use crate::{Backend, Env, Relocation};
use bumpalo::collections::Vec;
use roc_collections::all::{MutMap, MutSet};
use roc_module::symbol::Symbol;
use roc_mono::ir::{BranchInfo, Literal, Stmt};
use roc_mono::layout::{Builtin, Layout};
use std::marker::PhantomData;
use target_lexicon::Triple;

pub mod aarch64;
pub mod x86_64;

pub trait CallConv<GeneralReg: RegTrait, FloatReg: RegTrait> {
    const GENERAL_PARAM_REGS: &'static [GeneralReg];
    const GENERAL_RETURN_REGS: &'static [GeneralReg];
    const GENERAL_DEFAULT_FREE_REGS: &'static [GeneralReg];

    const FLOAT_PARAM_REGS: &'static [FloatReg];
    const FLOAT_RETURN_REGS: &'static [FloatReg];
    const FLOAT_DEFAULT_FREE_REGS: &'static [FloatReg];

    const SHADOW_SPACE_SIZE: u8;

    fn general_callee_saved(reg: &GeneralReg) -> bool;
    #[inline(always)]
    fn general_caller_saved(reg: &GeneralReg) -> bool {
        !Self::general_callee_saved(reg)
    }
    fn float_callee_saved(reg: &FloatReg) -> bool;
    #[inline(always)]
    fn float_caller_saved(reg: &FloatReg) -> bool {
        !Self::float_callee_saved(reg)
    }

    fn setup_stack<'a>(
        buf: &mut Vec<'a, u8>,
        general_saved_regs: &[GeneralReg],
        requested_stack_size: i32,
        fn_call_stack_size: i32,
    ) -> Result<i32, String>;
    fn cleanup_stack<'a>(
        buf: &mut Vec<'a, u8>,
        general_saved_regs: &[GeneralReg],
        aligned_stack_size: i32,
        fn_call_stack_size: i32,
    ) -> Result<(), String>;

    // load_args updates the symbol map to know where every arg is stored.
    fn load_args<'a>(
        symbol_map: &mut MutMap<Symbol, SymbolStorage<GeneralReg, FloatReg>>,
        args: &'a [(Layout<'a>, Symbol)],
    ) -> Result<(), String>;

    // store_args stores the args in registers and on the stack for function calling.
    // It returns the amount of stack space needed to temporarily store the args.
    fn store_args<'a>(
        buf: &mut Vec<'a, u8>,
        symbol_map: &MutMap<Symbol, SymbolStorage<GeneralReg, FloatReg>>,
        args: &'a [Symbol],
        arg_layouts: &[Layout<'a>],
        // ret_layout is needed because if it is a complex type, we pass a pointer as the first arg.
        ret_layout: &Layout<'a>,
    ) -> Result<u32, String>;
}

/// Assembler contains calls to the backend assembly generator.
/// These calls do not necessarily map directly to a single assembly instruction.
/// They are higher level in cases where an instruction would not be common and shared between multiple architectures.
/// Thus, some backends will need to use mulitiple instructions to preform a single one of this calls.
/// Generally, I prefer explicit sources, as opposed to dst being one of the sources. Ex: `x = x + y` would be `add x, x, y` instead of `add x, y`.
/// dst should always come before sources.
pub trait Assembler<GeneralReg: RegTrait, FloatReg: RegTrait> {
    fn abs_reg64_reg64(buf: &mut Vec<'_, u8>, dst: GeneralReg, src: GeneralReg);
    fn abs_freg64_freg64(
        buf: &mut Vec<'_, u8>,
        relocs: &mut Vec<'_, Relocation>,
        dst: FloatReg,
        src: FloatReg,
    );

    fn add_reg64_reg64_imm32(buf: &mut Vec<'_, u8>, dst: GeneralReg, src1: GeneralReg, imm32: i32);
    fn add_freg64_freg64_freg64(
        buf: &mut Vec<'_, u8>,
        dst: FloatReg,
        src1: FloatReg,
        src2: FloatReg,
    );
    fn add_reg64_reg64_reg64(
        buf: &mut Vec<'_, u8>,
        dst: GeneralReg,
        src1: GeneralReg,
        src2: GeneralReg,
    );

    fn call(buf: &mut Vec<'_, u8>, relocs: &mut Vec<'_, Relocation>, fn_name: String);

