const std = @import("std");
const builtin = @import("builtin");
const Compilation = @import("compilation.zig").Compilation;
const Scope = @import("scope.zig").Scope;
const ast = std.zig.ast;
const Allocator = std.mem.Allocator;
const Value = @import("value.zig").Value;
const Type = Value.Type;
const assert = std.debug.assert;
const Token = std.zig.Token;
const ParsedFile = @import("parsed_file.zig").ParsedFile;
const Span = @import("errmsg.zig").Span;

pub const LVal = enum {
    None,
    Ptr,
};

pub const IrVal = union(enum) {
    Unknown,
    KnownType: *Type,
    KnownValue: *Value,

    const Init = enum {
        Unknown,
        NoReturn,
        Void,
    };

    pub fn dump(self: IrVal) void {
        switch (self) {
            IrVal.Unknown => typeof.dump(),
            IrVal.KnownType => |typeof| {
                std.debug.warn("KnownType(");
                typeof.dump();
                std.debug.warn(")");
            },
            IrVal.KnownValue => |value| {
                std.debug.warn("KnownValue(");
                value.dump();
                std.debug.warn(")");
            },
        }
    }
};

pub const Instruction = struct {
    id: Id,
    scope: *Scope,
    debug_id: usize,
    val: IrVal,
    ref_count: usize,
    span: Span,

    /// true if this instruction was generated by zig and not from user code
    is_generated: bool,

    /// the instruction that is derived from this one in analysis
    child: ?*Instruction,

    /// the instruction that this one derives from in analysis
    parent: ?*Instruction,

    pub fn cast(base: *Instruction, comptime T: type) ?*T {
        if (base.id == comptime typeToId(T)) {
            return @fieldParentPtr(T, "base", base);
        }
        return null;
    }

    pub fn typeToId(comptime T: type) Id {
        comptime var i = 0;
        inline while (i < @memberCount(Id)) : (i += 1) {
            if (T == @field(Instruction, @memberName(Id, i))) {
                return @field(Id, @memberName(Id, i));
            }
        }
        unreachable;
    }

    pub fn dump(base: *const Instruction) void {
        comptime var i = 0;
        inline while (i < @memberCount(Id)) : (i += 1) {
            if (base.id == @field(Id, @memberName(Id, i))) {
                const T = @field(Instruction, @memberName(Id, i));
                std.debug.warn("#{} = {}(", base.debug_id, @tagName(base.id));
                @fieldParentPtr(T, "base", base).dump();
                std.debug.warn(")");
                return;
            }
        }
        unreachable;
    }

    pub fn hasSideEffects(base: *const Instruction) bool {
        comptime var i = 0;
        inline while (i < @memberCount(Id)) : (i += 1) {
            if (base.id == @field(Id, @memberName(Id, i))) {
                const T = @field(Instruction, @memberName(Id, i));
                return @fieldParentPtr(T, "base", base).hasSideEffects();
            }
        }
        unreachable;
    }

    pub fn analyze(base: *Instruction, ira: *Analyze) Analyze.Error!*Instruction {
        comptime var i = 0;
        inline while (i < @memberCount(Id)) : (i += 1) {
            if (base.id == @field(Id, @memberName(Id, i))) {
                const T = @field(Instruction, @memberName(Id, i));
                const new_inst = try @fieldParentPtr(T, "base", base).analyze(ira);
                new_inst.linkToParent(base);
                return new_inst;
            }
        }
        unreachable;
    }

    fn getAsParam(param: *Instruction) !*Instruction {
        const child = param.child orelse return error.SemanticAnalysisFailed;
        switch (child.val) {
            IrVal.Unknown => return error.SemanticAnalysisFailed,
            else => return child,
        }
    }

    /// asserts that the type is known
    fn getKnownType(self: *Instruction) *Type {
        switch (self.val) {
            IrVal.KnownType => |typeof| return typeof,
            IrVal.KnownValue => |value| return value.typeof,
            IrVal.Unknown => unreachable,
        }
    }

    pub fn setGenerated(base: *Instruction) void {
        base.is_generated = true;
    }

    pub fn isNoReturn(base: *const Instruction) bool {
        switch (base.val) {
            IrVal.Unknown => return false,
            IrVal.KnownValue => |x| return x.typeof.id == Type.Id.NoReturn,
            IrVal.KnownType => |typeof| return typeof.id == Type.Id.NoReturn,
        }
    }

    pub fn linkToParent(self: *Instruction, parent: *Instruction) void {
        assert(self.parent == null);
        assert(parent.child == null);
        self.parent = parent;
        parent.child = self;
    }

    pub const Id = enum {
        Return,
        Const,
        Ref,
        DeclVar,
        CheckVoidStmt,
        Phi,
        Br,
        AddImplicitReturnType,
    };

    pub const Const = struct {
        base: Instruction,
        params: Params,

        const Params = struct {};

