg) -> Result<(), RegisterMappingError> {
         // Pops in the epilogue are register restores, so record a "same value" for the register
         self.instructions.push((
             offset,

cranelift/codegen/src/isa/unwind/winx64.rs

@@ -226,13 +226,9 @@ impl UnwindInfo {
 
         let word_size: u32 = unwind.word_size.into();
         let mut unwind_codes = Vec::new();
-        for c in unwind.prologue_unwind_codes.iter() {
+        for (offset, c) in unwind.prologue_unwind_codes.iter() {
             match c {
-                InputUnwindCode::SaveRegister {
+                InputUnwindCode::SaveRegister { reg, stack_offset } => {
-                    offset,
-                    reg,
-                    stack_offset,
-                } => {
                     let reg = MR::map(*reg);
                     let offset = ensure_unwind_offset(*offset)?;
                     match reg {
@@ -269,7 +265,7 @@ impl UnwindInfo {
                         }
                     }
                 }
-                InputUnwindCode::StackAlloc { offset, size } => {
+                InputUnwindCode::StackAlloc { size } => {
                     unwind_codes.push(UnwindCode::StackAlloc {
                         offset: ensure_unwind_offset(*offset)?,
                         size: *size,

cranelift/codegen/src/isa/x64/inst/unwind.rs

@@ -1,36 +1,125 @@
-use super::Inst;
+use crate::isa::unwind::input::UnwindInfo;
-use crate::isa::unwind::UnwindInfo;
+use crate::isa::x64::inst::{
-use crate::machinst::{UnwindInfoContext, UnwindInfoGenerator, UnwindInfoKind};
+    args::{AluRmiROpcode, Amode, RegMemImm, SyntheticAmode},
+    regs, Inst,
+};
+use crate::machinst::{UnwindInfoContext, UnwindInfoGenerator};
 use crate::result::CodegenResult;
+use alloc::vec::Vec;
+use regalloc::Reg;
 
 #[cfg(feature = "unwind")]
-pub use self::systemv::create_cie;
+pub(crate) mod systemv;
-
-#[cfg(feature = "unwind")]
-mod systemv;
 
 pub struct X64UnwindInfo;
 
