cranelift_codegen/machinst/

isle.rs

use crate::ir::{BlockCall, Value, ValueList};
use alloc::boxed::Box;
use alloc::vec::Vec;
use smallvec::SmallVec;

pub use super::MachLabel;
use super::RetPair;
pub use crate::ir::{condcodes::CondCode, *};
pub use crate::isa::{TargetIsa, unwind::UnwindInst};
pub use crate::machinst::{
    ABIArg, ABIArgSlot, ABIMachineSpec, InputSourceInst, Lower, LowerBackend, RealReg, Reg,
    RelocDistance, Sig, TryCallInfo, VCodeInst, Writable,
};
pub use crate::settings::{StackSwitchModel, TlsModel};

pub type Unit = ();
pub type ValueSlice = (ValueList, usize);
pub type ValueArray2 = [Value; 2];
pub type ValueArray3 = [Value; 3];
pub type BlockArray2 = [BlockCall; 2];
pub type WritableReg = Writable<Reg>;
pub type VecRetPair = Vec<RetPair>;
pub type VecMask = Vec<u8>;
pub type ValueRegs = crate::machinst::ValueRegs<Reg>;
pub type WritableValueRegs = crate::machinst::ValueRegs<WritableReg>;
pub type ValueRegsVec = SmallVec<[ValueRegs; 2]>;
pub type InstOutput = SmallVec<[ValueRegs; 2]>;
pub type BoxExternalName = Box<ExternalName>;
pub type MachLabelSlice = [MachLabel];
pub type BoxVecMachLabel = Box<Vec<MachLabel>>;
pub type OptionTryCallInfo = Option<TryCallInfo>;

/// Helper macro to define methods in `prelude.isle` within `impl Context for
/// ...` for each backend. These methods are shared amongst all backends.
#[macro_export]
#[doc(hidden)]
macro_rules! isle_lower_prelude_methods {
    () => {
        crate::isle_lower_prelude_methods!(MInst);
    };
    ($inst:ty) => {
        crate::isle_common_prelude_methods!();

        #[inline]
        fn value_type(&mut self, val: Value) -> Type {
            self.lower_ctx.dfg().value_type(val)
        }

        #[inline]
        fn value_reg(&mut self, reg: Reg) -> ValueRegs {
            ValueRegs::one(reg)
        }

        #[inline]
        fn value_regs(&mut self, r1: Reg, r2: Reg) -> ValueRegs {
            ValueRegs::two(r1, r2)
        }

        #[inline]
        fn writable_value_regs(&mut self, r1: WritableReg, r2: WritableReg) -> WritableValueRegs {
            WritableValueRegs::two(r1, r2)
        }

        #[inline]
        fn writable_value_reg(&mut self, r: WritableReg) -> WritableValueRegs {
            WritableValueRegs::one(r)
        }

        #[inline]
        fn value_regs_invalid(&mut self) -> ValueRegs {
            ValueRegs::invalid()
        }

        #[inline]
        fn output_none(&mut self) -> InstOutput {
            smallvec::smallvec![]
        }

        #[inline]
        fn output(&mut self, regs: ValueRegs) -> InstOutput {
            smallvec::smallvec![regs]
        }

        #[inline]
        fn output_pair(&mut self, r1: ValueRegs, r2: ValueRegs) -> InstOutput {
            smallvec::smallvec![r1, r2]
        }

        #[inline]
        fn output_vec(&mut self, output: &ValueRegsVec) -> InstOutput {
            output.clone()
        }

        #[inline]
        fn temp_writable_reg(&mut self, ty: Type) -> WritableReg {
            let value_regs = self.lower_ctx.alloc_tmp(ty);
            value_regs.only_reg().unwrap()
        }

        #[inline]
        fn is_valid_reg(&mut self, reg: Reg) -> bool {
            use crate::machinst::valueregs::InvalidSentinel;
            !reg.is_invalid_sentinel()
        }

        #[inline]
        fn invalid_reg(&mut self) -> Reg {
            use crate::machinst::valueregs::InvalidSentinel;
            Reg::invalid_sentinel()
        }

