//! A frontend for building Cranelift IR from other languages.
use crate::ssa::{SSABuilder, SideEffects};
use crate::variable::Variable;
use core::fmt::{self, Debug};
use cranelift_codegen::cursor::{Cursor, FuncCursor};
use cranelift_codegen::entity::{EntityRef, EntitySet, SecondaryMap};
use cranelift_codegen::ir;
use cranelift_codegen::ir::condcodes::IntCC;
use cranelift_codegen::ir::{
    types, AbiParam, Block, DataFlowGraph, DynamicStackSlot, DynamicStackSlotData, ExtFuncData,
    ExternalName, FuncRef, Function, GlobalValue, GlobalValueData, Heap, HeapData, Inst,
    InstBuilder, InstBuilderBase, InstructionData, JumpTable, JumpTableData, LibCall, MemFlags,
    RelSourceLoc, SigRef, Signature, StackSlot, StackSlotData, Type, Value, ValueLabel,
    ValueLabelAssignments, ValueLabelStart,
};
use cranelift_codegen::isa::TargetFrontendConfig;
use cranelift_codegen::packed_option::PackedOption;
use std::convert::TryInto; // FIXME: Remove in edition2021

/// Structure used for translating a series of functions into Cranelift IR.
///
/// In order to reduce memory reallocations when compiling multiple functions,
/// `FunctionBuilderContext` holds various data structures which are cleared between
/// functions, rather than dropped, preserving the underlying allocations.
pub struct FunctionBuilderContext {
    ssa: SSABuilder,
    blocks: SecondaryMap<Block, BlockData>,
    types: SecondaryMap<Variable, Type>,
}

/// Temporary object used to build a single Cranelift IR `Function`.
pub struct FunctionBuilder<'a> {
    /// The function currently being built.
    /// This field is public so the function can be re-borrowed.
    pub func: &'a mut Function,

    /// Source location to assign to all new instructions.
    srcloc: ir::SourceLoc,

    func_ctx: &'a mut FunctionBuilderContext,
    position: PackedOption<Block>,
}

#[derive(Clone, Default)]
struct BlockData {
    /// A Block is "pristine" iff no instructions have been added since the last
    /// call to `switch_to_block()`.
    pristine: bool,

    /// A Block is "filled" iff a terminator instruction has been inserted since
    /// the last call to `switch_to_block()`.
    ///
    /// A filled block cannot be pristine.
    filled: bool,

    /// Count of parameters not supplied implicitly by the SSABuilder.
    user_param_count: usize,
}

impl FunctionBuilderContext {
    /// Creates a FunctionBuilderContext structure. The structure is automatically cleared after
    /// each [`FunctionBuilder`](struct.FunctionBuilder.html) completes translating a function.
    pub fn new() -> Self {
        Self {
            ssa: SSABuilder::new(),
            blocks: SecondaryMap::new(),
            types: SecondaryMap::new(),
        }
    }

    fn clear(&mut self) {
        self.ssa.clear();
        self.blocks.clear();
        self.types.clear();
    }

    fn is_empty(&self) -> bool {
        self.ssa.is_empty() && self.blocks.is_empty() && self.types.is_empty()
    }
}

/// Implementation of the [`InstBuilder`](cranelift_codegen::ir::InstBuilder) that has
/// one convenience method per Cranelift IR instruction.
pub struct FuncInstBuilder<'short, 'long: 'short> {
    builder: &'short mut FunctionBuilder<'long>,
    block: Block,
}

impl<'short, 'long> FuncInstBuilder<'short, 'long> {
    fn new(builder: &'short mut FunctionBuilder<'long>, block: Block) -> Self {
        Self { builder, block }
    }
}

impl<'short, 'long> InstBuilderBase<'short> for FuncInstBuilder<'short, 'long> {
    fn data_flow_graph(&self) -> &DataFlowGraph {
        &self.builder.func.dfg
    }

    fn data_flow_graph_mut(&mut self) -> &mut DataFlowGraph {
        &mut self.builder.func.dfg
    }

    // This implementation is richer than `InsertBuilder` because we use the data of the
    // instruction being inserted to add related info to the DFG and the SSA building system,
    // and perform debug sanity checks.
    fn build(self, data: InstructionData, ctrl_typevar: Type) -> (Inst, &'short mut DataFlowGraph) {
        // We only insert the Block in the layout when an instruction is added to it
        self.builder.ensure_inserted_block();

        let inst = self.builder.func.dfg.make_inst(data.clone());
        self.builder.func.dfg.make_inst_results(inst, ctrl_typevar);
        self.builder.func.layout.append_inst(inst, self.block);
        if !self.builder.srcloc.is_default() {
            self.builder.func.set_srcloc(inst, self.builder.srcloc);
        }

        if data.opcode().is_branch() {
            match data.branch_destination() {
                Some(dest_block) => {
                    // If the user has supplied jump arguments we must adapt the arguments of
                    // the destination block
                    self.builder.declare_successor(dest_block, inst);
                }
                None => {
                    // branch_destination() doesn't detect jump_tables
                    // If jump table we declare all entries successor
                    if let InstructionData::BranchTable {
                        table, destination, ..
                    } = data
                    {
                        // Unlike all other jumps/branches, jump tables are
                        // capable of having the same successor appear
                        // multiple times, so we must deduplicate.
                        let mut unique = EntitySet::<Block>::new();
                        for dest_block in self
                            .builder
                            .func
                            .jump_tables
                            .get(table)
                            .expect("you are referencing an undeclared jump table")
                            .iter()
                            .filter(|&dest_block| unique.insert(*dest_block))
                        {
                            // Call `declare_block_predecessor` instead of `declare_successor` for
                            // avoiding the borrow checker.
                            self.builder.func_ctx.ssa.declare_block_predecessor(
                                *dest_block,
                                self.builder.position.unwrap(),
                                inst,
                            );
                        }
                        self.builder.declare_successor(destination, inst);
                    }
                }
            }
        }

        if data.opcode().is_terminator() {
            self.builder.fill_current_block()
        }
        (inst, &mut self.builder.func.dfg)
    }
}

#[derive(Debug, Copy, Clone, PartialEq, Eq)]
/// An error encountered when calling [`FunctionBuilder::try_use_var`].
pub enum UseVariableError {
    UsedBeforeDeclared(Variable),
}

impl fmt::Display for UseVariableError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            UseVariableError::UsedBeforeDeclared(variable) => {
                write!(
                    f,
                    "variable {} was used before it was defined",
                    variable.index()
                )?;
            }
        }
        Ok(())
    }
}

impl std::error::Error for UseVariableError {}

#[derive(Debug, Copy, Clone, Eq, PartialEq)]
/// An error encountered when calling [`FunctionBuilder::try_declare_var`].
pub enum DeclareVariableError {
    DeclaredMultipleTimes(Variable),
}

impl std::error::Error for DeclareVariableError {}

impl fmt::Display for DeclareVariableError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            DeclareVariableError::DeclaredMultipleTimes(variable) => {
                write!(
                    f,
                    "variable {} was declared multiple times",
                    variable.index()
                )?;
            }
        }
        Ok(())
    }
}