        // Use Builder.buildConst* methods, or, after building a Const instruction,
        // manually set the ir_val field.
        const ir_val_init = IrVal.Init.Unknown;

        pub fn dump(self: *const Const) void {
            self.base.val.KnownValue.dump();
        }

        pub fn hasSideEffects(self: *const Const) bool {
            return false;
        }

        pub fn analyze(self: *const Const, ira: *Analyze) !*Instruction {
            const new_inst = try ira.irb.build(Const, self.base.scope, self.base.span, Params{});
            new_inst.val = IrVal{ .KnownValue = self.base.val.KnownValue.getRef() };
            return new_inst;
        }
    };

    pub const Return = struct {
        base: Instruction,
        params: Params,

        const Params = struct {
            return_value: *Instruction,
        };

        const ir_val_init = IrVal.Init.NoReturn;

        pub fn dump(self: *const Return) void {
            std.debug.warn("#{}", self.params.return_value.debug_id);
        }

        pub fn hasSideEffects(self: *const Return) bool {
            return true;
        }

        pub fn analyze(self: *const Return, ira: *Analyze) !*Instruction {
            const value = try self.params.return_value.getAsParam();
            const casted_value = try ira.implicitCast(value, ira.explicit_return_type);

            // TODO detect returning local variable address

            return ira.irb.build(Return, self.base.scope, self.base.span, Params{ .return_value = casted_value });
        }
    };

    pub const Ref = struct {
        base: Instruction,
        params: Params,

        const Params = struct {
            target: *Instruction,
            mut: Type.Pointer.Mut,
            volatility: Type.Pointer.Vol,
        };

        const ir_val_init = IrVal.Init.Unknown;

        pub fn dump(inst: *const Ref) void {}

        pub fn hasSideEffects(inst: *const Ref) bool {
            return false;
        }

        pub fn analyze(self: *const Ref, ira: *Analyze) !*Instruction {
            const target = try self.params.target.getAsParam();

            if (ira.getCompTimeValOrNullUndefOk(target)) |val| {
                return ira.getCompTimeRef(
                    val,
                    Value.Ptr.Mut.CompTimeConst,
                    self.params.mut,
                    self.params.volatility,
                    val.typeof.getAbiAlignment(ira.irb.comp),
                );
            }

            const new_inst = try ira.irb.build(Ref, self.base.scope, self.base.span, Params{
                .target = target,
                .mut = self.params.mut,
                .volatility = self.params.volatility,
            });
            const elem_type = target.getKnownType();
            const ptr_type = Type.Pointer.get(
                ira.irb.comp,
                elem_type,
                self.params.mut,
                self.params.volatility,
                Type.Pointer.Size.One,
                elem_type.getAbiAlignment(ira.irb.comp),
            );
            // TODO: potentially set the hint that this is a stack pointer. But it might not be - this
            // could be a ref of a global, for example
            new_inst.val = IrVal{ .KnownType = &ptr_type.base };
            // TODO potentially add an alloca entry here
            return new_inst;
        }
    };

    pub const DeclVar = struct {
        base: Instruction,
        params: Params,

        const Params = struct {
            variable: *Variable,
        };

        const ir_val_init = IrVal.Init.Unknown;

        pub fn dump(inst: *const DeclVar) void {}

        pub fn hasSideEffects(inst: *const DeclVar) bool {
            return true;
        }

        pub fn analyze(self: *const DeclVar, ira: *Analyze) !*Instruction {
            return error.Unimplemented; // TODO
        }
    };

    pub const CheckVoidStmt = struct {
        base: Instruction,
        params: Params,

        const Params = struct {
            target: *Instruction,
        };

        const ir_val_init = IrVal.Init.Unknown;

        pub fn dump(inst: *const CheckVoidStmt) void {}

        pub fn hasSideEffects(inst: *const CheckVoidStmt) bool {
            return true;
        }