        #[inline]
        fn mark_value_used(&mut self, val: Value) {
            self.lower_ctx.increment_lowered_uses(val);
        }

        #[inline]
        fn put_in_reg(&mut self, val: Value) -> Reg {
            self.put_in_regs(val).only_reg().unwrap()
        }

        #[inline]
        fn put_in_regs(&mut self, val: Value) -> ValueRegs {
            self.lower_ctx.put_value_in_regs(val)
        }

        #[inline]
        fn put_in_regs_vec(&mut self, (list, off): ValueSlice) -> ValueRegsVec {
            (off..list.len(&self.lower_ctx.dfg().value_lists))
                .map(|ix| {
                    let val = list.get(ix, &self.lower_ctx.dfg().value_lists).unwrap();
                    self.put_in_regs(val)
                })
                .collect()
        }

        #[inline]
        fn ensure_in_vreg(&mut self, reg: Reg, ty: Type) -> Reg {
            self.lower_ctx.ensure_in_vreg(reg, ty)
        }

        #[inline]
        fn value_regs_get(&mut self, regs: ValueRegs, i: usize) -> Reg {
            regs.regs()[i]
        }

        #[inline]
        fn value_regs_len(&mut self, regs: ValueRegs) -> usize {
            regs.regs().len()
        }

        #[inline]
        fn value_list_slice(&mut self, list: ValueList) -> ValueSlice {
            (list, 0)
        }

        #[inline]
        fn value_slice_empty(&mut self, slice: ValueSlice) -> Option<()> {
            let (list, off) = slice;
            if off >= list.len(&self.lower_ctx.dfg().value_lists) {
                Some(())
            } else {
                None
            }
        }

        #[inline]
        fn value_slice_unwrap(&mut self, slice: ValueSlice) -> Option<(Value, ValueSlice)> {
            let (list, off) = slice;
            if let Some(val) = list.get(off, &self.lower_ctx.dfg().value_lists) {
                Some((val, (list, off + 1)))
            } else {
                None
            }
        }

        #[inline]
        fn value_slice_len(&mut self, slice: ValueSlice) -> usize {
            let (list, off) = slice;
            list.len(&self.lower_ctx.dfg().value_lists) - off
        }

        #[inline]
        fn value_slice_get(&mut self, slice: ValueSlice, idx: usize) -> Value {
            let (list, off) = slice;
            list.get(off + idx, &self.lower_ctx.dfg().value_lists)
                .unwrap()
        }

        #[inline]
        fn writable_reg_to_reg(&mut self, r: WritableReg) -> Reg {
            r.to_reg()
        }

        #[inline]
        fn inst_results(&mut self, inst: Inst) -> ValueSlice {
            (self.lower_ctx.dfg().inst_results_list(inst), 0)
        }

        #[inline]
        fn first_result(&mut self, inst: Inst) -> Option<Value> {
            self.lower_ctx.dfg().inst_results(inst).first().copied()
        }

        #[inline]
        fn inst_data_value(&mut self, inst: Inst) -> InstructionData {
            self.lower_ctx.dfg().insts[inst]
        }

        #[inline]
        fn i64_from_iconst(&mut self, val: Value) -> Option<i64> {
            let inst = self.def_inst(val)?;
            let constant = match self.lower_ctx.data(inst) {
                InstructionData::UnaryImm {
                    opcode: Opcode::Iconst,
                    imm,
                } => imm.bits(),
                _ => return None,
            };
            let ty = self.lower_ctx.output_ty(inst, 0);
            let shift_amt = core::cmp::max(0, 64 - self.ty_bits(ty));
            Some((constant << shift_amt) >> shift_amt)
        }