 impl UnwindInfoGenerator<Inst> for X64UnwindInfo {
-    #[allow(unused_variables)]
     fn create_unwind_info(
         context: UnwindInfoContext<Inst>,
-        kind: UnwindInfoKind,
+    ) -> CodegenResult<Option<UnwindInfo<Reg>>> {
-    ) -> CodegenResult<Option<UnwindInfo>> {
+        use crate::isa::unwind::input::{self, UnwindCode};
-        // Assumption: RBP is being used as the frame pointer for both calling conventions
+        let mut codes = Vec::new();
-        // In the future, we should be omitting frame pointer as an optimization, so this will change
-        Ok(match kind {
-            #[cfg(feature = "unwind")]
-            UnwindInfoKind::SystemV => {
         const WORD_SIZE: u8 = 8;
-                systemv::create_unwind_info(context, WORD_SIZE)?.map(UnwindInfo::SystemV)
+
+        for i in context.prologue.clone() {
+            let i = i as usize;
+            let inst = &context.insts[i];
+            let offset = context.insts_layout[i];
+
+            match inst {
+                Inst::Push64 {
+                    src: RegMemImm::Reg { reg },
+                } => {
+                    codes.push((
+                        offset,
+                        UnwindCode::StackAlloc {
+                            size: WORD_SIZE.into(),
+                        },
+                    ));
+                    codes.push((
+                        offset,
+                        UnwindCode::SaveRegister {
+                            reg: *reg,
+                            stack_offset: 0,
+                        },
+                    ));
                 }
-            #[cfg(feature = "unwind")]
+                Inst::MovRR { src, dst, .. } => {
-            UnwindInfoKind::Windows => {
+                    if *src == regs::rsp() {
-                //TODO winx64::create_unwind_info(context)?.map(|u| UnwindInfo::WindowsX64(u))
+                        codes.push((offset, UnwindCode::SetFramePointer { reg: dst.to_reg() }));
-                panic!();
                     }
-            UnwindInfoKind::None => None,
+                }
+                Inst::AluRmiR {
+                    is_64: true,
+                    op: AluRmiROpcode::Sub,
+                    src: RegMemImm::Imm { simm32 },
+                    dst,
+                    ..
+                } if dst.to_reg() == regs::rsp() => {
+                    let imm = *simm32;
+                    codes.push((offset, UnwindCode::StackAlloc { size: imm }));
+                }
+                Inst::MovRM {
+                    src,
+                    dst: SyntheticAmode::Real(Amode::ImmReg { simm32, base }),
+                    ..
+                } if *base == regs::rsp() => {
+                    // `mov reg, imm(rsp)`
+                    let imm = *simm32;
+                    codes.push((
+                        offset,
+                        UnwindCode::SaveRegister {
+                            reg: *src,
+                            stack_offset: imm,
+                        },
+                    ));
+                }
+                Inst::AluRmiR {
+                    is_64: true,
+                    op: AluRmiROpcode::Add,
+                    src: RegMemImm::Imm { simm32 },
+                    dst,
+                    ..
+                } if dst.to_reg() == regs::rsp() => {
+                    let imm = *simm32;
+                    codes.push((offset, UnwindCode::StackDealloc { size: imm }));
+                }
+                _ => {}
+            }
+        }
+
+        let last_epilogue_end = context.len;
+        let epilogues_unwind_codes = context
+            .epilogues
+            .iter()
+            .map(|epilogue| {
+                // TODO add logic to process epilogue instruction instead of
+                // returning empty array.
+                let end = epilogue.end as usize - 1;
+                let end_offset = context.insts_layout[end];
+                if end_offset == last_epilogue_end {
+                    // Do not remember/restore for very last epilogue.
+                    return vec![];
+                }
+
+                let start = epilogue.start as usize;
+                let offset = context.insts_layout[start];
+                vec![
+                    (offset, UnwindCode::RememberState),
+                    // TODO epilogue instructions
+                    (end_offset, UnwindCode::RestoreState),
+                ]
             })
+            .collect();
+
+        let prologue_size = context.insts_layout[context.prologue.end as usize];
+        Ok(Some(input::UnwindInfo {
+            prologue_size,
+            prologue_unwind_codes: codes,
+            epilogues_unwind_codes,
+            function_size: context.len,
+            word_size: WORD_SIZE,
+            initial_sp_offset: WORD_SIZE,
+        }))
     }
 }

cranelift/codegen/src/isa/x64/inst/unwind/systemv.rs

@@ -1,13 +1,8 @@
 //! Unwind information for System V ABI (x86-64).
 
+use crate::isa::unwind::input;
 use crate::isa::unwind::systemv::{RegisterMappingError, UnwindInfo};
-use crate::isa::x64::inst::{
-    args::{AluRmiROpcode, Amode, RegMemImm, SyntheticAmode},
-    regs, Inst,
-};
-use crate::machinst::UnwindInfoContext;
 use crate::result::CodegenResult;
-use alloc::vec::Vec;
 use gimli::{write::CommonInformationEntry, Encoding, Format, Register, X86_64};
 use regalloc::{Reg, RegClass};
 
@@ -88,113 +83,15 @@ pub fn map_reg(reg: Reg) -> Result<Register, RegisterMappingError> {
 }
 