#[derive(Debug, Copy, Clone, Eq, PartialEq)]
/// An error encountered when defining the initial value of a variable.
pub enum DefVariableError {
    /// The variable was instantiated with a value of the wrong type.
    ///
    /// note: to obtain the type of the value, you can call
    /// [`cranelift_codegen::ir::dfg::DataFlowGraph::value_type`] (using the
    /// [`FunctionBuilder.func.dfg`] field)
    TypeMismatch(Variable, Value),
    /// The value was defined (in a call to [`FunctionBuilder::def_var`]) before
    /// it was declared (in a call to [`FunctionBuilder::declare_var`]).
    DefinedBeforeDeclared(Variable),
}

impl fmt::Display for DefVariableError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            DefVariableError::TypeMismatch(variable, value) => {
                write!(
                    f,
                    "the types of variable {} and value {} are not the same.
                    The `Value` supplied to `def_var` must be of the same type as
                    the variable was declared to be of in `declare_var`.",
                    variable.index(),
                    value.as_u32()
                )?;
            }
            DefVariableError::DefinedBeforeDeclared(variable) => {
                write!(
                    f,
                    "the value of variabe {} was declared before it was defined",
                    variable.index()
                )?;
            }
        }
        Ok(())
    }
}

/// This module allows you to create a function in Cranelift IR in a straightforward way, hiding
/// all the complexity of its internal representation.
///
/// The module is parametrized by one type which is the representation of variables in your
/// origin language. It offers a way to conveniently append instruction to your program flow.
/// You are responsible to split your instruction flow into extended blocks (declared with
/// `create_block`) whose properties are:
///
/// - branch and jump instructions can only point at the top of extended blocks;
/// - the last instruction of each block is a terminator instruction which has no natural successor,
///   and those instructions can only appear at the end of extended blocks.
///
/// The parameters of Cranelift IR instructions are Cranelift IR values, which can only be created
/// as results of other Cranelift IR instructions. To be able to create variables redefined multiple
/// times in your program, use the `def_var` and `use_var` command, that will maintain the
/// correspondence between your variables and Cranelift IR SSA values.
///
/// The first block for which you call `switch_to_block` will be assumed to be the beginning of
/// the function.
///
/// At creation, a `FunctionBuilder` instance borrows an already allocated `Function` which it
/// modifies with the information stored in the mutable borrowed
/// [`FunctionBuilderContext`](struct.FunctionBuilderContext.html). The function passed in
/// argument should be newly created with
/// [`Function::with_name_signature()`](Function::with_name_signature), whereas the
/// `FunctionBuilderContext` can be kept as is between two function translations.
///
/// # Errors
///
/// The functions below will panic in debug mode whenever you try to modify the Cranelift IR
/// function in a way that violate the coherence of the code. For instance: switching to a new
/// `Block` when you haven't filled the current one with a terminator instruction, inserting a
/// return instruction with arguments that don't match the function's signature.
impl<'a> FunctionBuilder<'a> {
    /// Creates a new FunctionBuilder structure that will operate on a `Function` using a
    /// `FunctionBuilderContext`.
    pub fn new(func: &'a mut Function, func_ctx: &'a mut FunctionBuilderContext) -> Self {
        debug_assert!(func_ctx.is_empty());
        Self {
            func,
            srcloc: Default::default(),
            func_ctx,
            position: Default::default(),
        }
    }

    /// Get the block that this builder is currently at.
    pub fn current_block(&self) -> Option<Block> {
        self.position.expand()
    }

    /// Set the source location that should be assigned to all new instructions.
    pub fn set_srcloc(&mut self, srcloc: ir::SourceLoc) {
        self.srcloc = srcloc;
    }

    /// Creates a new `Block` and returns its reference.
    pub fn create_block(&mut self) -> Block {
        let block = self.func.dfg.make_block();
        self.func_ctx.ssa.declare_block(block);
        self.func_ctx.blocks[block] = BlockData {
            filled: false,
            pristine: true,
            user_param_count: 0,
        };
        block
    }

    /// Mark a block as "cold".
    ///
    /// This will try to move it out of the ordinary path of execution
    /// when lowered to machine code.
    pub fn set_cold_block(&mut self, block: Block) {
        self.func.layout.set_cold(block);
    }

    /// Insert `block` in the layout *after* the existing block `after`.
    pub fn insert_block_after(&mut self, block: Block, after: Block) {
        self.func.layout.insert_block_after(block, after);
    }

    /// After the call to this function, new instructions will be inserted into the designated
    /// block, in the order they are declared. You must declare the types of the Block arguments
    /// you will use here.
    ///
    /// When inserting the terminator instruction (which doesn't have a fallthrough to its immediate
    /// successor), the block will be declared filled and it will not be possible to append
    /// instructions to it.
    pub fn switch_to_block(&mut self, block: Block) {
        // First we check that the previous block has been filled.
        debug_assert!(
            self.position.is_none()
                || self.is_unreachable()
                || self.is_pristine()
                || self.is_filled(),
            "you have to fill your block before switching"
        );
        // We cannot switch to a filled block
        debug_assert!(
            !self.func_ctx.blocks[block].filled,
            "you cannot switch to a block which is already filled"
        );

        // Then we change the cursor position.
        self.position = PackedOption::from(block);
    }

    /// Declares that all the predecessors of this block are known.
    ///
    /// Function to call with `block` as soon as the last branch instruction to `block` has been
    /// created. Forgetting to call this method on every block will cause inconsistencies in the
    /// produced functions.
    pub fn seal_block(&mut self, block: Block) {
        let side_effects = self.func_ctx.ssa.seal_block(block, self.func);
        self.handle_ssa_side_effects(side_effects);
    }

    /// Effectively calls seal_block on all unsealed blocks in the function.
    ///
    /// It's more efficient to seal `Block`s as soon as possible, during
    /// translation, but for frontends where this is impractical to do, this
    /// function can be used at the end of translating all blocks to ensure
    /// that everything is sealed.
    pub fn seal_all_blocks(&mut self) {
        let side_effects = self.func_ctx.ssa.seal_all_blocks(self.func);
        self.handle_ssa_side_effects(side_effects);
    }

    /// Declares the type of a variable, so that it can be used later (by calling
    /// [`FunctionBuilder::use_var`]). This function will return an error if it
    /// was not possible to use the variable.
    pub fn try_declare_var(&mut self, var: Variable, ty: Type) -> Result<(), DeclareVariableError> {
        if self.func_ctx.types[var] != types::INVALID {
            return Err(DeclareVariableError::DeclaredMultipleTimes(var));
        }
        self.func_ctx.types[var] = ty;
        Ok(())
    }

    /// In order to use a variable (by calling [`FunctionBuilder::use_var`]), you need
    /// to first declare its type with this method.
    pub fn declare_var(&mut self, var: Variable, ty: Type) {
        self.try_declare_var(var, ty)
            .unwrap_or_else(|_| panic!("the variable {:?} has been declared multiple times", var))
    }

    /// Returns the Cranelift IR necessary to use a previously defined user
    /// variable, returning an error if this is not possible.
    pub fn try_use_var(&mut self, var: Variable) -> Result<Value, UseVariableError> {
        let (val, side_effects) = {
            let ty = *self
                .func_ctx
                .types
                .get(var)
                .ok_or(UseVariableError::UsedBeforeDeclared(var))?;
            debug_assert_ne!(
                ty,
                types::INVALID,
                "variable {:?} is used but its type has not been declared",
                var
            );
            self.func_ctx
                .ssa
                .use_var(self.func, var, ty, self.position.unwrap())
        };
        self.handle_ssa_side_effects(side_effects);
        Ok(val)
    }

    /// Returns the Cranelift IR value corresponding to the utilization at the current program
    /// position of a previously defined user variable.
    pub fn use_var(&mut self, var: Variable) -> Value {
        self.try_use_var(var).unwrap_or_else(|_| {
            panic!(
                "variable {:?} is used but its type has not been declared",
                var
            )
        })
    }