        pub fn analyze(self: *const CheckVoidStmt, ira: *Analyze) !*Instruction {
            return error.Unimplemented; // TODO
        }
    };

    pub const Phi = struct {
        base: Instruction,
        params: Params,

        const Params = struct {
            incoming_blocks: []*BasicBlock,
            incoming_values: []*Instruction,
        };

        const ir_val_init = IrVal.Init.Unknown;

        pub fn dump(inst: *const Phi) void {}

        pub fn hasSideEffects(inst: *const Phi) bool {
            return false;
        }

        pub fn analyze(self: *const Phi, ira: *Analyze) !*Instruction {
            return error.Unimplemented; // TODO
        }
    };

    pub const Br = struct {
        base: Instruction,
        params: Params,

        const Params = struct {
            dest_block: *BasicBlock,
            is_comptime: *Instruction,
        };

        const ir_val_init = IrVal.Init.NoReturn;

        pub fn dump(inst: *const Br) void {}

        pub fn hasSideEffects(inst: *const Br) bool {
            return true;
        }

        pub fn analyze(self: *const Br, ira: *Analyze) !*Instruction {
            return error.Unimplemented; // TODO
        }
    };

    pub const AddImplicitReturnType = struct {
        base: Instruction,
        params: Params,

        pub const Params = struct {
            target: *Instruction,
        };

        const ir_val_init = IrVal.Init.Unknown;

        pub fn dump(inst: *const AddImplicitReturnType) void {
            std.debug.warn("#{}", inst.params.target.debug_id);
        }

        pub fn hasSideEffects(inst: *const AddImplicitReturnType) bool {
            return true;
        }

        pub fn analyze(self: *const AddImplicitReturnType, ira: *Analyze) !*Instruction {
            const target = try self.params.target.getAsParam();
            try ira.src_implicit_return_type_list.append(target);
            return ira.irb.buildConstVoid(self.base.scope, self.base.span, true);
        }
    };
};

pub const Variable = struct {
    child_scope: *Scope,
};

pub const BasicBlock = struct {
    ref_count: usize,
    name_hint: []const u8,
    debug_id: usize,
    scope: *Scope,
    instruction_list: std.ArrayList(*Instruction),
    ref_instruction: ?*Instruction,

    /// the basic block that is derived from this one in analysis
    child: ?*BasicBlock,

    /// the basic block that this one derives from in analysis
    parent: ?*BasicBlock,

    pub fn ref(self: *BasicBlock) void {
        self.ref_count += 1;
    }

    pub fn linkToParent(self: *BasicBlock, parent: *BasicBlock) void {
        assert(self.parent == null);
        assert(parent.child == null);
        self.parent = parent;
        parent.child = self;
    }
};

/// Stuff that survives longer than Builder
pub const Code = struct {
    basic_block_list: std.ArrayList(*BasicBlock),
    arena: std.heap.ArenaAllocator,
    return_type: ?*Type,

    /// allocator is comp.a()
    pub fn destroy(self: *Code, allocator: *Allocator) void {
        self.arena.deinit();
        allocator.destroy(self);
    }

    pub fn dump(self: *Code) void {
        var bb_i: usize = 0;
        for (self.basic_block_list.toSliceConst()) |bb| {
            std.debug.warn("{}_{}:\n", bb.name_hint, bb.debug_id);
            for (bb.instruction_list.toSliceConst()) |instr| {
                std.debug.warn("  ");
                instr.dump();
                std.debug.warn("\n");
            }
        }
    }
};

pub const Builder = struct {
    comp: *Compilation,
    code: *Code,
    current_basic_block: *BasicBlock,
    next_debug_id: usize,
    parsed_file: *ParsedFile,
    is_comptime: bool,

    pub const Error = Analyze.Error;

    pub fn init(comp: *Compilation, parsed_file: *ParsedFile) !Builder {
        const code = try comp.a().create(Code{
            .basic_block_list = undefined,
            .arena = std.heap.ArenaAllocator.init(comp.a()),
            .return_type = null,
        });
        code.basic_block_list = std.ArrayList(*BasicBlock).init(&code.arena.allocator);
        errdefer code.destroy(comp.a());

        return Builder{
            .comp = comp,
            .parsed_file = parsed_file,
            .current_basic_block = undefined,
            .code = code,
            .next_debug_id = 0,
            .is_comptime = false,
        };
    }

    pub fn abort(self: *Builder) void {
        self.code.destroy(self.comp.a());
    }