 pub(crate) fn create_unwind_info(
-    context: UnwindInfoContext<Inst>,
+    unwind: input::UnwindInfo<Reg>,
-    word_size: u8,
 ) -> CodegenResult<Option<UnwindInfo>> {
-    use crate::isa::unwind::input::{self, UnwindCode};
-    let mut codes = Vec::new();
-
-    for i in context.prologue.clone() {
-        let i = i as usize;
-        let inst = &context.insts[i];
-        let offset = context.insts_layout[i];
-
-        match inst {
-            Inst::Push64 {
-                src: RegMemImm::Reg { reg },
-            } => {
-                codes.push(UnwindCode::StackAlloc {
-                    offset,
-                    size: word_size.into(),
-                });
-                codes.push(UnwindCode::SaveRegister {
-                    offset,
-                    reg: *reg,
-                    stack_offset: 0,
-                });
-            }
-            Inst::MovRR { src, dst, .. } => {
-                if *src == regs::rsp() {
-                    codes.push(UnwindCode::SetFramePointer {
-                        offset,
-                        reg: dst.to_reg(),
-                    });
-                }
-            }
-            Inst::AluRmiR {
-                is_64: true,
-                op: AluRmiROpcode::Sub,
-                src: RegMemImm::Imm { simm32 },
-                dst,
-                ..
-            } if dst.to_reg() == regs::rsp() => {
-                let imm = *simm32;
-                codes.push(UnwindCode::StackAlloc { offset, size: imm });
-            }
-            Inst::MovRM {
-                src,
-                dst: SyntheticAmode::Real(Amode::ImmReg { simm32, base }),
-                ..
-            } if *base == regs::rsp() => {
-                // `mov reg, imm(rsp)`
-                let imm = *simm32;
-                codes.push(UnwindCode::SaveRegister {
-                    offset,
-                    reg: *src,
-                    stack_offset: imm,
-                });
-            }
-            Inst::AluRmiR {
-                is_64: true,
-                op: AluRmiROpcode::Add,
-                src: RegMemImm::Imm { simm32 },
-                dst,
-                ..
-            } if dst.to_reg() == regs::rsp() => {
-                let imm = *simm32;
-                codes.push(UnwindCode::StackDealloc { offset, size: imm });
-            }
-            _ => {}
-        }
-    }
-
-    let last_epilogue_end = context.len;
-    let epilogues_unwind_codes = context
-        .epilogues
-        .iter()
-        .map(|epilogue| {
-            let end = epilogue.end as usize - 1;
-            let end_offset = context.insts_layout[end];
-            if end_offset == last_epilogue_end {
-                // Do not remember/restore for very last epilogue.
-                return vec![];
-            }
-
-            let start = epilogue.start as usize;
-            let offset = context.insts_layout[start];
-            vec![
-                UnwindCode::RememberState { offset },
-                UnwindCode::RestoreState { offset: end_offset },
-            ]
-        })
-        .collect();
-
-    let prologue_size = context.insts_layout[context.prologue.end as usize];
-    let unwind = input::UnwindInfo {
-        prologue_size,
-        prologue_unwind_codes: codes,
-        epilogues_unwind_codes,
-        function_size: context.len,
-        word_size,
-    };
-
     struct RegisterMapper;
     impl crate::isa::unwind::systemv::RegisterMapper<Reg> for RegisterMapper {
         fn map(&self, reg: Reg) -> Result<u16, RegisterMappingError> {
             Ok(map_reg(reg)?.0)
         }
-        fn rsp(&self) -> u16 {
+        fn sp(&self) -> u16 {
-            map_reg(regs::rsp()).unwrap().0
+            X86_64::RSP.0
         }
     }
     let map = RegisterMapper;

cranelift/codegen/src/isa/x64/mod.rs

@@ -103,10 +103,29 @@ impl MachBackend for X64Backend {
         IntCC::UnsignedGreaterThanOrEqual
     }
 
+    #[cfg(feature = "unwind")]
+    fn emit_unwind_info(
+        &self,
+        result: &MachCompileResult,
+        kind: crate::machinst::UnwindInfoKind,
+    ) -> CodegenResult<Option<crate::isa::unwind::UnwindInfo>> {
+        use crate::isa::unwind::UnwindInfo;
+        use crate::machinst::UnwindInfoKind;
+        Ok(match (result.unwind_info.as_ref(), kind) {
+            (Some(info), UnwindInfoKind::SystemV) => {
+                inst::unwind::systemv::create_unwind_info(info.clone())?.map(UnwindInfo::SystemV)
+            }
+            (Some(_info), UnwindInfoKind::Windows) => {
+                //TODO inst::unwind::winx64::create_unwind_info(info.clone())?.map(|u| UnwindInfo::WindowsX64(u))
+                None
+            }
+            _ => None,
+        })
+    }
+
     #[cfg(feature = "unwind")]
     fn create_systemv_cie(&self) -> Option<gimli::write::CommonInformationEntry> {
-        // By default, an ISA cannot create a System V CIE
+        Some(inst::unwind::systemv::create_cie())
-        Some(inst::unwind::create_cie())
     }
 }
 