    // Jumps by an offset of offset bytes unconditionally.
    // It should always generate the same number of bytes to enable replacement if offset changes.
    // It returns the base offset to calculate the jump from (generally the instruction after the jump).
    fn jmp_imm32(buf: &mut Vec<'_, u8>, offset: i32) -> usize;

    fn tail_call(buf: &mut Vec<'_, u8>) -> u64;

    // Jumps by an offset of offset bytes if reg is not equal to imm.
    // It should always generate the same number of bytes to enable replacement if offset changes.
    // It returns the base offset to calculate the jump from (generally the instruction after the jump).
    fn jne_reg64_imm64_imm32(
        buf: &mut Vec<'_, u8>,
        reg: GeneralReg,
        imm: u64,
        offset: i32,
    ) -> usize;

    fn mov_freg64_imm64(
        buf: &mut Vec<'_, u8>,
        relocs: &mut Vec<'_, Relocation>,
        dst: FloatReg,
        imm: f64,
    );
    fn mov_reg64_imm64(buf: &mut Vec<'_, u8>, dst: GeneralReg, imm: i64);
    fn mov_freg64_freg64(buf: &mut Vec<'_, u8>, dst: FloatReg, src: FloatReg);
    fn mov_reg64_reg64(buf: &mut Vec<'_, u8>, dst: GeneralReg, src: GeneralReg);

    // base32 is similar to stack based instructions but they reference the base/frame pointer.
    fn mov_freg64_base32(buf: &mut Vec<'_, u8>, dst: FloatReg, offset: i32);
    fn mov_reg64_base32(buf: &mut Vec<'_, u8>, dst: GeneralReg, offset: i32);
    fn mov_base32_freg64(buf: &mut Vec<'_, u8>, offset: i32, src: FloatReg);
    fn mov_base32_reg64(buf: &mut Vec<'_, u8>, offset: i32, src: GeneralReg);

    fn mov_freg64_stack32(buf: &mut Vec<'_, u8>, dst: FloatReg, offset: i32);
    fn mov_reg64_stack32(buf: &mut Vec<'_, u8>, dst: GeneralReg, offset: i32);
    fn mov_stack32_freg64(buf: &mut Vec<'_, u8>, offset: i32, src: FloatReg);
    fn mov_stack32_reg64(buf: &mut Vec<'_, u8>, offset: i32, src: GeneralReg);

    fn imul_reg64_reg64_reg64(
        buf: &mut Vec<'_, u8>,
        dst: GeneralReg,
        src1: GeneralReg,
        src2: GeneralReg,
    );

    fn sub_reg64_reg64_imm32(buf: &mut Vec<'_, u8>, dst: GeneralReg, src1: GeneralReg, imm32: i32);
    fn sub_reg64_reg64_reg64(
        buf: &mut Vec<'_, u8>,
        dst: GeneralReg,
        src1: GeneralReg,
        src2: GeneralReg,
    );

    fn eq_reg64_reg64_reg64(
        buf: &mut Vec<'_, u8>,
        dst: GeneralReg,
        src1: GeneralReg,
        src2: GeneralReg,
    );

    fn ret(buf: &mut Vec<'_, u8>);
}

#[derive(Clone, Debug, PartialEq)]
#[allow(dead_code)]
pub enum SymbolStorage<GeneralReg: RegTrait, FloatReg: RegTrait> {
    // These may need layout, but I am not sure.
    // I think whenever a symbol would be used, we specify layout anyways.
    GeneralReg(GeneralReg),
    FloatReg(FloatReg),
    Base(i32),
    BaseAndGeneralReg(GeneralReg, i32),
    BaseAndFloatReg(FloatReg, i32),
}