        fn zero_value(&mut self, value: Value) -> Option<Value> {
            let insn = self.def_inst(value);
            if insn.is_some() {
                let insn = insn.unwrap();
                let inst_data = self.lower_ctx.data(insn);
                match inst_data {
                    InstructionData::Unary {
                        opcode: Opcode::Splat,
                        arg,
                    } => {
                        let arg = arg.clone();
                        return self.zero_value(arg);
                    }
                    InstructionData::UnaryConst {
                        opcode: Opcode::Vconst | Opcode::F128const,
                        constant_handle,
                    } => {
                        let constant_data =
                            self.lower_ctx.get_constant_data(*constant_handle).clone();
                        if constant_data.into_vec().iter().any(|&x| x != 0) {
                            return None;
                        } else {
                            return Some(value);
                        }
                    }
                    InstructionData::UnaryImm { imm, .. } => {
                        if imm.bits() == 0 {
                            return Some(value);
                        } else {
                            return None;
                        }
                    }
                    InstructionData::UnaryIeee16 { imm, .. } => {
                        if imm.bits() == 0 {
                            return Some(value);
                        } else {
                            return None;
                        }
                    }
                    InstructionData::UnaryIeee32 { imm, .. } => {
                        if imm.bits() == 0 {
                            return Some(value);
                        } else {
                            return None;
                        }
                    }
                    InstructionData::UnaryIeee64 { imm, .. } => {
                        if imm.bits() == 0 {
                            return Some(value);
                        } else {
                            return None;
                        }
                    }
                    _ => None,
                }
            } else {
                None
            }
        }

        #[inline]
        fn tls_model(&mut self, _: Type) -> TlsModel {
            self.backend.flags().tls_model()
        }

        #[inline]
        fn tls_model_is_elf_gd(&mut self) -> Option<()> {
            if self.backend.flags().tls_model() == TlsModel::ElfGd {
                Some(())
            } else {
                None
            }
        }

        #[inline]
        fn tls_model_is_macho(&mut self) -> Option<()> {
            if self.backend.flags().tls_model() == TlsModel::Macho {
                Some(())
            } else {
                None
            }
        }

        #[inline]
        fn tls_model_is_coff(&mut self) -> Option<()> {
            if self.backend.flags().tls_model() == TlsModel::Coff {
                Some(())
            } else {
                None
            }
        }

        #[inline]
        fn preserve_frame_pointers(&mut self) -> Option<()> {
            if self.backend.flags().preserve_frame_pointers() {
                Some(())
            } else {
                None
            }
        }

        #[inline]
        fn stack_switch_model(&mut self) -> Option<StackSwitchModel> {
            Some(self.backend.flags().stack_switch_model())
        }

        #[inline]
        fn func_ref_data(
            &mut self,
            func_ref: FuncRef,
        ) -> (SigRef, ExternalName, RelocDistance, bool) {
            let funcdata = &self.lower_ctx.dfg().ext_funcs[func_ref];
            let reloc_distance = if funcdata.colocated {
                RelocDistance::Near
            } else {
                RelocDistance::Far
            };
            (
                funcdata.signature,
                funcdata.name.clone(),
                reloc_distance,
                funcdata.patchable,
            )
        }

        #[inline]
        fn exception_sig(&mut self, et: ExceptionTable) -> SigRef {
            self.lower_ctx.dfg().exception_tables[et].signature()
        }

        #[inline]
        fn box_external_name(&mut self, extname: ExternalName) -> BoxExternalName {
            Box::new(extname)
        }

        #[inline]
        fn symbol_value_data(
            &mut self,
            global_value: GlobalValue,
        ) -> Option<(ExternalName, RelocDistance, i64)> {
            let (name, reloc, offset) = self.lower_ctx.symbol_value_data(global_value)?;
            Some((name.clone(), reloc, offset))
        }

        #[inline]
        fn u128_from_immediate(&mut self, imm: Immediate) -> Option<u128> {
            let bytes = self.lower_ctx.get_immediate_data(imm).as_slice();
            Some(u128::from_le_bytes(bytes.try_into().ok()?))
        }

        #[inline]
        fn vconst_from_immediate(&mut self, imm: Immediate) -> Option<VCodeConstant> {
            Some(self.lower_ctx.use_constant(VCodeConstantData::Generated(
                self.lower_ctx.get_immediate_data(imm).clone(),
            )))
        }

        #[inline]
        fn vec_mask_from_immediate(&mut self, imm: Immediate) -> Option<VecMask> {
            let data = self.lower_ctx.get_immediate_data(imm);
            if data.len() == 16 {
                Some(Vec::from(data.as_slice()))
            } else {
                None
            }
        }