    /// Call code.destroy() when done
    pub fn finish(self: *Builder) *Code {
        return self.code;
    }

    /// No need to clean up resources thanks to the arena allocator.
    pub fn createBasicBlock(self: *Builder, scope: *Scope, name_hint: []const u8) !*BasicBlock {
        const basic_block = try self.arena().create(BasicBlock{
            .ref_count = 0,
            .name_hint = name_hint,
            .debug_id = self.next_debug_id,
            .scope = scope,
            .instruction_list = std.ArrayList(*Instruction).init(self.arena()),
            .child = null,
            .parent = null,
            .ref_instruction = null,
        });
        self.next_debug_id += 1;
        return basic_block;
    }

    pub fn setCursorAtEndAndAppendBlock(self: *Builder, basic_block: *BasicBlock) !void {
        try self.code.basic_block_list.append(basic_block);
        self.setCursorAtEnd(basic_block);
    }

    pub fn setCursorAtEnd(self: *Builder, basic_block: *BasicBlock) void {
        self.current_basic_block = basic_block;
    }

    pub fn genNode(irb: *Builder, node: *ast.Node, scope: *Scope, lval: LVal) Error!*Instruction {
        switch (node.id) {
            ast.Node.Id.Root => unreachable,
            ast.Node.Id.Use => unreachable,
            ast.Node.Id.TestDecl => unreachable,
            ast.Node.Id.VarDecl => @panic("TODO"),
            ast.Node.Id.Defer => @panic("TODO"),
            ast.Node.Id.InfixOp => @panic("TODO"),
            ast.Node.Id.PrefixOp => @panic("TODO"),
            ast.Node.Id.SuffixOp => @panic("TODO"),
            ast.Node.Id.Switch => @panic("TODO"),
            ast.Node.Id.While => @panic("TODO"),
            ast.Node.Id.For => @panic("TODO"),
            ast.Node.Id.If => @panic("TODO"),
            ast.Node.Id.ControlFlowExpression => return error.Unimplemented,
            ast.Node.Id.Suspend => @panic("TODO"),
            ast.Node.Id.VarType => @panic("TODO"),
            ast.Node.Id.ErrorType => @panic("TODO"),
            ast.Node.Id.FnProto => @panic("TODO"),
            ast.Node.Id.PromiseType => @panic("TODO"),
            ast.Node.Id.IntegerLiteral => @panic("TODO"),
            ast.Node.Id.FloatLiteral => @panic("TODO"),
            ast.Node.Id.StringLiteral => @panic("TODO"),
            ast.Node.Id.MultilineStringLiteral => @panic("TODO"),
            ast.Node.Id.CharLiteral => @panic("TODO"),
            ast.Node.Id.BoolLiteral => @panic("TODO"),
            ast.Node.Id.NullLiteral => @panic("TODO"),
            ast.Node.Id.UndefinedLiteral => @panic("TODO"),
            ast.Node.Id.ThisLiteral => @panic("TODO"),
            ast.Node.Id.Unreachable => @panic("TODO"),
            ast.Node.Id.Identifier => @panic("TODO"),
            ast.Node.Id.GroupedExpression => {
                const grouped_expr = @fieldParentPtr(ast.Node.GroupedExpression, "base", node);
                return irb.genNode(grouped_expr.expr, scope, lval);
            },
            ast.Node.Id.BuiltinCall => @panic("TODO"),
            ast.Node.Id.ErrorSetDecl => @panic("TODO"),
            ast.Node.Id.ContainerDecl => @panic("TODO"),
            ast.Node.Id.Asm => @panic("TODO"),
            ast.Node.Id.Comptime => @panic("TODO"),
            ast.Node.Id.Block => {
                const block = @fieldParentPtr(ast.Node.Block, "base", node);
                return irb.lvalWrap(scope, try irb.genBlock(block, scope), lval);
            },
            ast.Node.Id.DocComment => @panic("TODO"),
            ast.Node.Id.SwitchCase => @panic("TODO"),
            ast.Node.Id.SwitchElse => @panic("TODO"),
            ast.Node.Id.Else => @panic("TODO"),
            ast.Node.Id.Payload => @panic("TODO"),
            ast.Node.Id.PointerPayload => @panic("TODO"),
            ast.Node.Id.PointerIndexPayload => @panic("TODO"),
            ast.Node.Id.StructField => @panic("TODO"),
            ast.Node.Id.UnionTag => @panic("TODO"),
            ast.Node.Id.EnumTag => @panic("TODO"),
            ast.Node.Id.ErrorTag => @panic("TODO"),
            ast.Node.Id.AsmInput => @panic("TODO"),
            ast.Node.Id.AsmOutput => @panic("TODO"),
            ast.Node.Id.AsyncAttribute => @panic("TODO"),
            ast.Node.Id.ParamDecl => @panic("TODO"),
            ast.Node.Id.FieldInitializer => @panic("TODO"),
        }
    }