pub trait RegTrait: Copy + Eq + std::hash::Hash + std::fmt::Debug + 'static {}

pub struct Backend64Bit<
    'a,
    GeneralReg: RegTrait,
    FloatReg: RegTrait,
    ASM: Assembler<GeneralReg, FloatReg>,
    CC: CallConv<GeneralReg, FloatReg>,
> {
    phantom_asm: PhantomData<ASM>,
    phantom_cc: PhantomData<CC>,
    env: &'a Env<'a>,
    buf: Vec<'a, u8>,
    relocs: Vec<'a, Relocation>,

    last_seen_map: MutMap<Symbol, *const Stmt<'a>>,
    free_map: MutMap<*const Stmt<'a>, Vec<'a, Symbol>>,
    symbols_map: MutMap<Symbol, SymbolStorage<GeneralReg, FloatReg>>,
    literal_map: MutMap<Symbol, Literal<'a>>,

    // This should probably be smarter than a vec.
    // There are certain registers we should always use first. With pushing and popping, this could get mixed.
    general_free_regs: Vec<'a, GeneralReg>,
    float_free_regs: Vec<'a, FloatReg>,

    // The last major thing we need is a way to decide what reg to free when all of them are full.
    // Theoretically we want a basic lru cache for the currently loaded symbols.
    // For now just a vec of used registers and the symbols they contain.
    general_used_regs: Vec<'a, (GeneralReg, Symbol)>,
    float_used_regs: Vec<'a, (FloatReg, Symbol)>,

    // used callee saved regs must be tracked for pushing and popping at the beginning/end of the function.
    general_used_callee_saved_regs: MutSet<GeneralReg>,
    float_used_callee_saved_regs: MutSet<FloatReg>,

    stack_size: u32,
    // The amount of stack space needed to pass args for function calling.
    fn_call_stack_size: u32,
}

impl<
        'a,
        GeneralReg: RegTrait,
        FloatReg: RegTrait,
        ASM: Assembler<GeneralReg, FloatReg>,
        CC: CallConv<GeneralReg, FloatReg>,
    > Backend<'a> for Backend64Bit<'a, GeneralReg, FloatReg, ASM, CC>
{
    fn new(env: &'a Env, _target: &Triple) -> Result<Self, String> {
        Ok(Backend64Bit {
            phantom_asm: PhantomData,
            phantom_cc: PhantomData,
            env,
            buf: bumpalo::vec!(in env.arena),
            relocs: bumpalo::vec!(in env.arena),
            last_seen_map: MutMap::default(),
            free_map: MutMap::default(),
            symbols_map: MutMap::default(),
            literal_map: MutMap::default(),
            general_free_regs: bumpalo::vec![in env.arena],
            general_used_regs: bumpalo::vec![in env.arena],
            general_used_callee_saved_regs: MutSet::default(),
            float_free_regs: bumpalo::vec![in env.arena],
            float_used_regs: bumpalo::vec![in env.arena],
            float_used_callee_saved_regs: MutSet::default(),
            stack_size: 0,
            fn_call_stack_size: 0,
        })
    }

    fn env(&self) -> &'a Env<'a> {
        self.env
    }

    fn reset(&mut self) {
        self.stack_size = 0;
        self.fn_call_stack_size = 0;
        self.last_seen_map.clear();
        self.free_map.clear();
        self.symbols_map.clear();
        self.buf.clear();
        self.general_used_callee_saved_regs.clear();
        self.general_free_regs.clear();
        self.general_used_regs.clear();
        self.general_free_regs
            .extend_from_slice(CC::GENERAL_DEFAULT_FREE_REGS);
        self.float_used_callee_saved_regs.clear();
        self.float_free_regs.clear();
        self.float_used_regs.clear();
        self.float_free_regs
            .extend_from_slice(CC::FLOAT_DEFAULT_FREE_REGS);
    }

    fn literal_map(&mut self) -> &mut MutMap<Symbol, Literal<'a>> {
        &mut self.literal_map
    }

    fn last_seen_map(&mut self) -> &mut MutMap<Symbol, *const Stmt<'a>> {
        &mut self.last_seen_map
    }

    fn set_free_map(&mut self, map: MutMap<*const Stmt<'a>, Vec<'a, Symbol>>) {
        self.free_map = map;
    }

    fn free_map(&mut self) -> &mut MutMap<*const Stmt<'a>, Vec<'a, Symbol>> {
        &mut self.free_map
    }

    fn finalize(&mut self) -> Result<(&'a [u8], &[Relocation]), String> {
        let mut out = bumpalo::vec![in self.env.arena];