        #[inline]
        fn u64_from_constant(&mut self, constant: Constant) -> Option<u64> {
            let bytes = self.lower_ctx.get_constant_data(constant).as_slice();
            Some(u64::from_le_bytes(bytes.try_into().ok()?))
        }

        #[inline]
        fn u128_from_constant(&mut self, constant: Constant) -> Option<u128> {
            let bytes = self.lower_ctx.get_constant_data(constant).as_slice();
            Some(u128::from_le_bytes(bytes.try_into().ok()?))
        }

        #[inline]
        fn emit_u64_le_const(&mut self, value: u64) -> VCodeConstant {
            let data = VCodeConstantData::U64(value.to_le_bytes());
            self.lower_ctx.use_constant(data)
        }

        #[inline]
        fn emit_u64_be_const(&mut self, value: u64) -> VCodeConstant {
            let data = VCodeConstantData::U64(value.to_be_bytes());
            self.lower_ctx.use_constant(data)
        }

        #[inline]
        fn emit_u128_le_const(&mut self, value: u128) -> VCodeConstant {
            let data = VCodeConstantData::Generated(value.to_le_bytes().as_slice().into());
            self.lower_ctx.use_constant(data)
        }

        #[inline]
        fn emit_u128_be_const(&mut self, value: u128) -> VCodeConstant {
            let data = VCodeConstantData::Generated(value.to_be_bytes().as_slice().into());
            self.lower_ctx.use_constant(data)
        }

        #[inline]
        fn const_to_vconst(&mut self, constant: Constant) -> VCodeConstant {
            self.lower_ctx.use_constant(VCodeConstantData::Pool(
                constant,
                self.lower_ctx.get_constant_data(constant).clone(),
            ))
        }

        fn only_writable_reg(&mut self, regs: WritableValueRegs) -> Option<WritableReg> {
            regs.only_reg()
        }

        fn writable_regs_get(&mut self, regs: WritableValueRegs, idx: usize) -> WritableReg {
            regs.regs()[idx]
        }

        fn abi_sig(&mut self, sig_ref: SigRef) -> Sig {
            self.lower_ctx.sigs().abi_sig_for_sig_ref(sig_ref)
        }

        fn abi_num_args(&mut self, abi: Sig) -> usize {
            self.lower_ctx.sigs().num_args(abi)
        }

        fn abi_get_arg(&mut self, abi: Sig, idx: usize) -> ABIArg {
            self.lower_ctx.sigs().get_arg(abi, idx)
        }

        fn abi_num_rets(&mut self, abi: Sig) -> usize {
            self.lower_ctx.sigs().num_rets(abi)
        }

        fn abi_get_ret(&mut self, abi: Sig, idx: usize) -> ABIArg {
            self.lower_ctx.sigs().get_ret(abi, idx)
        }

        fn abi_ret_arg(&mut self, abi: Sig) -> Option<ABIArg> {
            self.lower_ctx.sigs().get_ret_arg(abi)
        }

        fn abi_no_ret_arg(&mut self, abi: Sig) -> Option<()> {
            if let Some(_) = self.lower_ctx.sigs().get_ret_arg(abi) {
                None
            } else {
                Some(())
            }
        }

        fn abi_arg_only_slot(&mut self, arg: &ABIArg) -> Option<ABIArgSlot> {
            match arg {
                &ABIArg::Slots { ref slots, .. } => {
                    if slots.len() == 1 {
                        Some(slots[0])
                    } else {
                        None
                    }
                }
                _ => None,
            }
        }

        fn abi_arg_implicit_pointer(&mut self, arg: &ABIArg) -> Option<(ABIArgSlot, i64, Type)> {
            match arg {
                &ABIArg::ImplicitPtrArg {
                    pointer,
                    offset,
                    ty,
                    ..
                } => Some((pointer, offset, ty)),
                _ => None,
            }
        }

        fn abi_unwrap_ret_area_ptr(&mut self) -> Reg {
            self.lower_ctx.abi().ret_area_ptr().unwrap()
        }