    /// Registers a new definition of a user variable. This function will return
    /// an error if the value supplied does not match the type the variable was
    /// declared to have.
    pub fn try_def_var(&mut self, var: Variable, val: Value) -> Result<(), DefVariableError> {
        let var_ty = *self
            .func_ctx
            .types
            .get(var)
            .ok_or(DefVariableError::DefinedBeforeDeclared(var))?;
        if var_ty != self.func.dfg.value_type(val) {
            return Err(DefVariableError::TypeMismatch(var, val));
        }

        self.func_ctx.ssa.def_var(var, val, self.position.unwrap());
        Ok(())
    }

    /// Register a new definition of a user variable. The type of the value must be
    /// the same as the type registered for the variable.
    pub fn def_var(&mut self, var: Variable, val: Value) {
        self.try_def_var(var, val)
            .unwrap_or_else(|error| match error {
                DefVariableError::TypeMismatch(var, val) => {
                    panic!(
                        "declared type of variable {:?} doesn't match type of value {}",
                        var, val
                    );
                }
                DefVariableError::DefinedBeforeDeclared(var) => {
                    panic!(
                        "variable {:?} is used but its type has not been declared",
                        var
                    );
                }
            })
    }

    /// Set label for Value
    ///
    /// This will not do anything unless `func.dfg.collect_debug_info` is called first.
    pub fn set_val_label(&mut self, val: Value, label: ValueLabel) {
        if let Some(values_labels) = self.func.stencil.dfg.values_labels.as_mut() {
            use alloc::collections::btree_map::Entry;

            let start = ValueLabelStart {
                from: RelSourceLoc::from_base_offset(self.func.params.base_srcloc(), self.srcloc),
                label,
            };

            match values_labels.entry(val) {
                Entry::Occupied(mut e) => match e.get_mut() {
                    ValueLabelAssignments::Starts(starts) => starts.push(start),
                    _ => panic!("Unexpected ValueLabelAssignments at this stage"),
                },
                Entry::Vacant(e) => {
                    e.insert(ValueLabelAssignments::Starts(vec![start]));
                }
            }
        }
    }

    /// Creates a jump table in the function, to be used by `br_table` instructions.
    pub fn create_jump_table(&mut self, data: JumpTableData) -> JumpTable {
        self.func.create_jump_table(data)
    }

    /// Creates a sized stack slot in the function, to be used by `stack_load`, `stack_store` and
    /// `stack_addr` instructions.
    pub fn create_sized_stack_slot(&mut self, data: StackSlotData) -> StackSlot {
        self.func.create_sized_stack_slot(data)
    }

    /// Creates a dynamic stack slot in the function, to be used by `dynamic_stack_load`,
    /// `dynamic_stack_store` and `dynamic_stack_addr` instructions.
    pub fn create_dynamic_stack_slot(&mut self, data: DynamicStackSlotData) -> DynamicStackSlot {
        self.func.create_dynamic_stack_slot(data)
    }

    /// Adds a signature which can later be used to declare an external function import.
    pub fn import_signature(&mut self, signature: Signature) -> SigRef {
        self.func.import_signature(signature)
    }

    /// Declare an external function import.
    pub fn import_function(&mut self, data: ExtFuncData) -> FuncRef {
        self.func.import_function(data)
    }

    /// Declares a global value accessible to the function.
    pub fn create_global_value(&mut self, data: GlobalValueData) -> GlobalValue {
        self.func.create_global_value(data)
    }

    /// Declares a heap accessible to the function.
    pub fn create_heap(&mut self, data: HeapData) -> Heap {
        self.func.create_heap(data)
    }

    /// Returns an object with the [`InstBuilder`](cranelift_codegen::ir::InstBuilder)
    /// trait that allows to conveniently append an instruction to the current `Block` being built.
    pub fn ins<'short>(&'short mut self) -> FuncInstBuilder<'short, 'a> {
        let block = self
            .position
            .expect("Please call switch_to_block before inserting instructions");
        FuncInstBuilder::new(self, block)
    }

    /// Make sure that the current block is inserted in the layout.
    pub fn ensure_inserted_block(&mut self) {
        let block = self.position.unwrap();
        if self.func_ctx.blocks[block].pristine {
            if !self.func.layout.is_block_inserted(block) {
                self.func.layout.append_block(block);
            }
            self.func_ctx.blocks[block].pristine = false;
        } else {
            debug_assert!(
                !self.func_ctx.blocks[block].filled,
                "you cannot add an instruction to a block already filled"
            );
        }
    }

    /// Returns a `FuncCursor` pointed at the current position ready for inserting instructions.
    ///
    /// This can be used to insert SSA code that doesn't need to access locals and that doesn't
    /// need to know about `FunctionBuilder` at all.
    pub fn cursor(&mut self) -> FuncCursor {
        self.ensure_inserted_block();
        FuncCursor::new(self.func)
            .with_srcloc(self.srcloc)
            .at_bottom(self.position.unwrap())
    }

    /// Append parameters to the given `Block` corresponding to the function
    /// parameters. This can be used to set up the block parameters for the
    /// entry block.
    pub fn append_block_params_for_function_params(&mut self, block: Block) {
        debug_assert!(
            !self.func_ctx.ssa.has_any_predecessors(block),
            "block parameters for function parameters should only be added to the entry block"
        );

        // These parameters count as "user" parameters here because they aren't
        // inserted by the SSABuilder.
        let user_param_count = &mut self.func_ctx.blocks[block].user_param_count;
        for argtyp in &self.func.stencil.signature.params {
            *user_param_count += 1;
            self.func
                .stencil
                .dfg
                .append_block_param(block, argtyp.value_type);
        }
    }

    /// Append parameters to the given `Block` corresponding to the function
    /// return values. This can be used to set up the block parameters for a
    /// function exit block.
    pub fn append_block_params_for_function_returns(&mut self, block: Block) {
        // These parameters count as "user" parameters here because they aren't
        // inserted by the SSABuilder.
        let user_param_count = &mut self.func_ctx.blocks[block].user_param_count;
        for argtyp in &self.func.stencil.signature.returns {
            *user_param_count += 1;
            self.func
                .stencil
                .dfg
                .append_block_param(block, argtyp.value_type);
        }
    }

    /// Declare that translation of the current function is complete. This
    /// resets the state of the `FunctionBuilder` in preparation to be used
    /// for another function.
    pub fn finalize(&mut self) {
        // Check that all the `Block`s are filled and sealed.
        #[cfg(debug_assertions)]
        {
            for (block, block_data) in self.func_ctx.blocks.iter() {
                assert!(
                    block_data.pristine || self.func_ctx.ssa.is_sealed(block),
                    "FunctionBuilder finalized, but block {} is not sealed",
                    block,
                );
                assert!(
                    block_data.pristine || block_data.filled,
                    "FunctionBuilder finalized, but block {} is not filled",
                    block,
                );
            }
        }

        // In debug mode, check that all blocks are valid basic blocks.
        #[cfg(debug_assertions)]
        {
            // Iterate manually to provide more helpful error messages.
            for block in self.func_ctx.blocks.keys() {
                if let Err((inst, msg)) = self.func.is_block_basic(block) {
                    let inst_str = self.func.dfg.display_inst(inst);
                    panic!(
                        "{} failed basic block invariants on {}: {}",
                        block, inst_str, msg
                    );
                }
            }
        }

        // Clear the state (but preserve the allocated buffers) in preparation
        // for translation another function.
        self.func_ctx.clear();

        // Reset srcloc and position to initial states.
        self.srcloc = Default::default();
        self.position = Default::default();
    }
}