        // Setup stack.
        let mut used_regs = bumpalo::vec![in self.env.arena];
        used_regs.extend(&self.general_used_callee_saved_regs);
        let aligned_stack_size = CC::setup_stack(
            &mut out,
            &used_regs,
            self.stack_size as i32,
            self.fn_call_stack_size as i32,
        )?;
        let setup_offset = out.len();

        // Add function body.
        out.extend(&self.buf);

        // Cleanup stack.
        CC::cleanup_stack(
            &mut out,
            &used_regs,
            aligned_stack_size,
            self.fn_call_stack_size as i32,
        )?;
        ASM::ret(&mut out);

        // Update relocs to include stack setup offset.
        let mut out_relocs = bumpalo::vec![in self.env.arena];
        let old_relocs = std::mem::replace(&mut self.relocs, bumpalo::vec![in self.env.arena]);
        out_relocs.extend(old_relocs.into_iter().map(|reloc| match reloc {
            Relocation::LocalData { offset, data } => Relocation::LocalData {
                offset: offset + setup_offset as u64,
                data,
            },
            Relocation::LinkedData { offset, name } => Relocation::LinkedData {
                offset: offset + setup_offset as u64,
                name,
            },
            Relocation::LinkedFunction { offset, name } => Relocation::LinkedFunction {
                offset: offset + setup_offset as u64,
                name,
            },
        }));
        Ok((out.into_bump_slice(), out_relocs.into_bump_slice()))
    }

    fn load_args(&mut self, args: &'a [(Layout<'a>, Symbol)]) -> Result<(), String> {
        CC::load_args(&mut self.symbols_map, args)?;
        // Update used and free regs.
        for (sym, storage) in &self.symbols_map {
            match storage {
                SymbolStorage::GeneralReg(reg) | SymbolStorage::BaseAndGeneralReg(reg, _) => {
                    self.general_free_regs.retain(|r| *r != *reg);
                    self.general_used_regs.push((*reg, *sym));
                }
                SymbolStorage::FloatReg(reg) | SymbolStorage::BaseAndFloatReg(reg, _) => {
                    self.float_free_regs.retain(|r| *r != *reg);
                    self.float_used_regs.push((*reg, *sym));
                }
                SymbolStorage::Base(_) => {}
            }
        }
        Ok(())
    }

    /// Used for generating wrappers for malloc/realloc/free
    fn build_wrapped_jmp(&mut self) -> Result<(&'a [u8], u64), String> {
        let mut out = bumpalo::vec![in self.env.arena];
        let offset = ASM::tail_call(&mut out);

        Ok((out.into_bump_slice(), offset))
    }

    fn build_fn_call(
        &mut self,
        dst: &Symbol,
        fn_name: String,
        args: &'a [Symbol],
        arg_layouts: &[Layout<'a>],
        ret_layout: &Layout<'a>,
    ) -> Result<(), String> {
        // Save used caller saved regs.
        let old_general_used_regs = std::mem::replace(
            &mut self.general_used_regs,
            bumpalo::vec![in self.env.arena],
        );
        for (reg, saved_sym) in old_general_used_regs.into_iter() {
            if CC::general_caller_saved(&reg) {
                self.general_free_regs.push(reg);
                self.free_to_stack(&saved_sym)?;
            } else {
                self.general_used_regs.push((reg, saved_sym));
            }
        }
        let old_float_used_regs =
            std::mem::replace(&mut self.float_used_regs, bumpalo::vec![in self.env.arena]);
        for (reg, saved_sym) in old_float_used_regs.into_iter() {
            if CC::float_caller_saved(&reg) {
                self.float_free_regs.push(reg);
                self.free_to_stack(&saved_sym)?;
            } else {
                self.float_used_regs.push((reg, saved_sym));
            }
        }