        fn abi_stackslot_addr(
            &mut self,
            dst: WritableReg,
            stack_slot: StackSlot,
            offset: Offset32,
        ) -> MInst {
            let offset = u32::try_from(i32::from(offset)).unwrap();
            self.lower_ctx
                .abi()
                .sized_stackslot_addr(stack_slot, offset, dst)
                .into()
        }

        fn abi_stackslot_offset_into_slot_region(
            &mut self,
            stack_slot: StackSlot,
            offset1: Offset32,
            offset2: Offset32,
        ) -> i32 {
            let offset1 = i32::from(offset1);
            let offset2 = i32::from(offset2);
            i32::try_from(self.lower_ctx.abi().sized_stackslot_offset(stack_slot))
                .expect("Stack slot region cannot be larger than 2GiB")
                .checked_add(offset1)
                .expect("Stack slot region cannot be larger than 2GiB")
                .checked_add(offset2)
                .expect("Stack slot region cannot be larger than 2GiB")
        }

        fn abi_dynamic_stackslot_addr(
            &mut self,
            dst: WritableReg,
            stack_slot: DynamicStackSlot,
        ) -> MInst {
            assert!(
                self.lower_ctx
                    .abi()
                    .dynamic_stackslot_offsets()
                    .is_valid(stack_slot)
            );
            self.lower_ctx
                .abi()
                .dynamic_stackslot_addr(stack_slot, dst)
                .into()
        }

        fn real_reg_to_reg(&mut self, reg: RealReg) -> Reg {
            Reg::from(reg)
        }

        fn real_reg_to_writable_reg(&mut self, reg: RealReg) -> WritableReg {
            Writable::from_reg(Reg::from(reg))
        }

        fn is_sinkable_inst(&mut self, val: Value) -> Option<Inst> {
            let input = self.lower_ctx.get_value_as_source_or_const(val);

            if let InputSourceInst::UniqueUse(inst, _) = input.inst {
                Some(inst)
            } else {
                None
            }
        }

        #[inline]
        fn sink_inst(&mut self, inst: Inst) {
            self.lower_ctx.sink_inst(inst);
        }

        #[inline]
        fn maybe_uextend(&mut self, value: Value) -> Option<Value> {
            if let Some(def_inst) = self.def_inst(value) {
                if let InstructionData::Unary {
                    opcode: Opcode::Uextend,
                    arg,
                } = self.lower_ctx.data(def_inst)
                {
                    return Some(*arg);
                }
            }

            Some(value)
        }

        #[inline]
        fn uimm8(&mut self, x: Imm64) -> Option<u8> {
            let x64: i64 = x.into();
            let x8: u8 = x64.try_into().ok()?;
            Some(x8)
        }

        #[inline]
        fn preg_to_reg(&mut self, preg: PReg) -> Reg {
            preg.into()
        }

        #[inline]
        fn gen_move(&mut self, ty: Type, dst: WritableReg, src: Reg) -> MInst {
            <$inst>::gen_move(dst, src, ty).into()
        }

        /// Generate the return instruction.
        fn gen_return(&mut self, rets: &ValueRegsVec) {
            self.lower_ctx.gen_return(rets);
        }

        fn gen_call_output(&mut self, sig_ref: SigRef) -> ValueRegsVec {
            self.lower_ctx.gen_call_output_from_sig_ref(sig_ref)
        }

        fn gen_call_args(&mut self, sig: Sig, inputs: &ValueRegsVec) -> CallArgList {
            self.lower_ctx.gen_call_args(sig, inputs)
        }

        fn gen_return_call_args(&mut self, sig: Sig, inputs: &ValueRegsVec) -> CallArgList {
            self.lower_ctx.gen_return_call_args(sig, inputs)
        }

        fn gen_call_rets(&mut self, sig: Sig, outputs: &ValueRegsVec) -> CallRetList {
            self.lower_ctx.gen_call_rets(sig, &outputs)
        }

        fn gen_try_call_rets(&mut self, sig: Sig) -> CallRetList {
            self.lower_ctx.gen_try_call_rets(sig)
        }

        fn gen_patchable_call_rets(&mut self) -> CallRetList {
            smallvec::smallvec![]
        }