    fn isCompTime(irb: *Builder, target_scope: *Scope) bool {
        if (irb.is_comptime)
            return true;

        var scope = target_scope;
        while (true) {
            switch (scope.id) {
                Scope.Id.CompTime => return true,
                Scope.Id.FnDef => return false,
                Scope.Id.Decls => unreachable,
                Scope.Id.Block,
                Scope.Id.Defer,
                Scope.Id.DeferExpr,
                => scope = scope.parent orelse return false,
            }
        }
    }

    pub fn genBlock(irb: *Builder, block: *ast.Node.Block, parent_scope: *Scope) !*Instruction {
        const block_scope = try Scope.Block.create(irb.comp, parent_scope);

        const outer_block_scope = &block_scope.base;
        var child_scope = outer_block_scope;

        if (parent_scope.findFnDef()) |fndef_scope| {
            if (fndef_scope.fn_val.child_scope == parent_scope) {
                fndef_scope.fn_val.block_scope = block_scope;
            }
        }

        if (block.statements.len == 0) {
            // {}
            return irb.buildConstVoid(child_scope, Span.token(block.lbrace), false);
        }

        if (block.label) |label| {
            block_scope.incoming_values = std.ArrayList(*Instruction).init(irb.arena());
            block_scope.incoming_blocks = std.ArrayList(*BasicBlock).init(irb.arena());
            block_scope.end_block = try irb.createBasicBlock(parent_scope, "BlockEnd");
            block_scope.is_comptime = try irb.buildConstBool(
                parent_scope,
                Span.token(block.lbrace),
                irb.isCompTime(parent_scope),
            );
        }

        var is_continuation_unreachable = false;
        var noreturn_return_value: ?*Instruction = null;

        var stmt_it = block.statements.iterator(0);
        while (stmt_it.next()) |statement_node_ptr| {
            const statement_node = statement_node_ptr.*;

            if (statement_node.cast(ast.Node.Defer)) |defer_node| {
                // defer starts a new scope
                const defer_token = irb.parsed_file.tree.tokens.at(defer_node.defer_token);
                const kind = switch (defer_token.id) {
                    Token.Id.Keyword_defer => Scope.Defer.Kind.ScopeExit,
                    Token.Id.Keyword_errdefer => Scope.Defer.Kind.ErrorExit,
                    else => unreachable,
                };
                const defer_expr_scope = try Scope.DeferExpr.create(irb.comp, parent_scope, defer_node.expr);
                const defer_child_scope = try Scope.Defer.create(irb.comp, parent_scope, kind, defer_expr_scope);
                child_scope = &defer_child_scope.base;
                continue;
            }
            const statement_value = try irb.genNode(statement_node, child_scope, LVal.None);

            is_continuation_unreachable = statement_value.isNoReturn();
            if (is_continuation_unreachable) {
                // keep the last noreturn statement value around in case we need to return it
                noreturn_return_value = statement_value;
            }

            if (statement_value.cast(Instruction.DeclVar)) |decl_var| {
                // variable declarations start a new scope
                child_scope = decl_var.params.variable.child_scope;
            } else if (!is_continuation_unreachable) {
                // this statement's value must be void
                _ = irb.build(
                    Instruction.CheckVoidStmt,
                    child_scope,
                    statement_value.span,
                    Instruction.CheckVoidStmt.Params{ .target = statement_value },
                );
            }
        }

        if (is_continuation_unreachable) {
            assert(noreturn_return_value != null);
            if (block.label == null or block_scope.incoming_blocks.len == 0) {
                return noreturn_return_value.?;
            }

            try irb.setCursorAtEndAndAppendBlock(block_scope.end_block);
            return irb.build(Instruction.Phi, parent_scope, Span.token(block.rbrace), Instruction.Phi.Params{
                .incoming_blocks = block_scope.incoming_blocks.toOwnedSlice(),
                .incoming_values = block_scope.incoming_values.toOwnedSlice(),
            });
        }

        if (block.label) |label| {
            try block_scope.incoming_blocks.append(irb.current_basic_block);
            try block_scope.incoming_values.append(
                try irb.buildConstVoid(parent_scope, Span.token(block.rbrace), true),
            );
            _ = try irb.genDefersForBlock(child_scope, outer_block_scope, Scope.Defer.Kind.ScopeExit);

