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//! Windows Arm64 ABI unwind information.
use alloc::vec::Vec;
#[cfg(feature = "enable-serde")]
use serde_derive::{Deserialize, Serialize};
use crate::binemit::CodeOffset;
use crate::isa::unwind::UnwindInst;
use crate::result::CodegenResult;
use super::Writer;
/// The supported unwind codes for the Arm64 Windows ABI.
///
/// See: <https://learn.microsoft.com/en-us/cpp/build/arm64-exception-handling>
/// Only what is needed to describe the prologues generated by the Cranelift AArch64 ISA are represented here.
#[derive(Clone, Debug, PartialEq, Eq)]
#[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
pub(crate) enum UnwindCode {
/// Save int register, or register pair.
SaveReg {
reg: u8,
stack_offset: u16,
is_pair: bool,
},
/// Save floating point register, or register pair.
SaveFReg {
reg: u8,
stack_offset: u16,
is_pair: bool,
},
/// Save frame-pointer register (X29) and LR register pair.
SaveFpLrPair {
stack_offset: u16,
},
// Small (<512b) stack allocation.
AllocS {
size: u16,
},
// Medium (<32Kb) stack allocation.
AllocM {
size: u16,
},
// Large (<256Mb) stack allocation.
AllocL {
size: u32,
},
/// PAC sign the LR register.
PacSignLr,
/// Set the frame-pointer register to the stack-pointer register.
SetFp,
/// Set the frame-pointer register to the stack-pointer register with an
/// offset.
AddFp {
offset: u16,
},
}
/// Represents Windows Arm64 unwind information.
///
/// For information about Windows Arm64 unwind info, see:
/// <https://learn.microsoft.com/en-us/cpp/build/arm64-exception-handling>
#[derive(Clone, Debug, PartialEq, Eq)]
#[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
pub struct UnwindInfo {
pub(crate) unwind_codes: Vec<UnwindCode>,
}
impl UnwindInfo {
/// Calculate the number of words needed to encode the unwind codes.
pub fn code_words(&self) -> u8 {
let mut bytes = 0u16;
for code in self.unwind_codes.iter() {
let next_bytes = match code {
UnwindCode::SaveFpLrPair { .. }
| UnwindCode::AllocS { .. }
| UnwindCode::PacSignLr
| UnwindCode::SetFp => 1,
UnwindCode::SaveReg { .. }
| UnwindCode::SaveFReg { .. }
| UnwindCode::AllocM { .. }
| UnwindCode::AddFp { .. } => 2,
UnwindCode::AllocL { .. } => 4,
};
bytes = bytes.checked_add(next_bytes).unwrap();
}
bytes.div_ceil(4).try_into().unwrap()
}
/// Emits the unwind information into the given mutable byte slice.
///
/// This function will panic if the slice is not at least `emit_size` in length.
pub fn emit(&self, buf: &mut [u8]) {
fn encode_stack_offset<const BITS: u8>(stack_offset: u16) -> u16 {
let encoded = (stack_offset / 8) - 1;
assert!(encoded < (1 << BITS), "Stack offset too large");
encoded
}
// NOTE: Unwind codes are written in big-endian!
let mut writer = Writer::new(buf);
for code in self.unwind_codes.iter().rev() {
match code {
&UnwindCode::SaveReg {
reg,
stack_offset,
is_pair,
} => {
assert!(reg >= 19, "Can't save registers before X19");
let reg = u16::from(reg - 19);
let encoding = if is_pair {
let mut encoding = 0b11001100_00000000u16;
encoding |= reg << 6;
encoding |= encode_stack_offset::<6>(stack_offset);
encoding
} else {
let mut encoding = 0b11010100_00000000u16;
encoding |= reg << 5;
encoding |= encode_stack_offset::<5>(stack_offset);
encoding
};
writer.write_u16_be(encoding);
}
&UnwindCode::SaveFReg {
reg,
stack_offset,
is_pair,
} => {
assert!(reg >= 8, "Can't save registers before D8");
let reg = u16::from(reg - 8);
let encoding = if is_pair {
let mut encoding = 0b11011010_00000000u16;
encoding |= reg << 6;
encoding |= encode_stack_offset::<6>(stack_offset);
encoding
} else {
let mut encoding = 0b11011110_00000000u16;
encoding |= reg << 5;
encoding |= encode_stack_offset::<5>(stack_offset);
encoding
};
writer.write_u16_be(encoding);
}
&UnwindCode::SaveFpLrPair { stack_offset } => {
if stack_offset == 0 {
writer.write_u8(0b01000000);
} else {
let encoding = 0b10000000u8
| u8::try_from(encode_stack_offset::<6>(stack_offset)).unwrap();
writer.write_u8(encoding);
}
}
&UnwindCode::AllocS { size } => {
// Size is measured in double 64-bit words.
let encoding = size / 16;
assert!(encoding < (1 << 5), "Stack alloc size too large");
// Tag is 0b000, so we don't need to encode that.
writer.write_u8(encoding.try_into().unwrap());
}
&UnwindCode::AllocM { size } => {
// Size is measured in double 64-bit words.
let mut encoding = size / 16;
assert!(encoding < (1 << 11), "Stack alloc size too large");
encoding |= 0b11000 << 11;
writer.write_u16_be(encoding);
}
&UnwindCode::AllocL { size } => {
// Size is measured in double 64-bit words.