/// All the functions documented in the previous block are write-only and help you build a valid
/// Cranelift IR functions via multiple debug asserts. However, you might need to improve the
/// performance of your translation perform more complex transformations to your Cranelift IR
/// function. The functions below help you inspect the function you're creating and modify it
/// in ways that can be unsafe if used incorrectly.
impl<'a> FunctionBuilder<'a> {
    /// Retrieves all the parameters for a `Block` currently inferred from the jump instructions
    /// inserted that target it and the SSA construction.
    pub fn block_params(&self, block: Block) -> &[Value] {
        self.func.dfg.block_params(block)
    }

    /// Retrieves the signature with reference `sigref` previously added with `import_signature`.
    pub fn signature(&self, sigref: SigRef) -> Option<&Signature> {
        self.func.dfg.signatures.get(sigref)
    }

    /// Creates a parameter for a specific `Block` by appending it to the list of already existing
    /// parameters.
    ///
    /// **Note:** this function has to be called at the creation of the `Block` before adding
    /// instructions to it, otherwise this could interfere with SSA construction.
    pub fn append_block_param(&mut self, block: Block, ty: Type) -> Value {
        debug_assert!(
            self.func_ctx.blocks[block].pristine,
            "You can't add block parameters after adding any instruction"
        );
        debug_assert_eq!(
            self.func_ctx.blocks[block].user_param_count,
            self.func.dfg.num_block_params(block)
        );
        self.func_ctx.blocks[block].user_param_count += 1;
        self.func.dfg.append_block_param(block, ty)
    }

    /// Returns the result values of an instruction.
    pub fn inst_results(&self, inst: Inst) -> &[Value] {
        self.func.dfg.inst_results(inst)
    }

    /// Changes the destination of a jump instruction after creation.
    ///
    /// **Note:** You are responsible for maintaining the coherence with the arguments of
    /// other jump instructions.
    pub fn change_jump_destination(&mut self, inst: Inst, new_dest: Block) {
        let old_dest = self.func.dfg[inst]
            .branch_destination_mut()
            .expect("you want to change the jump destination of a non-jump instruction");
        let pred = self.func_ctx.ssa.remove_block_predecessor(*old_dest, inst);
        *old_dest = new_dest;
        self.func_ctx
            .ssa
            .declare_block_predecessor(new_dest, pred, inst);
    }

    /// Returns `true` if and only if the current `Block` is sealed and has no predecessors declared.
    ///
    /// The entry block of a function is never unreachable.
    pub fn is_unreachable(&self) -> bool {
        let is_entry = match self.func.layout.entry_block() {
            None => false,
            Some(entry) => self.position.unwrap() == entry,
        };
        !is_entry
            && self.func_ctx.ssa.is_sealed(self.position.unwrap())
            && !self
                .func_ctx
                .ssa
                .has_any_predecessors(self.position.unwrap())
    }

    /// Returns `true` if and only if no instructions have been added since the last call to
    /// `switch_to_block`.
    pub fn is_pristine(&self) -> bool {
        self.func_ctx.blocks[self.position.unwrap()].pristine
    }

    /// Returns `true` if and only if a terminator instruction has been inserted since the
    /// last call to `switch_to_block`.
    pub fn is_filled(&self) -> bool {
        self.func_ctx.blocks[self.position.unwrap()].filled
    }
}

/// Helper functions
impl<'a> FunctionBuilder<'a> {
    /// Calls libc.memcpy
    ///
    /// Copies the `size` bytes from `src` to `dest`, assumes that `src + size`
    /// won't overlap onto `dest`. If `dest` and `src` overlap, the behavior is
    /// undefined. Applications in which `dest` and `src` might overlap should
    /// use `call_memmove` instead.
    pub fn call_memcpy(
        &mut self,
        config: TargetFrontendConfig,
        dest: Value,
        src: Value,
        size: Value,
    ) {
        let pointer_type = config.pointer_type();
        let signature = {
            let mut s = Signature::new(config.default_call_conv);
            s.params.push(AbiParam::new(pointer_type));
            s.params.push(AbiParam::new(pointer_type));
            s.params.push(AbiParam::new(pointer_type));
            self.import_signature(s)
        };

        let libc_memcpy = self.import_function(ExtFuncData {
            name: ExternalName::LibCall(LibCall::Memcpy),
            signature,
            colocated: false,
        });

        self.ins().call(libc_memcpy, &[dest, src, size]);
    }

    /// Optimised memcpy or memmove for small copies.
    ///
    /// # Codegen safety
    ///
    /// The following properties must hold to prevent UB:
    ///
    /// * `src_align` and `dest_align` are an upper-bound on the alignment of `src` respectively `dest`.
    /// * If `non_overlapping` is true, then this must be correct.
    pub fn emit_small_memory_copy(
        &mut self,
        config: TargetFrontendConfig,
        dest: Value,
        src: Value,
        size: u64,
        dest_align: u8,
        src_align: u8,
        non_overlapping: bool,
        mut flags: MemFlags,
    ) {
        // Currently the result of guess work, not actual profiling.
        const THRESHOLD: u64 = 4;

        if size == 0 {
            return;
        }

        let access_size = greatest_divisible_power_of_two(size);
        assert!(
            access_size.is_power_of_two(),
            "`size` is not a power of two"
        );
        assert!(
            access_size >= u64::from(::core::cmp::min(src_align, dest_align)),
            "`size` is smaller than `dest` and `src`'s alignment value."
        );

        let (access_size, int_type) = if access_size <= 8 {
            (access_size, Type::int((access_size * 8) as u16).unwrap())
        } else {
            (8, types::I64)
        };

        let load_and_store_amount = size / access_size;

        if load_and_store_amount > THRESHOLD {
            let size_value = self.ins().iconst(config.pointer_type(), size as i64);
            if non_overlapping {
                self.call_memcpy(config, dest, src, size_value);
            } else {
                self.call_memmove(config, dest, src, size_value);
            }
            return;
        }

        if u64::from(src_align) >= access_size && u64::from(dest_align) >= access_size {
            flags.set_aligned();
        }

        // Load all of the memory first. This is necessary in case `dest` overlaps.
        // It can also improve performance a bit.
        let registers: smallvec::SmallVec<[_; THRESHOLD as usize]> = (0..load_and_store_amount)
            .map(|i| {
                let offset = (access_size * i) as i32;
                (self.ins().load(int_type, flags, src, offset), offset)
            })
            .collect();

        for (value, offset) in registers {
            self.ins().store(flags, value, dest, offset);
        }
    }

    /// Calls libc.memset
    ///
    /// Writes `size` bytes of i8 value `ch` to memory starting at `buffer`.
    pub fn call_memset(
        &mut self,
        config: TargetFrontendConfig,
        buffer: Value,
        ch: Value,
        size: Value,
    ) {
        let pointer_type = config.pointer_type();
        let signature = {
            let mut s = Signature::new(config.default_call_conv);
            s.params.push(AbiParam::new(pointer_type));
            s.params.push(AbiParam::new(types::I32));
            s.params.push(AbiParam::new(pointer_type));
            self.import_signature(s)
        };

        let libc_memset = self.import_function(ExtFuncData {
            name: ExternalName::LibCall(LibCall::Memset),
            signature,
            colocated: false,
        });

        let ch = self.ins().uextend(types::I32, ch);
        self.ins().call(libc_memset, &[buffer, ch, size]);
    }