        // Put values in param regs or on top of the stack.
        let tmp_stack_size = CC::store_args(
            &mut self.buf,
            &self.symbols_map,
            args,
            arg_layouts,
            ret_layout,
        )?;
        self.fn_call_stack_size = std::cmp::max(self.fn_call_stack_size, tmp_stack_size);

        // Call function and generate reloc.
        ASM::call(&mut self.buf, &mut self.relocs, fn_name);

        // move return value to dst.
        match ret_layout {
            Layout::Builtin(Builtin::Int64) => {
                let dst_reg = self.claim_general_reg(dst)?;
                ASM::mov_reg64_reg64(&mut self.buf, dst_reg, CC::GENERAL_RETURN_REGS[0]);
                Ok(())
            }
            Layout::Builtin(Builtin::Float64) => {
                let dst_reg = self.claim_float_reg(dst)?;
                ASM::mov_freg64_freg64(&mut self.buf, dst_reg, CC::FLOAT_RETURN_REGS[0]);
                Ok(())
            }
            x => Err(format!(
                "receiving return type, {:?}, is not yet implemented",
                x
            )),
        }
    }

    fn build_switch(
        &mut self,
        cond_symbol: &Symbol,
        _cond_layout: &Layout<'a>, // cond_layout must be a integer due to potential jump table optimizations.
        branches: &'a [(u64, BranchInfo<'a>, Stmt<'a>)],
        default_branch: &(BranchInfo<'a>, &'a Stmt<'a>),
        _ret_layout: &Layout<'a>,
    ) -> Result<(), String> {
        // Switches are a little complex due to keeping track of jumps.
        // In general I am trying to not have to loop over things multiple times or waste memory.
        // The basic plan is to make jumps to nowhere and then correct them once we know the correct address.
        let cond_reg = self.load_to_general_reg(cond_symbol)?;

        let mut ret_jumps = bumpalo::vec![in self.env.arena];
        let mut tmp = bumpalo::vec![in self.env.arena];
        for (val, branch_info, stmt) in branches.iter() {
            tmp.clear();
            if let BranchInfo::None = branch_info {
                // Create jump to next branch if not cond_sym not equal to value.
                // Since we don't know the offset yet, set it to 0 and overwrite later.
                let jne_location = self.buf.len();
                let start_offset = ASM::jne_reg64_imm64_imm32(&mut self.buf, cond_reg, *val, 0);

                // Build all statements in this branch.
                self.build_stmt(stmt)?;

                // Build unconditional jump to the end of this switch.
                // Since we don't know the offset yet, set it to 0 and overwrite later.
                let jmp_location = self.buf.len();
                let jmp_offset = ASM::jmp_imm32(&mut self.buf, 0);
                ret_jumps.push((jmp_location, jmp_offset));

                // Overwite the original jne with the correct offset.
                let end_offset = self.buf.len();
                let jne_offset = end_offset - start_offset;
                ASM::jne_reg64_imm64_imm32(&mut tmp, cond_reg, *val, jne_offset as i32);
                for (i, byte) in tmp.iter().enumerate() {
                    self.buf[jne_location + i] = *byte;
                }
            } else {
                return Err(format!(
                    "branch info, {:?}, is not yet implemented in switch statemens",
                    branch_info
                ));
            }
        }
        let (branch_info, stmt) = default_branch;
        if let BranchInfo::None = branch_info {
            self.build_stmt(stmt)?;