            _ = try irb.buildGen(Instruction.Br, parent_scope, Span.token(block.rbrace), Instruction.Br.Params{
                .dest_block = block_scope.end_block,
                .is_comptime = block_scope.is_comptime,
            });

            try irb.setCursorAtEndAndAppendBlock(block_scope.end_block);

            return irb.build(Instruction.Phi, parent_scope, Span.token(block.rbrace), Instruction.Phi.Params{
                .incoming_blocks = block_scope.incoming_blocks.toOwnedSlice(),
                .incoming_values = block_scope.incoming_values.toOwnedSlice(),
            });
        }

        _ = try irb.genDefersForBlock(child_scope, outer_block_scope, Scope.Defer.Kind.ScopeExit);
        return irb.buildConstVoid(child_scope, Span.token(block.rbrace), true);
    }

    fn genDefersForBlock(
        irb: *Builder,
        inner_scope: *Scope,
        outer_scope: *Scope,
        gen_kind: Scope.Defer.Kind,
    ) !bool {
        var scope = inner_scope;
        var is_noreturn = false;
        while (true) {
            switch (scope.id) {
                Scope.Id.Defer => {
                    const defer_scope = @fieldParentPtr(Scope.Defer, "base", scope);
                    const generate = switch (defer_scope.kind) {
                        Scope.Defer.Kind.ScopeExit => true,
                        Scope.Defer.Kind.ErrorExit => gen_kind == Scope.Defer.Kind.ErrorExit,
                    };
                    if (generate) {
                        const defer_expr_scope = defer_scope.defer_expr_scope;
                        const instruction = try irb.genNode(
                            defer_expr_scope.expr_node,
                            &defer_expr_scope.base,
                            LVal.None,
                        );
                        if (instruction.isNoReturn()) {
                            is_noreturn = true;
                        } else {
                            _ = try irb.build(
                                Instruction.CheckVoidStmt,
                                &defer_expr_scope.base,
                                Span.token(defer_expr_scope.expr_node.lastToken()),
                                Instruction.CheckVoidStmt.Params{ .target = instruction },
                            );
                        }
                    }
                },
                Scope.Id.FnDef,
                Scope.Id.Decls,
                => return is_noreturn,

                Scope.Id.CompTime,
                Scope.Id.Block,
                => scope = scope.parent orelse return is_noreturn,

                Scope.Id.DeferExpr => unreachable,
            }
        }
    }

    pub fn lvalWrap(irb: *Builder, scope: *Scope, instruction: *Instruction, lval: LVal) !*Instruction {
        switch (lval) {
            LVal.None => return instruction,
            LVal.Ptr => {
                // We needed a pointer to a value, but we got a value. So we create
                // an instruction which just makes a const pointer of it.
                return irb.build(Instruction.Ref, scope, instruction.span, Instruction.Ref.Params{
                    .target = instruction,
                    .mut = Type.Pointer.Mut.Const,
                    .volatility = Type.Pointer.Vol.Non,
                });
            },
        }
    }

    fn arena(self: *Builder) *Allocator {
        return &self.code.arena.allocator;
    }

    fn buildExtra(
        self: *Builder,
        comptime I: type,
        scope: *Scope,
        span: Span,
        params: I.Params,
        is_generated: bool,
    ) !*Instruction {
        const inst = try self.arena().create(I{
            .base = Instruction{
                .id = Instruction.typeToId(I),
                .is_generated = is_generated,
                .scope = scope,
                .debug_id = self.next_debug_id,
                .val = switch (I.ir_val_init) {
                    IrVal.Init.Unknown => IrVal.Unknown,
                    IrVal.Init.NoReturn => IrVal{ .KnownValue = &Value.NoReturn.get(self.comp).base },
                    IrVal.Init.Void => IrVal{ .KnownValue = &Value.Void.get(self.comp).base },
                },
                .ref_count = 0,
                .span = span,
                .child = null,
                .parent = null,
            },
            .params = params,
        });