    /// Calls libc.memset
    ///
    /// Writes `size` bytes of value `ch` to memory starting at `buffer`.
    pub fn emit_small_memset(
        &mut self,
        config: TargetFrontendConfig,
        buffer: Value,
        ch: u8,
        size: u64,
        buffer_align: u8,
        mut flags: MemFlags,
    ) {
        // Currently the result of guess work, not actual profiling.
        const THRESHOLD: u64 = 4;

        if size == 0 {
            return;
        }

        let access_size = greatest_divisible_power_of_two(size);
        assert!(
            access_size.is_power_of_two(),
            "`size` is not a power of two"
        );
        assert!(
            access_size >= u64::from(buffer_align),
            "`size` is smaller than `dest` and `src`'s alignment value."
        );

        let (access_size, int_type) = if access_size <= 8 {
            (access_size, Type::int((access_size * 8) as u16).unwrap())
        } else {
            (8, types::I64)
        };

        let load_and_store_amount = size / access_size;

        if load_and_store_amount > THRESHOLD {
            let ch = self.ins().iconst(types::I8, i64::from(ch));
            let size = self.ins().iconst(config.pointer_type(), size as i64);
            self.call_memset(config, buffer, ch, size);
        } else {
            if u64::from(buffer_align) >= access_size {
                flags.set_aligned();
            }

            let ch = u64::from(ch);
            let raw_value = if int_type == types::I64 {
                ch * 0x0101010101010101_u64
            } else if int_type == types::I32 {
                ch * 0x01010101_u64
            } else if int_type == types::I16 {
                (ch << 8) | ch
            } else {
                assert_eq!(int_type, types::I8);
                ch
            };

            let value = self.ins().iconst(int_type, raw_value as i64);
            for i in 0..load_and_store_amount {
                let offset = (access_size * i) as i32;
                self.ins().store(flags, value, buffer, offset);
            }
        }
    }

    /// Calls libc.memmove
    ///
    /// Copies `size` bytes from memory starting at `source` to memory starting
    /// at `dest`. `source` is always read before writing to `dest`.
    pub fn call_memmove(
        &mut self,
        config: TargetFrontendConfig,
        dest: Value,
        source: Value,
        size: Value,
    ) {
        let pointer_type = config.pointer_type();
        let signature = {
            let mut s = Signature::new(config.default_call_conv);
            s.params.push(AbiParam::new(pointer_type));
            s.params.push(AbiParam::new(pointer_type));
            s.params.push(AbiParam::new(pointer_type));
            self.import_signature(s)
        };

        let libc_memmove = self.import_function(ExtFuncData {
            name: ExternalName::LibCall(LibCall::Memmove),
            signature,
            colocated: false,
        });

        self.ins().call(libc_memmove, &[dest, source, size]);
    }

    /// Calls libc.memcmp
    ///
    /// Compares `size` bytes from memory starting at `left` to memory starting
    /// at `right`. Returns `0` if all `n` bytes are equal.  If the first difference
    /// is at offset `i`, returns a positive integer if `ugt(left[i], right[i])`
    /// and a negative integer if `ult(left[i], right[i])`.
    ///
    /// Returns a C `int`, which is currently always [`types::I32`].
    pub fn call_memcmp(
        &mut self,
        config: TargetFrontendConfig,
        left: Value,
        right: Value,
        size: Value,
    ) -> Value {
        let pointer_type = config.pointer_type();
        let signature = {
            let mut s = Signature::new(config.default_call_conv);
            s.params.reserve(3);
            s.params.push(AbiParam::new(pointer_type));
            s.params.push(AbiParam::new(pointer_type));
            s.params.push(AbiParam::new(pointer_type));
            s.returns.push(AbiParam::new(types::I32));
            self.import_signature(s)
        };

        let libc_memcmp = self.import_function(ExtFuncData {
            name: ExternalName::LibCall(LibCall::Memcmp),
            signature,
            colocated: false,
        });

        let call = self.ins().call(libc_memcmp, &[left, right, size]);
        self.func.dfg.first_result(call)
    }

    /// Optimised [`Self::call_memcmp`] for small copies.
    ///
    /// This implements the byte slice comparison `int_cc(left[..size], right[..size])`.
    ///
    /// `left_align` and `right_align` are the statically-known alignments of the
    /// `left` and `right` pointers respectively.  These are used to know whether
    /// to mark `load`s as aligned.  It's always fine to pass `1` for these, but
    /// passing something higher than the true alignment may trap or otherwise
    /// misbehave as described in [`MemFlags::aligned`].
    ///
    /// Note that `memcmp` is a *big-endian* and *unsigned* comparison.
    /// As such, this panics when called with `IntCC::Signed*` or `IntCC::*Overflow`.
    pub fn emit_small_memory_compare(
        &mut self,
        config: TargetFrontendConfig,
        int_cc: IntCC,
        left: Value,
        right: Value,
        size: u64,
        left_align: std::num::NonZeroU8,
        right_align: std::num::NonZeroU8,
        flags: MemFlags,
    ) -> Value {
        use IntCC::*;
        let (zero_cc, empty_imm) = match int_cc {
            //
            Equal => (Equal, true),
            NotEqual => (NotEqual, false),

            UnsignedLessThan => (SignedLessThan, false),
            UnsignedGreaterThanOrEqual => (SignedGreaterThanOrEqual, true),
            UnsignedGreaterThan => (SignedGreaterThan, false),
            UnsignedLessThanOrEqual => (SignedLessThanOrEqual, true),

            SignedLessThan
            | SignedGreaterThanOrEqual
            | SignedGreaterThan
            | SignedLessThanOrEqual => {
                panic!("Signed comparison {} not supported by memcmp", int_cc)
            }
            Overflow | NotOverflow => {
                panic!("Overflow comparison {} not supported by memcmp", int_cc)
            }
        };

        if size == 0 {
            return self.ins().bconst(types::B1, empty_imm);
        }

        // Future work could consider expanding this to handle more-complex scenarios.
        if let Some(small_type) = size.try_into().ok().and_then(Type::int_with_byte_size) {
            if let Equal | NotEqual = zero_cc {
                let mut left_flags = flags;
                if size == left_align.get() as u64 {
                    left_flags.set_aligned();
                }
                let mut right_flags = flags;
                if size == right_align.get() as u64 {
                    right_flags.set_aligned();
                }
                let left_val = self.ins().load(small_type, left_flags, left, 0);
                let right_val = self.ins().load(small_type, right_flags, right, 0);
                return self.ins().icmp(int_cc, left_val, right_val);
            } else if small_type == types::I8 {
                // Once the big-endian loads from wasmtime#2492 are implemented in
                // the backends, we could easily handle comparisons for more sizes here.
                // But for now, just handle single bytes where we don't need to worry.

                let mut aligned_flags = flags;
                aligned_flags.set_aligned();
                let left_val = self.ins().load(small_type, aligned_flags, left, 0);
                let right_val = self.ins().load(small_type, aligned_flags, right, 0);
                return self.ins().icmp(int_cc, left_val, right_val);
            }
        }

        let pointer_type = config.pointer_type();
        let size = self.ins().iconst(pointer_type, size as i64);
        let cmp = self.call_memcmp(config, left, right, size);
        self.ins().icmp_imm(zero_cc, cmp, 0)
    }
}

fn greatest_divisible_power_of_two(size: u64) -> u64 {
    (size as i64 & -(size as i64)) as u64
}

// Helper functions
impl<'a> FunctionBuilder<'a> {
    /// A Block is 'filled' when a terminator instruction is present.
    fn fill_current_block(&mut self) {
        self.func_ctx.blocks[self.position.unwrap()].filled = true;
    }

    fn declare_successor(&mut self, dest_block: Block, jump_inst: Inst) {
        self.func_ctx
            .ssa
            .declare_block_predecessor(dest_block, self.position.unwrap(), jump_inst);
    }

    fn handle_ssa_side_effects(&mut self, side_effects: SideEffects) {
        for split_block in side_effects.split_blocks_created {
            self.func_ctx.blocks[split_block].filled = true
        }
        for modified_block in side_effects.instructions_added_to_blocks {
            self.func_ctx.blocks[modified_block].pristine = false
        }
    }
}