        fn try_call_none(&mut self) -> OptionTryCallInfo {
            None
        }

        fn try_call_info(
            &mut self,
            et: ExceptionTable,
            labels: &MachLabelSlice,
        ) -> OptionTryCallInfo {
            let mut exception_handlers = vec![];
            let mut labels = labels.iter().cloned();
            for item in self.lower_ctx.dfg().exception_tables[et].clone().items() {
                match item {
                    crate::ir::ExceptionTableItem::Tag(tag, _) => {
                        exception_handlers.push(crate::machinst::abi::TryCallHandler::Tag(
                            tag,
                            labels.next().unwrap(),
                        ));
                    }
                    crate::ir::ExceptionTableItem::Default(_) => {
                        exception_handlers.push(crate::machinst::abi::TryCallHandler::Default(
                            labels.next().unwrap(),
                        ));
                    }
                    crate::ir::ExceptionTableItem::Context(ctx) => {
                        let reg = self.put_in_reg(ctx);
                        exception_handlers.push(crate::machinst::abi::TryCallHandler::Context(reg));
                    }
                }
            }

            let continuation = labels.next().unwrap();
            assert_eq!(labels.next(), None);

            let exception_handlers = exception_handlers.into_boxed_slice();

            Some(TryCallInfo {
                continuation,
                exception_handlers,
            })
        }

        /// Same as `shuffle32_from_imm`, but for 64-bit lane shuffles.
        fn shuffle64_from_imm(&mut self, imm: Immediate) -> Option<(u8, u8)> {
            use crate::machinst::isle::shuffle_imm_as_le_lane_idx;

            let bytes = self.lower_ctx.get_immediate_data(imm).as_slice();
            Some((
                shuffle_imm_as_le_lane_idx(8, &bytes[0..8])?,
                shuffle_imm_as_le_lane_idx(8, &bytes[8..16])?,
            ))
        }

        /// Attempts to interpret the shuffle immediate `imm` as a shuffle of
        /// 32-bit lanes, returning four integers, each of which is less than 8,
        /// which represents a permutation of 32-bit lanes as specified by
        /// `imm`.
        ///
        /// For example the shuffle immediate
        ///
        /// `0 1 2 3 8 9 10 11 16 17 18 19 24 25 26 27`
        ///
        /// would return `Some((0, 2, 4, 6))`.
        fn shuffle32_from_imm(&mut self, imm: Immediate) -> Option<(u8, u8, u8, u8)> {
            use crate::machinst::isle::shuffle_imm_as_le_lane_idx;

            let bytes = self.lower_ctx.get_immediate_data(imm).as_slice();
            Some((
                shuffle_imm_as_le_lane_idx(4, &bytes[0..4])?,
                shuffle_imm_as_le_lane_idx(4, &bytes[4..8])?,
                shuffle_imm_as_le_lane_idx(4, &bytes[8..12])?,
                shuffle_imm_as_le_lane_idx(4, &bytes[12..16])?,
            ))
        }

        /// Same as `shuffle32_from_imm`, but for 16-bit lane shuffles.
        fn shuffle16_from_imm(
            &mut self,
            imm: Immediate,
        ) -> Option<(u8, u8, u8, u8, u8, u8, u8, u8)> {
            use crate::machinst::isle::shuffle_imm_as_le_lane_idx;
            let bytes = self.lower_ctx.get_immediate_data(imm).as_slice();
            Some((
                shuffle_imm_as_le_lane_idx(2, &bytes[0..2])?,
                shuffle_imm_as_le_lane_idx(2, &bytes[2..4])?,
                shuffle_imm_as_le_lane_idx(2, &bytes[4..6])?,
                shuffle_imm_as_le_lane_idx(2, &bytes[6..8])?,
                shuffle_imm_as_le_lane_idx(2, &bytes[8..10])?,
                shuffle_imm_as_le_lane_idx(2, &bytes[10..12])?,
                shuffle_imm_as_le_lane_idx(2, &bytes[12..14])?,
                shuffle_imm_as_le_lane_idx(2, &bytes[14..16])?,
            ))
        }