            // Update all return jumps to jump past the default case.
            let ret_offset = self.buf.len();
            for (jmp_location, start_offset) in ret_jumps.into_iter() {
                tmp.clear();
                let jmp_offset = ret_offset - start_offset;
                ASM::jmp_imm32(&mut tmp, jmp_offset as i32);
                for (i, byte) in tmp.iter().enumerate() {
                    self.buf[jmp_location + i] = *byte;
                }
            }
            Ok(())
        } else {
            Err(format!(
                "branch info, {:?}, is not yet implemented in switch statemens",
                branch_info
            ))
        }
    }

    fn build_num_abs_i64(&mut self, dst: &Symbol, src: &Symbol) -> Result<(), String> {
        let dst_reg = self.claim_general_reg(dst)?;
        let src_reg = self.load_to_general_reg(src)?;
        ASM::abs_reg64_reg64(&mut self.buf, dst_reg, src_reg);
        Ok(())
    }

    fn build_num_abs_f64(&mut self, dst: &Symbol, src: &Symbol) -> Result<(), String> {
        let dst_reg = self.claim_float_reg(dst)?;
        let src_reg = self.load_to_float_reg(src)?;

        ASM::abs_freg64_freg64(&mut self.buf, &mut self.relocs, dst_reg, src_reg);

        Ok(())
    }

    fn build_num_add_i64(
        &mut self,
        dst: &Symbol,
        src1: &Symbol,
        src2: &Symbol,
    ) -> Result<(), String> {
        let dst_reg = self.claim_general_reg(dst)?;
        let src1_reg = self.load_to_general_reg(src1)?;
        let src2_reg = self.load_to_general_reg(src2)?;
        ASM::add_reg64_reg64_reg64(&mut self.buf, dst_reg, src1_reg, src2_reg);
        Ok(())
    }

    fn build_num_add_f64(
        &mut self,
        dst: &Symbol,
        src1: &Symbol,
        src2: &Symbol,
    ) -> Result<(), String> {
        let dst_reg = self.claim_float_reg(dst)?;
        let src1_reg = self.load_to_float_reg(src1)?;
        let src2_reg = self.load_to_float_reg(src2)?;
        ASM::add_freg64_freg64_freg64(&mut self.buf, dst_reg, src1_reg, src2_reg);
        Ok(())
    }

    fn build_num_mul_i64(
        &mut self,
        dst: &Symbol,
        src1: &Symbol,
        src2: &Symbol,
    ) -> Result<(), String> {
        let dst_reg = self.claim_general_reg(dst)?;
        let src1_reg = self.load_to_general_reg(src1)?;
        let src2_reg = self.load_to_general_reg(src2)?;
        ASM::imul_reg64_reg64_reg64(&mut self.buf, dst_reg, src1_reg, src2_reg);
        Ok(())
    }

    fn build_num_sub_i64(
        &mut self,
        dst: &Symbol,
        src1: &Symbol,
        src2: &Symbol,
    ) -> Result<(), String> {
        let dst_reg = self.claim_general_reg(dst)?;
        let src1_reg = self.load_to_general_reg(src1)?;
        let src2_reg = self.load_to_general_reg(src2)?;
        ASM::sub_reg64_reg64_reg64(&mut self.buf, dst_reg, src1_reg, src2_reg);
        Ok(())
    }

    fn build_eq_i64(&mut self, dst: &Symbol, src1: &Symbol, src2: &Symbol) -> Result<(), String> {
        let dst_reg = self.claim_general_reg(dst)?;
        let src1_reg = self.load_to_general_reg(src1)?;
        let src2_reg = self.load_to_general_reg(src2)?;
        ASM::eq_reg64_reg64_reg64(&mut self.buf, dst_reg, src1_reg, src2_reg);
        Ok(())
    }

    fn load_literal(&mut self, sym: &Symbol, lit: &Literal<'a>) -> Result<(), String> {
        match lit {
            Literal::Int(x) => {
                let reg = self.claim_general_reg(sym)?;
                let val = *x;
                ASM::mov_reg64_imm64(&mut self.buf, reg, val as i64);
                Ok(())
            }
            Literal::Float(x) => {
                let reg = self.claim_float_reg(sym)?;
                let val = *x;
                ASM::mov_freg64_imm64(&mut self.buf, &mut self.relocs, reg, val);
                Ok(())
            }
            x => Err(format!("loading literal, {:?}, is not yet implemented", x)),
        }
    }