#[cfg(test)]
mod tests {
    use super::greatest_divisible_power_of_two;
    use crate::frontend::{
        DeclareVariableError, DefVariableError, FunctionBuilder, FunctionBuilderContext,
        UseVariableError,
    };
    use crate::Variable;
    use alloc::string::ToString;
    use cranelift_codegen::entity::EntityRef;
    use cranelift_codegen::ir::condcodes::IntCC;
    use cranelift_codegen::ir::{types::*, UserFuncName};
    use cranelift_codegen::ir::{AbiParam, Function, InstBuilder, MemFlags, Signature, Value};
    use cranelift_codegen::isa::{CallConv, TargetFrontendConfig, TargetIsa};
    use cranelift_codegen::settings;
    use cranelift_codegen::verifier::verify_function;
    use target_lexicon::PointerWidth;

    fn sample_function(lazy_seal: bool) {
        let mut sig = Signature::new(CallConv::SystemV);
        sig.returns.push(AbiParam::new(I32));
        sig.params.push(AbiParam::new(I32));

        let mut fn_ctx = FunctionBuilderContext::new();
        let mut func = Function::with_name_signature(UserFuncName::testcase("sample"), sig);
        {
            let mut builder = FunctionBuilder::new(&mut func, &mut fn_ctx);

            let block0 = builder.create_block();
            let block1 = builder.create_block();
            let block2 = builder.create_block();
            let block3 = builder.create_block();
            let x = Variable::new(0);
            let y = Variable::new(1);
            let z = Variable::new(2);
            builder.declare_var(x, I32);
            builder.declare_var(y, I32);
            builder.declare_var(z, I32);
            builder.append_block_params_for_function_params(block0);

            builder.switch_to_block(block0);
            if !lazy_seal {
                builder.seal_block(block0);
            }
            {
                let tmp = builder.block_params(block0)[0]; // the first function parameter
                builder.def_var(x, tmp);
            }
            {
                let tmp = builder.ins().iconst(I32, 2);
                builder.def_var(y, tmp);
            }
            {
                let arg1 = builder.use_var(x);
                let arg2 = builder.use_var(y);
                let tmp = builder.ins().iadd(arg1, arg2);
                builder.def_var(z, tmp);
            }
            builder.ins().jump(block1, &[]);

            builder.switch_to_block(block1);
            {
                let arg1 = builder.use_var(y);
                let arg2 = builder.use_var(z);
                let tmp = builder.ins().iadd(arg1, arg2);
                builder.def_var(z, tmp);
            }
            {
                let arg = builder.use_var(y);
                builder.ins().brnz(arg, block3, &[]);
            }
            builder.ins().jump(block2, &[]);

            builder.switch_to_block(block2);
            if !lazy_seal {
                builder.seal_block(block2);
            }
            {
                let arg1 = builder.use_var(z);
                let arg2 = builder.use_var(x);
                let tmp = builder.ins().isub(arg1, arg2);
                builder.def_var(z, tmp);
            }
            {
                let arg = builder.use_var(y);
                builder.ins().return_(&[arg]);
            }

            builder.switch_to_block(block3);
            if !lazy_seal {
                builder.seal_block(block3);
            }

            {
                let arg1 = builder.use_var(y);
                let arg2 = builder.use_var(x);
                let tmp = builder.ins().isub(arg1, arg2);
                builder.def_var(y, tmp);
            }
            builder.ins().jump(block1, &[]);
            if !lazy_seal {
                builder.seal_block(block1);
            }

            if lazy_seal {
                builder.seal_all_blocks();
            }

            builder.finalize();
        }

        let flags = settings::Flags::new(settings::builder());
        // println!("{}", func.display(None));
        if let Err(errors) = verify_function(&func, &flags) {
            panic!("{}\n{}", func.display(), errors)
        }
    }

    #[test]
    fn sample() {
        sample_function(false)
    }

    #[test]
    fn sample_with_lazy_seal() {
        sample_function(true)
    }

    #[track_caller]
    fn check(func: &Function, expected_ir: &str) {
        let actual_ir = func.display().to_string();
        assert!(
            expected_ir == actual_ir,
            "Expected:\n{}\nGot:\n{}",
            expected_ir,
            actual_ir
        );
    }

    /// Helper function to construct a fixed frontend configuration.
    fn systemv_frontend_config() -> TargetFrontendConfig {
        TargetFrontendConfig {
            default_call_conv: CallConv::SystemV,
            pointer_width: PointerWidth::U64,
        }
    }

    #[test]
    fn memcpy() {
        let frontend_config = systemv_frontend_config();
        let mut sig = Signature::new(frontend_config.default_call_conv);
        sig.returns.push(AbiParam::new(I32));

        let mut fn_ctx = FunctionBuilderContext::new();
        let mut func = Function::with_name_signature(UserFuncName::testcase("sample"), sig);
        {
            let mut builder = FunctionBuilder::new(&mut func, &mut fn_ctx);

            let block0 = builder.create_block();
            let x = Variable::new(0);
            let y = Variable::new(1);
            let z = Variable::new(2);
            builder.declare_var(x, frontend_config.pointer_type());
            builder.declare_var(y, frontend_config.pointer_type());
            builder.declare_var(z, I32);
            builder.append_block_params_for_function_params(block0);
            builder.switch_to_block(block0);

            let src = builder.use_var(x);
            let dest = builder.use_var(y);
            let size = builder.use_var(y);
            builder.call_memcpy(frontend_config, dest, src, size);
            builder.ins().return_(&[size]);

            builder.seal_all_blocks();
            builder.finalize();
        }

        check(
            &func,
            "function %sample() -> i32 system_v {
    sig0 = (i64, i64, i64) system_v
    fn0 = %Memcpy sig0

block0:
    v3 = iconst.i64 0
    v1 -> v3
    v2 = iconst.i64 0
    v0 -> v2
    call fn0(v1, v0, v1)  ; v1 = 0, v0 = 0, v1 = 0
    return v1  ; v1 = 0
}
",
        );
    }

    #[test]
    fn small_memcpy() {
        let frontend_config = systemv_frontend_config();
        let mut sig = Signature::new(frontend_config.default_call_conv);
        sig.returns.push(AbiParam::new(I32));

        let mut fn_ctx = FunctionBuilderContext::new();
        let mut func = Function::with_name_signature(UserFuncName::testcase("sample"), sig);
        {
            let mut builder = FunctionBuilder::new(&mut func, &mut fn_ctx);

            let block0 = builder.create_block();
            let x = Variable::new(0);
            let y = Variable::new(16);
            builder.declare_var(x, frontend_config.pointer_type());
            builder.declare_var(y, frontend_config.pointer_type());
            builder.append_block_params_for_function_params(block0);
            builder.switch_to_block(block0);

            let src = builder.use_var(x);
            let dest = builder.use_var(y);
            let size = 8;
            builder.emit_small_memory_copy(
                frontend_config,
                dest,
                src,
                size,
                8,
                8,
                true,
                MemFlags::new(),
            );
            builder.ins().return_(&[dest]);

            builder.seal_all_blocks();
            builder.finalize();
        }

        check(
            &func,
            "function %sample() -> i32 system_v {
block0:
    v4 = iconst.i64 0
    v1 -> v4
    v3 = iconst.i64 0
    v0 -> v3
    v2 = load.i64 aligned v0  ; v0 = 0
    store aligned v2, v1  ; v1 = 0
    return v1  ; v1 = 0
}
",
        );
    }