cranelift/codegen/src/isa/x86/unwind.rs

@@ -81,7 +81,7 @@ pub(crate) fn create_unwind_info(
                 unwind_codes = epilogues_unwind_codes.last_mut().unwrap();
 
                 if !is_last_block {
-                    unwind_codes.push(UnwindCode::RememberState { offset });
+                    unwind_codes.push((offset, UnwindCode::RememberState));
                 }
             } else if in_epilogue {
                 unwind_codes = epilogues_unwind_codes.last_mut().unwrap();
@@ -95,15 +95,19 @@ pub(crate) fn create_unwind_info(
                     match opcode {
                         Opcode::X86Push => {
                             let reg = func.locations[arg].unwrap_reg();
-                            unwind_codes.push(UnwindCode::StackAlloc {
+                            unwind_codes.push((
                                 offset,
+                                UnwindCode::StackAlloc {
                                     size: word_size.into(),
-                            });
+                                },
-                            unwind_codes.push(UnwindCode::SaveRegister {
+                            ));
+                            unwind_codes.push((
                                 offset,
+                                UnwindCode::SaveRegister {
                                     reg,
                                     stack_offset: 0,
-                            });
+                                },
+                            ));
                         }
                         Opcode::AdjustSpDown => {
                             let stack_size =
@@ -111,10 +115,8 @@ pub(crate) fn create_unwind_info(
 
                             // This is used when calling a stack check function
                             // We need to track the assignment to RAX which has the size of the stack
-                            unwind_codes.push(UnwindCode::StackAlloc {
+                            unwind_codes
-                                offset,
+                                .push((offset, UnwindCode::StackAlloc { size: stack_size }));
-                                size: stack_size,
-                            });
                         }
                         _ => {}
                     }
@@ -138,10 +140,8 @@ pub(crate) fn create_unwind_info(
 
                             stack_size = Some(imm as u32);
 
-                            unwind_codes.push(UnwindCode::StackAlloc {
+                            unwind_codes
-                                offset,
+                                .push((offset, UnwindCode::StackAlloc { size: imm as u32 }));
-                                size: imm as u32,
-                            });
                         }
                         Opcode::AdjustSpUpImm => {
                             let imm: i64 = imm.into();
@@ -149,10 +149,8 @@ pub(crate) fn create_unwind_info(
 
                             stack_size = Some(imm as u32);
 
-                            unwind_codes.push(UnwindCode::StackDealloc {
+                            unwind_codes
-                                offset,
+                                .push((offset, UnwindCode::StackDealloc { size: imm as u32 }));
-                                size: imm as u32,
-                            });
                         }
                         _ => {}
                     }
@@ -170,18 +168,20 @@ pub(crate) fn create_unwind_info(
                         // Note: the stack_offset here is relative to an adjusted SP
                         if dst == (RU::rsp as RegUnit) && FPR.contains(src) {
                             let stack_offset: i32 = stack_offset.into();
-                            unwind_codes.push(UnwindCode::SaveRegister {
+                            unwind_codes.push((
                                 offset,
+                                UnwindCode::SaveRegister {
                                     reg: src,
                                     stack_offset: stack_offset as u32,
-                            });
+                                },
+                            ));
                         }
                     }
                 }
                 InstructionData::CopySpecial { src, dst, .. } if frame_register.is_none() => {
                     // Check for change in CFA register (RSP is always the starting CFA)
                     if src == (RU::rsp as RegUnit) {
-                        unwind_codes.push(UnwindCode::SetFramePointer { offset, reg: dst });
+                        unwind_codes.push((offset, UnwindCode::SetFramePointer { reg: dst }));
                         frame_register = Some(dst);
                     }
                 }
@@ -203,18 +203,25 @@ pub(crate) fn create_unwind_info(
                             let offset = epilogue_pop_offsets[i];
 