    fn free_symbol(&mut self, sym: &Symbol) {
        self.symbols_map.remove(sym);
        for i in 0..self.general_used_regs.len() {
            let (reg, saved_sym) = self.general_used_regs[i];
            if saved_sym == *sym {
                self.general_free_regs.push(reg);
                self.general_used_regs.remove(i);
                break;
            }
        }
    }

    fn return_symbol(&mut self, sym: &Symbol) -> Result<(), String> {
        let val = self.symbols_map.get(sym);
        match val {
            Some(SymbolStorage::GeneralReg(reg)) if *reg == CC::GENERAL_RETURN_REGS[0] => Ok(()),
            Some(SymbolStorage::GeneralReg(reg)) => {
                // If it fits in a general purpose register, just copy it over to.
                // Technically this can be optimized to produce shorter instructions if less than 64bits.
                ASM::mov_reg64_reg64(&mut self.buf, CC::GENERAL_RETURN_REGS[0], *reg);
                Ok(())
            }
            Some(SymbolStorage::FloatReg(reg)) if *reg == CC::FLOAT_RETURN_REGS[0] => Ok(()),
            Some(SymbolStorage::FloatReg(reg)) => {
                ASM::mov_freg64_freg64(&mut self.buf, CC::FLOAT_RETURN_REGS[0], *reg);
                Ok(())
            }
            Some(x) => Err(format!(
                "returning symbol storage, {:?}, is not yet implemented",
                x
            )),
            None => Err(format!("Unknown return symbol: {}", sym)),
        }
    }
}

/// This impl block is for ir related instructions that need backend specific information.
/// For example, loading a symbol for doing a computation.
impl<
        'a,
        FloatReg: RegTrait,
        GeneralReg: RegTrait,
        ASM: Assembler<GeneralReg, FloatReg>,
        CC: CallConv<GeneralReg, FloatReg>,
    > Backend64Bit<'a, GeneralReg, FloatReg, ASM, CC>
{
    fn claim_general_reg(&mut self, sym: &Symbol) -> Result<GeneralReg, String> {
        let reg = if !self.general_free_regs.is_empty() {
            let free_reg = self.general_free_regs.pop().unwrap();
            if CC::general_callee_saved(&free_reg) {
                self.general_used_callee_saved_regs.insert(free_reg);
            }
            Ok(free_reg)
        } else if !self.general_used_regs.is_empty() {
            let (reg, sym) = self.general_used_regs.remove(0);
            self.free_to_stack(&sym)?;
            Ok(reg)
        } else {
            Err("completely out of general purpose registers".to_string())
        }?;

        self.general_used_regs.push((reg, *sym));
        self.symbols_map
            .insert(*sym, SymbolStorage::GeneralReg(reg));
        Ok(reg)
    }

    fn claim_float_reg(&mut self, sym: &Symbol) -> Result<FloatReg, String> {
        let reg = if !self.float_free_regs.is_empty() {
            let free_reg = self.float_free_regs.pop().unwrap();
            if CC::float_callee_saved(&free_reg) {
                self.float_used_callee_saved_regs.insert(free_reg);
            }
            Ok(free_reg)
        } else if !self.float_used_regs.is_empty() {
            let (reg, sym) = self.float_used_regs.remove(0);
            self.free_to_stack(&sym)?;
            Ok(reg)
        } else {
            Err("completely out of floating point registers".to_string())
        }?;

        self.float_used_regs.push((reg, *sym));
        self.symbols_map.insert(*sym, SymbolStorage::FloatReg(reg));
        Ok(reg)
    }