    #[test]
    fn not_so_small_memcpy() {
        let frontend_config = systemv_frontend_config();
        let mut sig = Signature::new(frontend_config.default_call_conv);
        sig.returns.push(AbiParam::new(I32));

        let mut fn_ctx = FunctionBuilderContext::new();
        let mut func = Function::with_name_signature(UserFuncName::testcase("sample"), sig);
        {
            let mut builder = FunctionBuilder::new(&mut func, &mut fn_ctx);

            let block0 = builder.create_block();
            let x = Variable::new(0);
            let y = Variable::new(16);
            builder.declare_var(x, frontend_config.pointer_type());
            builder.declare_var(y, frontend_config.pointer_type());
            builder.append_block_params_for_function_params(block0);
            builder.switch_to_block(block0);

            let src = builder.use_var(x);
            let dest = builder.use_var(y);
            let size = 8192;
            builder.emit_small_memory_copy(
                frontend_config,
                dest,
                src,
                size,
                8,
                8,
                true,
                MemFlags::new(),
            );
            builder.ins().return_(&[dest]);

            builder.seal_all_blocks();
            builder.finalize();
        }

        check(
            &func,
            "function %sample() -> i32 system_v {
    sig0 = (i64, i64, i64) system_v
    fn0 = %Memcpy sig0

block0:
    v4 = iconst.i64 0
    v1 -> v4
    v3 = iconst.i64 0
    v0 -> v3
    v2 = iconst.i64 8192
    call fn0(v1, v0, v2)  ; v1 = 0, v0 = 0, v2 = 8192
    return v1  ; v1 = 0
}
",
        );
    }

    #[test]
    fn small_memset() {
        let frontend_config = systemv_frontend_config();
        let mut sig = Signature::new(frontend_config.default_call_conv);
        sig.returns.push(AbiParam::new(I32));

        let mut fn_ctx = FunctionBuilderContext::new();
        let mut func = Function::with_name_signature(UserFuncName::testcase("sample"), sig);
        {
            let mut builder = FunctionBuilder::new(&mut func, &mut fn_ctx);

            let block0 = builder.create_block();
            let y = Variable::new(16);
            builder.declare_var(y, frontend_config.pointer_type());
            builder.append_block_params_for_function_params(block0);
            builder.switch_to_block(block0);

            let dest = builder.use_var(y);
            let size = 8;
            builder.emit_small_memset(frontend_config, dest, 1, size, 8, MemFlags::new());
            builder.ins().return_(&[dest]);

            builder.seal_all_blocks();
            builder.finalize();
        }

        check(
            &func,
            "function %sample() -> i32 system_v {
block0:
    v2 = iconst.i64 0
    v0 -> v2
    v1 = iconst.i64 0x0101_0101_0101_0101
    store aligned v1, v0  ; v1 = 0x0101_0101_0101_0101, v0 = 0
    return v0  ; v0 = 0
}
",
        );
    }

    #[test]
    fn not_so_small_memset() {
        let frontend_config = systemv_frontend_config();
        let mut sig = Signature::new(frontend_config.default_call_conv);
        sig.returns.push(AbiParam::new(I32));

        let mut fn_ctx = FunctionBuilderContext::new();
        let mut func = Function::with_name_signature(UserFuncName::testcase("sample"), sig);
        {
            let mut builder = FunctionBuilder::new(&mut func, &mut fn_ctx);

            let block0 = builder.create_block();
            let y = Variable::new(16);
            builder.declare_var(y, frontend_config.pointer_type());
            builder.append_block_params_for_function_params(block0);
            builder.switch_to_block(block0);

            let dest = builder.use_var(y);
            let size = 8192;
            builder.emit_small_memset(frontend_config, dest, 1, size, 8, MemFlags::new());
            builder.ins().return_(&[dest]);

            builder.seal_all_blocks();
            builder.finalize();
        }

        check(
            &func,
            "function %sample() -> i32 system_v {
    sig0 = (i64, i32, i64) system_v
    fn0 = %Memset sig0

block0:
    v4 = iconst.i64 0
    v0 -> v4
    v1 = iconst.i8 1
    v2 = iconst.i64 8192
    v3 = uextend.i32 v1  ; v1 = 1
    call fn0(v0, v3, v2)  ; v0 = 0, v2 = 8192
    return v0  ; v0 = 0
}
",
        );
    }

    #[test]
    fn memcmp() {
        use core::str::FromStr;
        use cranelift_codegen::isa;

        let shared_builder = settings::builder();
        let shared_flags = settings::Flags::new(shared_builder);

        let triple =
            ::target_lexicon::Triple::from_str("x86_64").expect("Couldn't create x86_64 triple");

        let target = isa::lookup(triple)
            .ok()
            .map(|b| b.finish(shared_flags))
            .expect("This test requires x86_64 support.")
            .expect("Should be able to create backend with default flags");

        let mut sig = Signature::new(target.default_call_conv());
        sig.returns.push(AbiParam::new(I32));

        let mut fn_ctx = FunctionBuilderContext::new();
        let mut func = Function::with_name_signature(UserFuncName::testcase("sample"), sig);
        {
            let mut builder = FunctionBuilder::new(&mut func, &mut fn_ctx);

            let block0 = builder.create_block();
            let x = Variable::new(0);
            let y = Variable::new(1);
            let z = Variable::new(2);
            builder.declare_var(x, target.pointer_type());
            builder.declare_var(y, target.pointer_type());
            builder.declare_var(z, target.pointer_type());
            builder.append_block_params_for_function_params(block0);
            builder.switch_to_block(block0);

            let left = builder.use_var(x);
            let right = builder.use_var(y);
            let size = builder.use_var(z);
            let cmp = builder.call_memcmp(target.frontend_config(), left, right, size);
            builder.ins().return_(&[cmp]);

            builder.seal_all_blocks();
            builder.finalize();
        }

        check(
            &func,
            "function %sample() -> i32 system_v {
    sig0 = (i64, i64, i64) -> i32 system_v
    fn0 = %Memcmp sig0

block0:
    v6 = iconst.i64 0
    v2 -> v6
    v5 = iconst.i64 0
    v1 -> v5
    v4 = iconst.i64 0
    v0 -> v4
    v3 = call fn0(v0, v1, v2)  ; v0 = 0, v1 = 0, v2 = 0
    return v3
}
",
        );
    }

    #[test]
    fn small_memcmp_zero_size() {
        let align_eight = std::num::NonZeroU8::new(8).unwrap();
        small_memcmp_helper(
            "
block0:
    v4 = iconst.i64 0
    v1 -> v4
    v3 = iconst.i64 0
    v0 -> v3
    v2 = bconst.b1 true
    return v2  ; v2 = true",
            |builder, target, x, y| {
                builder.emit_small_memory_compare(
                    target.frontend_config(),
                    IntCC::UnsignedGreaterThanOrEqual,
                    x,
                    y,
                    0,
                    align_eight,
                    align_eight,
                    MemFlags::new(),
                )
            },
        );
    }