        // Look at the params and ref() other instructions
        comptime var i = 0;
        inline while (i < @memberCount(I.Params)) : (i += 1) {
            const FieldType = comptime @typeOf(@field(I.Params(undefined), @memberName(I.Params, i)));
            switch (FieldType) {
                *Instruction => @field(inst.params, @memberName(I.Params, i)).ref_count += 1,
                ?*Instruction => if (@field(inst.params, @memberName(I.Params, i))) |other| other.ref_count += 1,
                else => {},
            }
        }

        self.next_debug_id += 1;
        try self.current_basic_block.instruction_list.append(&inst.base);
        return &inst.base;
    }

    fn build(
        self: *Builder,
        comptime I: type,
        scope: *Scope,
        span: Span,
        params: I.Params,
    ) !*Instruction {
        return self.buildExtra(I, scope, span, params, false);
    }

    fn buildGen(
        self: *Builder,
        comptime I: type,
        scope: *Scope,
        span: Span,
        params: I.Params,
    ) !*Instruction {
        return self.buildExtra(I, scope, span, params, true);
    }

    fn buildConstBool(self: *Builder, scope: *Scope, span: Span, x: bool) !*Instruction {
        const inst = try self.build(Instruction.Const, scope, span, Instruction.Const.Params{});
        inst.val = IrVal{ .KnownValue = &Value.Bool.get(self.comp, x).base };
        return inst;
    }

    fn buildConstVoid(self: *Builder, scope: *Scope, span: Span, is_generated: bool) !*Instruction {
        const inst = try self.buildExtra(Instruction.Const, scope, span, Instruction.Const.Params{}, is_generated);
        inst.val = IrVal{ .KnownValue = &Value.Void.get(self.comp).base };
        return inst;
    }
};

const Analyze = struct {
    irb: Builder,
    old_bb_index: usize,
    const_predecessor_bb: ?*BasicBlock,
    parent_basic_block: *BasicBlock,
    instruction_index: usize,
    src_implicit_return_type_list: std.ArrayList(*Instruction),
    explicit_return_type: ?*Type,

    pub const Error = error{
        /// This is only for when we have already reported a compile error. It is the poison value.
        SemanticAnalysisFailed,

        /// This is a placeholder - it is useful to use instead of panicking but once the compiler is
        /// done this error code will be removed.
        Unimplemented,

        OutOfMemory,
    };

    pub fn init(comp: *Compilation, parsed_file: *ParsedFile, explicit_return_type: ?*Type) !Analyze {
        var irb = try Builder.init(comp, parsed_file);
        errdefer irb.abort();

        return Analyze{
            .irb = irb,
            .old_bb_index = 0,
            .const_predecessor_bb = null,
            .parent_basic_block = undefined, // initialized with startBasicBlock
            .instruction_index = undefined, // initialized with startBasicBlock
            .src_implicit_return_type_list = std.ArrayList(*Instruction).init(irb.arena()),
            .explicit_return_type = explicit_return_type,
        };
    }

    pub fn abort(self: *Analyze) void {
        self.irb.abort();
    }

    pub fn getNewBasicBlock(self: *Analyze, old_bb: *BasicBlock, ref_old_instruction: ?*Instruction) !*BasicBlock {
        if (old_bb.child) |child| {
            if (ref_old_instruction == null or child.ref_instruction != ref_old_instruction)
                return child;
        }

        const new_bb = try self.irb.createBasicBlock(old_bb.scope, old_bb.name_hint);
        new_bb.linkToParent(old_bb);
        new_bb.ref_instruction = ref_old_instruction;
        return new_bb;
    }

    pub fn startBasicBlock(self: *Analyze, old_bb: *BasicBlock, const_predecessor_bb: ?*BasicBlock) void {
        self.instruction_index = 0;
        self.parent_basic_block = old_bb;
        self.const_predecessor_bb = const_predecessor_bb;
    }

    pub fn finishBasicBlock(ira: *Analyze, old_code: *Code) !void {
        try ira.irb.code.basic_block_list.append(ira.irb.current_basic_block);
        ira.instruction_index += 1;

        while (ira.instruction_index < ira.parent_basic_block.instruction_list.len) {
            const next_instruction = ira.parent_basic_block.instruction_list.at(ira.instruction_index);

            if (!next_instruction.is_generated) {
                try ira.addCompileError(next_instruction.span, "unreachable code");
                break;
            }
            ira.instruction_index += 1;
        }

        ira.old_bb_index += 1;