    fn load_to_general_reg(&mut self, sym: &Symbol) -> Result<GeneralReg, String> {
        let val = self.symbols_map.remove(sym);
        match val {
            Some(SymbolStorage::GeneralReg(reg)) => {
                self.symbols_map
                    .insert(*sym, SymbolStorage::GeneralReg(reg));
                Ok(reg)
            }
            Some(SymbolStorage::Base(offset)) => {
                let reg = self.claim_general_reg(sym)?;
                self.symbols_map
                    .insert(*sym, SymbolStorage::BaseAndGeneralReg(reg, offset));
                ASM::mov_reg64_base32(&mut self.buf, reg, offset as i32);
                Ok(reg)
            }
            Some(SymbolStorage::BaseAndGeneralReg(reg, offset)) => {
                self.symbols_map
                    .insert(*sym, SymbolStorage::BaseAndGeneralReg(reg, offset));
                Ok(reg)
            }
            Some(SymbolStorage::FloatReg(_)) | Some(SymbolStorage::BaseAndFloatReg(_, _)) => {
                Err("Cannot load floating point symbol into GeneralReg".to_string())
            }
            None => Err(format!("Unknown symbol: {}", sym)),
        }
    }

    fn load_to_float_reg(&mut self, sym: &Symbol) -> Result<FloatReg, String> {
        let val = self.symbols_map.remove(sym);
        match val {
            Some(SymbolStorage::FloatReg(reg)) => {
                self.symbols_map.insert(*sym, SymbolStorage::FloatReg(reg));
                Ok(reg)
            }
            Some(SymbolStorage::Base(offset)) => {
                let reg = self.claim_float_reg(sym)?;
                self.symbols_map
                    .insert(*sym, SymbolStorage::BaseAndFloatReg(reg, offset));
                ASM::mov_freg64_base32(&mut self.buf, reg, offset as i32);
                Ok(reg)
            }
            Some(SymbolStorage::BaseAndFloatReg(reg, offset)) => {
                self.symbols_map
                    .insert(*sym, SymbolStorage::BaseAndFloatReg(reg, offset));
                Ok(reg)
            }
            Some(SymbolStorage::GeneralReg(_)) | Some(SymbolStorage::BaseAndGeneralReg(_, _)) => {
                Err("Cannot load integer point symbol into FloatReg".to_string())
            }
            None => Err(format!("Unknown symbol: {}", sym)),
        }
    }

    fn free_to_stack(&mut self, sym: &Symbol) -> Result<(), String> {
        let val = self.symbols_map.remove(sym);
        match val {
            Some(SymbolStorage::GeneralReg(reg)) => {
                let offset = self.increase_stack_size(8)? as i32;
                // For base addresssing, use the negative offset - 8.
                ASM::mov_base32_reg64(&mut self.buf, -offset - 8, reg);
                self.symbols_map
                    .insert(*sym, SymbolStorage::Base(-offset - 8));
                Ok(())
            }
            Some(SymbolStorage::FloatReg(reg)) => {
                let offset = self.increase_stack_size(8)? as i32;
                // For base addresssing, use the negative offset.
                ASM::mov_base32_freg64(&mut self.buf, -offset - 8, reg);
                self.symbols_map
                    .insert(*sym, SymbolStorage::Base(-offset - 8));
                Ok(())
            }
            Some(SymbolStorage::Base(offset)) => {
                self.symbols_map.insert(*sym, SymbolStorage::Base(offset));
                Ok(())
            }
            Some(SymbolStorage::BaseAndGeneralReg(_, offset)) => {
                self.symbols_map.insert(*sym, SymbolStorage::Base(offset));
                Ok(())
            }
            Some(SymbolStorage::BaseAndFloatReg(_, offset)) => {
                self.symbols_map.insert(*sym, SymbolStorage::Base(offset));
                Ok(())
            }
            None => Err(format!("Unknown symbol: {}", sym)),
        }
    }

    /// increase_stack_size increase the current stack size and returns the offset of the stack.
    fn increase_stack_size(&mut self, amount: u32) -> Result<u32, String> {
        debug_assert!(amount > 0);
        let offset = self.stack_size;
        if let Some(new_size) = self.stack_size.checked_add(amount) {
            // Since stack size is u32, but the max offset is i32, if we pass i32 max, we have overflowed.
            if new_size > i32::MAX as u32 {
                Err("Ran out of stack space".to_string())
            } else {
                self.stack_size = new_size;
                Ok(offset)
            }
        } else {
            Err("Ran out of stack space".to_string())
        }
    }
}