                             let reg = func.locations[*arg].unwrap_reg();
-                            unwind_codes.push(UnwindCode::RestoreRegister { offset, reg });
+                            unwind_codes.push((offset, UnwindCode::RestoreRegister { reg }));
-                            unwind_codes.push(UnwindCode::StackDealloc {
+                            unwind_codes.push((
                                 offset,
+                                UnwindCode::StackDealloc {
                                     size: word_size.into(),
-                            });
+                                },
+                            ));
+
+                            if Some(reg) == frame_register {
+                                unwind_codes.push((offset, UnwindCode::RestoreFramePointer));
+                                // Keep frame_register assigned for next epilogue.
+                            }
                         }
                         epilogue_pop_offsets.clear();
 
                         // TODO ensure unwind codes sorted by offsets ?
 
                         if !is_last_block {
-                            unwind_codes.push(UnwindCode::RestoreState { offset });
+                            unwind_codes.push((offset, UnwindCode::RestoreState));
                         }
 
                         in_epilogue = false;
@@ -232,6 +239,7 @@ pub(crate) fn create_unwind_info(
         epilogues_unwind_codes,
         function_size,
         word_size,
+        initial_sp_offset: word_size,
     }))
 }
 
@@ -271,37 +279,36 @@ mod tests {
             UnwindInfo {
                 prologue_size: 9,
                 prologue_unwind_codes: vec![
-                    UnwindCode::StackAlloc { offset: 2, size: 8 },
+                    (2, UnwindCode::StackAlloc { size: 8 }),
+                    (
+                        2,
                         UnwindCode::SaveRegister {
-                        offset: 2,
                             reg: RU::rbp.into(),
                             stack_offset: 0,
-                    },
-                    UnwindCode::SetFramePointer {
-                        offset: 5,
-                        reg: RU::rbp.into(),
-                    },
-                    UnwindCode::StackAlloc {
-                        offset: 9,
-                        size: 64
                         }
+                    ),
+                    (
+                        5,
+                        UnwindCode::SetFramePointer {
+                            reg: RU::rbp.into(),
+                        }
+                    ),
+                    (9, UnwindCode::StackAlloc { size: 64 })
                 ],
                 epilogues_unwind_codes: vec![vec![
-                    UnwindCode::StackDealloc {
+                    (13, UnwindCode::StackDealloc { size: 64 }),
-                        offset: 13,
+                    (
-                        size: 64
+                        15,
-                    },
                         UnwindCode::RestoreRegister {
-                        offset: 15,
                             reg: RU::rbp.into()
-                    },
-                    UnwindCode::StackDealloc {
-                        offset: 15,
-                        size: 8
                         }
+                    ),
+                    (15, UnwindCode::StackDealloc { size: 8 }),
+                    (15, UnwindCode::RestoreFramePointer)
                 ]],
                 function_size: 16,
                 word_size: 8,
+                initial_sp_offset: 8,
             }
         );
     }
@@ -329,37 +336,36 @@ mod tests {
             UnwindInfo {
                 prologue_size: 27,
                 prologue_unwind_codes: vec![
-                    UnwindCode::StackAlloc { offset: 2, size: 8 },
+                    (2, UnwindCode::StackAlloc { size: 8 }),
+                    (
+                        2,
                         UnwindCode::SaveRegister {
-                        offset: 2,
                             reg: RU::rbp.into(),
                             stack_offset: 0,
-                    },
-                    UnwindCode::SetFramePointer {
-                        offset: 5,
-                        reg: RU::rbp.into(),
-                    },
-                    UnwindCode::StackAlloc {
-                        offset: 27,
-                        size: 10000
                         }
+                    ),
+                    (
+                        5,
+                        UnwindCode::SetFramePointer {
+                            reg: RU::rbp.into(),
+                        }
+                    ),
+                    (27, UnwindCode::StackAlloc { size: 10000 })
                 ],
                 epilogues_unwind_codes: vec![vec![
-                    UnwindCode::StackDealloc {
+                    (34, UnwindCode::StackDealloc { size: 10000 }),
-                        offset: 34,
+                    (
-                        size: 10000
+                        36,
-                    },
                         UnwindCode::RestoreRegister {
-                        offset: 36,
                             reg: RU::rbp.into()
-                    },
-                    UnwindCode::StackDealloc {
-                        offset: 36,
-                        size: 8
                         }