        var need_repeat = true;
        while (true) {
            while (ira.old_bb_index < old_code.basic_block_list.len) {
                const old_bb = old_code.basic_block_list.at(ira.old_bb_index);
                const new_bb = old_bb.child orelse {
                    ira.old_bb_index += 1;
                    continue;
                };
                if (new_bb.instruction_list.len != 0) {
                    ira.old_bb_index += 1;
                    continue;
                }
                ira.irb.current_basic_block = new_bb;

                ira.startBasicBlock(old_bb, null);
                return;
            }
            if (!need_repeat)
                return;
            need_repeat = false;
            ira.old_bb_index = 0;
            continue;
        }
    }

    fn addCompileError(self: *Analyze, span: Span, comptime fmt: []const u8, args: ...) !void {
        return self.irb.comp.addCompileError(self.irb.parsed_file, span, fmt, args);
    }

    fn resolvePeerTypes(self: *Analyze, expected_type: ?*Type, peers: []const *Instruction) Analyze.Error!*Type {
        // TODO actual implementation
        return &Type.Void.get(self.irb.comp).base;
    }

    fn implicitCast(self: *Analyze, target: *Instruction, optional_dest_type: ?*Type) Analyze.Error!*Instruction {
        const dest_type = optional_dest_type orelse return target;
        @panic("TODO implicitCast");
    }

    fn getCompTimeValOrNullUndefOk(self: *Analyze, target: *Instruction) ?*Value {
        @panic("TODO getCompTimeValOrNullUndefOk");
    }

    fn getCompTimeRef(
        self: *Analyze,
        value: *Value,
        ptr_mut: Value.Ptr.Mut,
        mut: Type.Pointer.Mut,
        volatility: Type.Pointer.Vol,
        ptr_align: u32,
    ) Analyze.Error!*Instruction {
        @panic("TODO getCompTimeRef");
    }
};

pub async fn gen(
    comp: *Compilation,
    body_node: *ast.Node,
    scope: *Scope,
    end_span: Span,
    parsed_file: *ParsedFile,
) !*Code {
    var irb = try Builder.init(comp, parsed_file);
    errdefer irb.abort();

    const entry_block = try irb.createBasicBlock(scope, "Entry");
    entry_block.ref(); // Entry block gets a reference because we enter it to begin.
    try irb.setCursorAtEndAndAppendBlock(entry_block);

    const result = try irb.genNode(body_node, scope, LVal.None);
    if (!result.isNoReturn()) {
        _ = irb.buildGen(
            Instruction.AddImplicitReturnType,
            scope,
            end_span,
            Instruction.AddImplicitReturnType.Params{ .target = result },
        );
        _ = irb.buildGen(
            Instruction.Return,
            scope,
            end_span,
            Instruction.Return.Params{ .return_value = result },
        );
    }

    return irb.finish();
}

pub async fn analyze(comp: *Compilation, parsed_file: *ParsedFile, old_code: *Code, expected_type: ?*Type) !*Code {
    var ira = try Analyze.init(comp, parsed_file, expected_type);
    errdefer ira.abort();

    const old_entry_bb = old_code.basic_block_list.at(0);

    const new_entry_bb = try ira.getNewBasicBlock(old_entry_bb, null);
    new_entry_bb.ref();

    ira.irb.current_basic_block = new_entry_bb;

    ira.startBasicBlock(old_entry_bb, null);

    while (ira.old_bb_index < old_code.basic_block_list.len) {
        const old_instruction = ira.parent_basic_block.instruction_list.at(ira.instruction_index);

        if (old_instruction.ref_count == 0 and !old_instruction.hasSideEffects()) {
            ira.instruction_index += 1;
            continue;
        }

        const return_inst = try old_instruction.analyze(&ira);
        // Note: if we ever modify the above to handle error.CompileError by continuing analysis,
        // then here we want to check if ira.isCompTime() and return early if true

        if (return_inst.isNoReturn()) {
            try ira.finishBasicBlock(old_code);
            continue;
        }

        ira.instruction_index += 1;
    }

    if (ira.src_implicit_return_type_list.len == 0) {
        ira.irb.code.return_type = &Type.NoReturn.get(comp).base;
        return ira.irb.finish();
    }

    ira.irb.code.return_type = try ira.resolvePeerTypes(expected_type, ira.src_implicit_return_type_list.toSliceConst());
    return ira.irb.finish();
}