        fn safe_divisor_from_imm64(&mut self, ty: Type, val: Imm64) -> Option<u64> {
            let minus_one = if ty.bytes() == 8 {
                -1
            } else {
                (1 << (ty.bytes() * 8)) - 1
            };
            let bits = val.bits() & minus_one;
            if bits == 0 || bits == minus_one {
                None
            } else {
                Some(bits as u64)
            }
        }

        fn single_target(&mut self, targets: &MachLabelSlice) -> Option<MachLabel> {
            if targets.len() == 1 {
                Some(targets[0])
            } else {
                None
            }
        }

        fn two_targets(&mut self, targets: &MachLabelSlice) -> Option<(MachLabel, MachLabel)> {
            if targets.len() == 2 {
                Some((targets[0], targets[1]))
            } else {
                None
            }
        }

        fn jump_table_targets(
            &mut self,
            targets: &MachLabelSlice,
        ) -> Option<(MachLabel, BoxVecMachLabel)> {
            use alloc::boxed::Box;
            if targets.is_empty() {
                return None;
            }

            let default_label = targets[0];
            let jt_targets = Box::new(targets[1..].to_vec());
            Some((default_label, jt_targets))
        }

        fn jump_table_size(&mut self, targets: &BoxVecMachLabel) -> u32 {
            targets.len() as u32
        }

        fn add_range_fact(&mut self, reg: Reg, bits: u16, min: u64, max: u64) -> Reg {
            self.lower_ctx.add_range_fact(reg, bits, min, max);
            reg
        }

        fn value_is_unused(&mut self, val: Value) -> bool {
            self.lower_ctx.value_is_unused(val)
        }

        fn block_exn_successor_label(&mut self, block: &Block, exn_succ: u64) -> MachLabel {
            // The first N successors are the exceptional edges, and
            // the normal return is last; so the `exn_succ`'th
            // exceptional edge is just the `exn_succ`'th edge overall.
            let succ = usize::try_from(exn_succ).unwrap();
            self.lower_ctx.block_successor_label(*block, succ)
        }
    };
}

/// Returns the `size`-byte lane referred to by the shuffle immediate specified
/// in `bytes`.
///
/// This helper is used by `shuffleNN_from_imm` above and is used to interpret a
/// byte-based shuffle as a higher-level shuffle of bigger lanes. This will see
/// if the `bytes` specified, which must have `size` length, specifies a lane in
/// vectors aligned to a `size`-byte boundary.
///
/// Returns `None` if `bytes` doesn't specify a `size`-byte lane aligned
/// appropriately, or returns `Some(n)` where `n` is the index of the lane being
/// shuffled.
pub fn shuffle_imm_as_le_lane_idx(size: u8, bytes: &[u8]) -> Option<u8> {
    assert_eq!(bytes.len(), usize::from(size));

    // The first index in `bytes` must be aligned to a `size` boundary for the
    // bytes to be a valid specifier for a lane of `size` bytes.
    if bytes[0] % size != 0 {
        return None;
    }

    // Afterwards the bytes must all be one larger than the prior to specify a
    // contiguous sequence of bytes that's being shuffled. Basically `bytes`
    // must refer to the entire `size`-byte lane, in little-endian order.
    for i in 0..size - 1 {
        let idx = usize::from(i);
        if bytes[idx] + 1 != bytes[idx + 1] {
            return None;
        }
    }

    // All of the `bytes` are in-order, meaning that this is a valid shuffle
    // immediate to specify a lane of `size` bytes. The index, when viewed as
    // `size`-byte immediates, will be the first byte divided by the byte size.
    Some(bytes[0] / size)
}

/// This structure is used to implement the ISLE-generated `Context` trait and
/// internally has a temporary reference to a machinst `LowerCtx`.
pub(crate) struct IsleContext<'a, 'b, I, B>
where
    I: VCodeInst,
    B: LowerBackend,
{
    pub lower_ctx: &'a mut Lower<'b, I>,
    pub backend: &'a B,
}

impl<I, B> IsleContext<'_, '_, I, B>
where
    I: VCodeInst,
    B: LowerBackend,
{
    pub(crate) fn dfg(&self) -> &crate::ir::DataFlowGraph {
        &self.lower_ctx.f.dfg
    }
}