+                    ),
+                    (36, UnwindCode::StackDealloc { size: 8 }),
+                    (36, UnwindCode::RestoreFramePointer)
                 ]],
                 function_size: 37,
                 word_size: 8,
+                initial_sp_offset: 8,
             }
         );
     }
@@ -387,37 +393,36 @@ mod tests {
             UnwindInfo {
                 prologue_size: 27,
                 prologue_unwind_codes: vec![
-                    UnwindCode::StackAlloc { offset: 2, size: 8 },
+                    (2, UnwindCode::StackAlloc { size: 8 }),
+                    (
+                        2,
                         UnwindCode::SaveRegister {
-                        offset: 2,
                             reg: RU::rbp.into(),
                             stack_offset: 0,
-                    },
-                    UnwindCode::SetFramePointer {
-                        offset: 5,
-                        reg: RU::rbp.into(),
-                    },
-                    UnwindCode::StackAlloc {
-                        offset: 27,
-                        size: 1000000
                         }
+                    ),
+                    (
+                        5,
+                        UnwindCode::SetFramePointer {
+                            reg: RU::rbp.into(),
+                        }
+                    ),
+                    (27, UnwindCode::StackAlloc { size: 1000000 })
                 ],
                 epilogues_unwind_codes: vec![vec![
-                    UnwindCode::StackDealloc {
+                    (34, UnwindCode::StackDealloc { size: 1000000 }),
-                        offset: 34,
+                    (
-                        size: 1000000
+                        36,
-                    },
                         UnwindCode::RestoreRegister {
-                        offset: 36,
                             reg: RU::rbp.into()
-                    },
-                    UnwindCode::StackDealloc {
-                        offset: 36,
-                        size: 8
                         }
+                    ),
+                    (36, UnwindCode::StackDealloc { size: 8 }),
+                    (36, UnwindCode::RestoreFramePointer)
                 ]],
                 function_size: 37,
                 word_size: 8,
+                initial_sp_offset: 8,
             }
         );
     }
@@ -458,43 +463,48 @@ mod tests {
             UnwindInfo {
                 prologue_size: 5,
                 prologue_unwind_codes: vec![
-                    UnwindCode::StackAlloc { offset: 2, size: 8 },
+                    (2, UnwindCode::StackAlloc { size: 8 }),
+                    (
+                        2,
                         UnwindCode::SaveRegister {
-                        offset: 2,
                             reg: RU::rbp.into(),
                             stack_offset: 0,
-                    },
+                        }
+                    ),
+                    (
+                        5,
                         UnwindCode::SetFramePointer {
-                        offset: 5,
                             reg: RU::rbp.into()
                         }
+                    )
                 ],
                 epilogues_unwind_codes: vec![
                     vec![
-                        UnwindCode::RememberState { offset: 12 },
+                        (12, UnwindCode::RememberState),
+                        (
+                            12,
                             UnwindCode::RestoreRegister {
-                            offset: 12,
                                 reg: RU::rbp.into()
-                        },
+                            }
-                        UnwindCode::StackDealloc {
+                        ),
-                            offset: 12,
+                        (12, UnwindCode::StackDealloc { size: 8 }),
-                            size: 8
+                        (12, UnwindCode::RestoreFramePointer),
-                        },
+                        (13, UnwindCode::RestoreState)
-                        UnwindCode::RestoreState { offset: 13 }
                     ],
                     vec![
+                        (
+                            15,
                             UnwindCode::RestoreRegister {
-                            offset: 15,
                                 reg: RU::rbp.into()
-                        },
-                        UnwindCode::StackDealloc {
-                            offset: 15,
-                            size: 8
                             }
+                        ),
+                        (15, UnwindCode::StackDealloc { size: 8 }),
+                        (15, UnwindCode::RestoreFramePointer)
                     ]
                 ],
                 function_size: 16,
                 word_size: 8,
+                initial_sp_offset: 8,
             }
         );
     }

cranelift/codegen/src/isa/x86/unwind/systemv.rs

@@ -118,7 +118,7 @@ pub(crate) fn create_unwind_info(
         fn map(&self, reg: RegUnit) -> Result<u16, RegisterMappingError> {
             Ok(map_reg(self.0, reg)?.0)
         }
-        fn rsp(&self) -> u16 {
+        fn sp(&self) -> u16 {
             X86_64::RSP.0
         }
     }
@@ -165,7 +165,7 @@ mod tests {
             _ => panic!("expected unwind information"),
         };
 