let mut encoding = size / 16;
assert!(encoding < (1 << 24), "Stack alloc size too large");
encoding |= 0b11100000 << 24;
writer.write_u32_be(encoding);
}
UnwindCode::PacSignLr => {
writer.write_u8(0b11111100);
}
UnwindCode::SetFp => {
writer.write_u8(0b11100001);
}
&UnwindCode::AddFp { mut offset } => {
offset /= 8;
assert!(offset & !0xFF == 0, "Offset too large");
let encoding = (0b11100010 << 8) | offset;
writer.write_u16_be(encoding);
}
}
}
}
}
pub(crate) fn create_unwind_info_from_insts(
insts: &[(CodeOffset, UnwindInst)],
) -> CodegenResult<UnwindInfo> {
let mut unwind_codes = vec![];
let mut last_stackalloc = None;
let mut last_clobber_offset = None;
for &(_, ref inst) in insts {
match inst {
&UnwindInst::PushFrameRegs { .. } => {
unwind_codes.push(UnwindCode::SaveFpLrPair { stack_offset: 16 });
unwind_codes.push(UnwindCode::SetFp);
}
&UnwindInst::DefineNewFrame {
offset_downward_to_clobbers,
..
} => {
assert!(last_clobber_offset.is_none(), "More than one frame defined");
last_clobber_offset = Some(offset_downward_to_clobbers);
// If we've seen a stackalloc, then we were adjusting the stack
// to make space for additional arguments, so encode that now.
if let &Some(last_stackalloc) = &last_stackalloc {
assert!(last_stackalloc < (1u32 << 8) * 8);
unwind_codes.push(UnwindCode::AddFp {
offset: u16::try_from(last_stackalloc).unwrap(),
});
unwind_codes.push(UnwindCode::SaveFpLrPair { stack_offset: 0 });
unwind_codes.push(UnwindCode::SetFp);
}
}
&UnwindInst::StackAlloc { size } => {
last_stackalloc = Some(size);
assert!(size % 16 == 0, "Size must be a multiple of 16");
const SMALL_STACK_ALLOC_MAX: u32 = (1 << 5) * 16 - 1;
const MEDIUM_STACK_ALLOC_MIN: u32 = SMALL_STACK_ALLOC_MAX + 1;
const MEDIUM_STACK_ALLOC_MAX: u32 = (1 << 11) * 16 - 1;
const LARGE_STACK_ALLOC_MIN: u32 = MEDIUM_STACK_ALLOC_MAX + 1;
const LARGE_STACK_ALLOC_MAX: u32 = (1 << 24) * 16 - 1;
match size {
0..=SMALL_STACK_ALLOC_MAX => unwind_codes.push(UnwindCode::AllocS {
size: size.try_into().unwrap(),
}),
MEDIUM_STACK_ALLOC_MIN..=MEDIUM_STACK_ALLOC_MAX => {
unwind_codes.push(UnwindCode::AllocM {
size: size.try_into().unwrap(),
})
}
LARGE_STACK_ALLOC_MIN..=LARGE_STACK_ALLOC_MAX => {
unwind_codes.push(UnwindCode::AllocL { size })
}
_ => panic!("Stack allocation size too large"),
}
}
&UnwindInst::SaveReg {
clobber_offset,
reg,
} => {
// We're given the clobber offset, but we need to encode how far
// the stack was adjusted, so calculate that based on the last
// clobber offset we saw.
let last_clobber_offset = last_clobber_offset.as_mut().expect("No frame defined");
if *last_clobber_offset > clobber_offset {
let stack_offset = *last_clobber_offset - clobber_offset;
*last_clobber_offset = clobber_offset;
assert!(stack_offset % 8 == 0, "Offset must be a multiple of 8");
match reg.class() {
regalloc2::RegClass::Int => {
let reg = reg.hw_enc();
if reg < 19 {
panic!("Can't save registers before X19");
}
unwind_codes.push(UnwindCode::SaveReg {
reg,
stack_offset: stack_offset.try_into().unwrap(),
is_pair: false,
});
}
regalloc2::RegClass::Float => {
let reg = reg.hw_enc();
if reg < 8 {
panic!("Can't save registers before D8");
}
unwind_codes.push(UnwindCode::SaveFReg {
reg,
stack_offset: stack_offset.try_into().unwrap(),
is_pair: false,
});
}
regalloc2::RegClass::Vector => unreachable!(),
}
} else {
// If we see a clobber offset within the last offset amount,
// then we're actually saving a pair of registers.
let last_unwind_code = unwind_codes.last_mut().unwrap();
match last_unwind_code {
UnwindCode::SaveReg { is_pair, .. } => {
assert_eq!(reg.class(), regalloc2::RegClass::Int);
assert!(!*is_pair);
*is_pair = true;
}
UnwindCode::SaveFReg { is_pair, .. } => {
assert_eq!(reg.class(), regalloc2::RegClass::Float);
assert!(!*is_pair);
*is_pair = true;
}
_ => unreachable!("Previous code should have been a register save"),
}
}
}
&UnwindInst::RegStackOffset { .. } => {
unreachable!("only supported with DWARF");
}
&UnwindInst::Aarch64SetPointerAuth { return_addresses } => {
assert!(
return_addresses,
"Windows doesn't support explicitly disabling return address signing"
);
unwind_codes.push(UnwindCode::PacSignLr);
}
}
}
Ok(UnwindInfo { unwind_codes })
}