    #[test]
    fn small_memcmp_byte_ugt() {
        let align_one = std::num::NonZeroU8::new(1).unwrap();
        small_memcmp_helper(
            "
block0:
    v6 = iconst.i64 0
    v1 -> v6
    v5 = iconst.i64 0
    v0 -> v5
    v2 = load.i8 aligned v0  ; v0 = 0
    v3 = load.i8 aligned v1  ; v1 = 0
    v4 = icmp ugt v2, v3
    return v4",
            |builder, target, x, y| {
                builder.emit_small_memory_compare(
                    target.frontend_config(),
                    IntCC::UnsignedGreaterThan,
                    x,
                    y,
                    1,
                    align_one,
                    align_one,
                    MemFlags::new(),
                )
            },
        );
    }

    #[test]
    fn small_memcmp_aligned_eq() {
        let align_four = std::num::NonZeroU8::new(4).unwrap();
        small_memcmp_helper(
            "
block0:
    v6 = iconst.i64 0
    v1 -> v6
    v5 = iconst.i64 0
    v0 -> v5
    v2 = load.i32 aligned v0  ; v0 = 0
    v3 = load.i32 aligned v1  ; v1 = 0
    v4 = icmp eq v2, v3
    return v4",
            |builder, target, x, y| {
                builder.emit_small_memory_compare(
                    target.frontend_config(),
                    IntCC::Equal,
                    x,
                    y,
                    4,
                    align_four,
                    align_four,
                    MemFlags::new(),
                )
            },
        );
    }

    #[test]
    fn small_memcmp_ipv6_ne() {
        let align_two = std::num::NonZeroU8::new(2).unwrap();
        small_memcmp_helper(
            "
block0:
    v6 = iconst.i64 0
    v1 -> v6
    v5 = iconst.i64 0
    v0 -> v5
    v2 = load.i128 v0  ; v0 = 0
    v3 = load.i128 v1  ; v1 = 0
    v4 = icmp ne v2, v3
    return v4",
            |builder, target, x, y| {
                builder.emit_small_memory_compare(
                    target.frontend_config(),
                    IntCC::NotEqual,
                    x,
                    y,
                    16,
                    align_two,
                    align_two,
                    MemFlags::new(),
                )
            },
        );
    }

    #[test]
    fn small_memcmp_odd_size_uge() {
        let one = std::num::NonZeroU8::new(1).unwrap();
        small_memcmp_helper(
            "
    sig0 = (i64, i64, i64) -> i32 system_v
    fn0 = %Memcmp sig0

block0:
    v6 = iconst.i64 0
    v1 -> v6
    v5 = iconst.i64 0
    v0 -> v5
    v2 = iconst.i64 3
    v3 = call fn0(v0, v1, v2)  ; v0 = 0, v1 = 0, v2 = 3
    v4 = icmp_imm sge v3, 0
    return v4",
            |builder, target, x, y| {
                builder.emit_small_memory_compare(
                    target.frontend_config(),
                    IntCC::UnsignedGreaterThanOrEqual,
                    x,
                    y,
                    3,
                    one,
                    one,
                    MemFlags::new(),
                )
            },
        );
    }

    fn small_memcmp_helper(
        expected: &str,
        f: impl FnOnce(&mut FunctionBuilder, &dyn TargetIsa, Value, Value) -> Value,
    ) {
        use core::str::FromStr;
        use cranelift_codegen::isa;

        let shared_builder = settings::builder();
        let shared_flags = settings::Flags::new(shared_builder);

        let triple =
            ::target_lexicon::Triple::from_str("x86_64").expect("Couldn't create x86_64 triple");

        let target = isa::lookup(triple)
            .ok()
            .map(|b| b.finish(shared_flags))
            .expect("This test requires x86_64 support.")
            .expect("Should be able to create backend with default flags");

        let mut sig = Signature::new(target.default_call_conv());
        sig.returns.push(AbiParam::new(B1));

        let mut fn_ctx = FunctionBuilderContext::new();
        let mut func = Function::with_name_signature(UserFuncName::testcase("sample"), sig);
        {
            let mut builder = FunctionBuilder::new(&mut func, &mut fn_ctx);

            let block0 = builder.create_block();
            let x = Variable::new(0);
            let y = Variable::new(1);
            builder.declare_var(x, target.pointer_type());
            builder.declare_var(y, target.pointer_type());
            builder.append_block_params_for_function_params(block0);
            builder.switch_to_block(block0);

            let left = builder.use_var(x);
            let right = builder.use_var(y);
            let ret = f(&mut builder, &*target, left, right);
            builder.ins().return_(&[ret]);

            builder.seal_all_blocks();
            builder.finalize();
        }

        check(
            &func,
            &format!("function %sample() -> b1 system_v {{{}\n}}\n", expected),
        );
    }

    #[test]
    fn undef_vector_vars() {
        let mut sig = Signature::new(CallConv::SystemV);
        sig.returns.push(AbiParam::new(I8X16));
        sig.returns.push(AbiParam::new(B8X16));
        sig.returns.push(AbiParam::new(F32X4));

        let mut fn_ctx = FunctionBuilderContext::new();
        let mut func = Function::with_name_signature(UserFuncName::testcase("sample"), sig);
        {
            let mut builder = FunctionBuilder::new(&mut func, &mut fn_ctx);

            let block0 = builder.create_block();
            let a = Variable::new(0);
            let b = Variable::new(1);
            let c = Variable::new(2);
            builder.declare_var(a, I8X16);
            builder.declare_var(b, B8X16);
            builder.declare_var(c, F32X4);
            builder.switch_to_block(block0);

            let a = builder.use_var(a);
            let b = builder.use_var(b);
            let c = builder.use_var(c);
            builder.ins().return_(&[a, b, c]);

            builder.seal_all_blocks();
            builder.finalize();
        }

        check(
            &func,
            "function %sample() -> i8x16, b8x16, f32x4 system_v {
    const0 = 0x00000000000000000000000000000000

block0:
    v5 = f32const 0.0
    v6 = splat.f32x4 v5  ; v5 = 0.0
    v2 -> v6
    v4 = vconst.b8x16 const0
    v1 -> v4
    v3 = vconst.i8x16 const0
    v0 -> v3
    return v0, v1, v2  ; v0 = const0, v1 = const0
}
",
        );
    }

    #[test]
    fn test_greatest_divisible_power_of_two() {
        assert_eq!(64, greatest_divisible_power_of_two(64));
        assert_eq!(16, greatest_divisible_power_of_two(48));
        assert_eq!(8, greatest_divisible_power_of_two(24));
        assert_eq!(1, greatest_divisible_power_of_two(25));
    }

    #[test]
    fn try_use_var() {
        let sig = Signature::new(CallConv::SystemV);

        let mut fn_ctx = FunctionBuilderContext::new();
        let mut func = Function::with_name_signature(UserFuncName::testcase("sample"), sig);
        {
            let mut builder = FunctionBuilder::new(&mut func, &mut fn_ctx);

            let block0 = builder.create_block();
            builder.append_block_params_for_function_params(block0);
            builder.switch_to_block(block0);

            assert_eq!(
                builder.try_use_var(Variable::with_u32(0)),
                Err(UseVariableError::UsedBeforeDeclared(Variable::with_u32(0)))
            );

            let value = builder.ins().iconst(cranelift_codegen::ir::types::I32, 0);

            assert_eq!(
                builder.try_def_var(Variable::with_u32(0), value),
                Err(DefVariableError::DefinedBeforeDeclared(Variable::with_u32(
                    0
                )))
            );

            builder.declare_var(Variable::with_u32(0), cranelift_codegen::ir::types::I32);
            assert_eq!(
                builder.try_declare_var(Variable::with_u32(0), cranelift_codegen::ir::types::I32),
                Err(DeclareVariableError::DeclaredMultipleTimes(
                    Variable::with_u32(0)
                ))
            );
        }
    }
}