-        assert_eq!(format!("{:?}", fde), "FrameDescriptionEntry { address: Constant(1234), length: 16, lsda: None, instructions: [(2, CfaOffset(16)), (2, Offset(Register(6), -16)), (5, CfaRegister(Register(6))), (15, Cfa(Register(7), 8)), (15, SameValue(Register(6)))] }");
+        assert_eq!(format!("{:?}", fde), "FrameDescriptionEntry { address: Constant(1234), length: 16, lsda: None, instructions: [(2, CfaOffset(16)), (2, Offset(Register(6), -16)), (5, CfaRegister(Register(6))), (15, SameValue(Register(6))), (15, Cfa(Register(7), 8))] }");
     }
 
     fn create_function(call_conv: CallConv, stack_slot: Option<StackSlotData>) -> Function {
@@ -205,7 +205,7 @@ mod tests {
             _ => panic!("expected unwind information"),
         };
 
-        assert_eq!(format!("{:?}", fde), "FrameDescriptionEntry { address: Constant(4321), length: 16, lsda: None, instructions: [(2, CfaOffset(16)), (2, Offset(Register(6), -16)), (5, CfaRegister(Register(6))), (12, RememberState), (12, Cfa(Register(7), 8)), (12, SameValue(Register(6))), (13, RestoreState), (15, Cfa(Register(7), 8)), (15, SameValue(Register(6)))] }");
+        assert_eq!(format!("{:?}", fde), "FrameDescriptionEntry { address: Constant(4321), length: 16, lsda: None, instructions: [(2, CfaOffset(16)), (2, Offset(Register(6), -16)), (5, CfaRegister(Register(6))), (12, RememberState), (12, SameValue(Register(6))), (12, Cfa(Register(7), 8)), (13, RestoreState), (15, SameValue(Register(6))), (15, Cfa(Register(7), 8))] }");
     }
 
     fn create_multi_return_function(call_conv: CallConv) -> Function {

cranelift/codegen/src/machinst/adapter.rs

@@ -129,7 +129,7 @@ impl TargetIsa for TargetIsaAdapter {
         self.backend.unsigned_sub_overflow_condition()
     }
 
-    #[cfg(feature = "unwind")]
+    #[cfg(all(feature = "unwind", feature = "x64"))]
     fn create_systemv_cie(&self) -> Option<gimli::write::CommonInformationEntry> {
         self.backend.create_systemv_cie()
     }

cranelift/codegen/src/machinst/mod.rs

@@ -99,7 +99,7 @@
 use crate::binemit::{CodeInfo, CodeOffset, StackMap};
 use crate::ir::condcodes::IntCC;
 use crate::ir::{Function, Type};
-use crate::isa::unwind;
+use crate::isa::unwind::input as unwind_input;
 use crate::result::CodegenResult;
 use crate::settings::Flags;
 
@@ -314,7 +314,7 @@ pub struct MachCompileResult {
     /// Disassembly, if requested.
     pub disasm: Option<String>,
     /// Unwind info.
-    pub unwind_info: Option<unwind::UnwindInfo>,
+    pub unwind_info: Option<unwind_input::UnwindInfo<Reg>>,
 }
 
 impl MachCompileResult {
@@ -360,6 +360,17 @@ pub trait MachBackend {
     /// Condition that will be true when an IsubIfcout overflows.
     fn unsigned_sub_overflow_condition(&self) -> IntCC;
 
+    /// Produces unwind info based on backend results.
+    #[cfg(feature = "unwind")]
+    fn emit_unwind_info(
+        &self,
+        _result: &MachCompileResult,
+        _kind: UnwindInfoKind,
+    ) -> CodegenResult<Option<crate::isa::unwind::UnwindInfo>> {
+        // By default, an backend cannot produce unwind info.
+        Ok(None)
+    }
+
     /// Machine-specific condcode info needed by TargetIsa.
     /// Creates a new System V Common Information Entry for the ISA.
     #[cfg(feature = "unwind")]
@@ -403,6 +414,5 @@ pub trait UnwindInfoGenerator<I: MachInstEmit> {
     /// emitted instructions.
     fn create_unwind_info(
         context: UnwindInfoContext<I>,
-        kind: UnwindInfoKind,
+    ) -> CodegenResult<Option<unwind_input::UnwindInfo<Reg>>>;
-    ) -> CodegenResult<Option<unwind::UnwindInfo>>;
 }

cranelift/codegen/src/machinst/vcode.rs

@@ -560,7 +560,7 @@ impl<I: VCodeInst> VCode<I> {
     /// Generates unwind info.
     pub fn unwind_info(
         &self,
-    ) -> crate::result::CodegenResult<Option<crate::isa::unwind::UnwindInfo>> {
+    ) -> crate::result::CodegenResult<Option<crate::isa::unwind::input::UnwindInfo<Reg>>> {
         let layout = &self.insts_layout.borrow();
         let (prologue, epilogues) = self.prologue_epilogue_ranges.as_ref().unwrap();
         let context = UnwindInfoContext {
@@ -570,7 +570,7 @@ impl<I: VCodeInst> VCode<I> {
             prologue: prologue.clone(),
             epilogues,
         };
-        I::UnwindInfo::create_unwind_info(context, self.abi.unwind_info_kind())
+        I::UnwindInfo::create_unwind_info(context)
     }
 
     /// Get the IR block for a BlockIndex, if one exists.

cranelift/filetests/filetests/isa/x86/systemv_x64_unwind.clif

@@ -37,8 +37,8 @@ block0:
 ; nextln:                 DW_CFA_advance_loc (3)
 ; nextln:                 DW_CFA_def_cfa_register (r6)
 ; nextln:                 DW_CFA_advance_loc (1)
-; nextln:                 DW_CFA_def_cfa (r7, 8)
 ; nextln:                 DW_CFA_same_value (r6)
+; nextln:                 DW_CFA_def_cfa (r7, 8)
 ; nextln:                 DW_CFA_nop
 
 ; check a function with medium-sized stack alloc
@@ -76,8 +76,8 @@ block0:
 ; nextln:                 DW_CFA_advance_loc (3)
 ; nextln:                 DW_CFA_def_cfa_register (r6)
 ; nextln:                 DW_CFA_advance_loc (21)
-; nextln:                 DW_CFA_def_cfa (r7, 8)
 ; nextln:                 DW_CFA_same_value (r6)
+; nextln:                 DW_CFA_def_cfa (r7, 8)
 ; nextln:                 DW_CFA_nop
 
 ; check a function with large-sized stack alloc
@@ -115,8 +115,8 @@ block0:
 ; nextln:                 DW_CFA_advance_loc (3)
 ; nextln:                 DW_CFA_def_cfa_register (r6)
 ; nextln:                 DW_CFA_advance_loc (21)
-; nextln:                 DW_CFA_def_cfa (r7, 8)
 ; nextln:                 DW_CFA_same_value (r6)
+; nextln:                 DW_CFA_def_cfa (r7, 8)
 ; nextln:                 DW_CFA_nop
 ; nextln: 
 
@@ -200,7 +200,6 @@ block0(v0: i64, v1: i64):
 ; nextln:                 DW_CFA_advance_loc (1)
 ; nextln:                 DW_CFA_same_value (r3)
 ; nextln:                 DW_CFA_advance_loc (1)
-; nextln:                 DW_CFA_def_cfa (r7, 8)
 ; nextln:                 DW_CFA_same_value (r6)
-; nextln:                 DW_CFA_nop
+; nextln:                 DW_CFA_def_cfa (r7, 8)
 ; nextln:                 DW_CFA_nop