package lower

import "../hir"
import "../ir"
import "../source"
import "../target"
import "../types"
import "core:fmt"
import "core:mem"

State :: struct {
	hir_module:   ^hir.Module,
	instructions: [dynamic]ir.Instruction,
	local_values: []ir.Instruction_Id,
	local_slots:  []ir.Instruction_Id,
	func_locals:  []hir.Local,
	func_result:  types.Type,
	expr_stack:   [dynamic]Lower_Expr_Frame,
	next_label:   i64,
	allocator:    mem.Allocator,
}

fresh_label :: proc(state: ^State) -> i64 {
	id := state.next_label
	state.next_label += 1
	return id
}

append_instruction :: proc(state: ^State, instruction: ir.Instruction) -> ir.Instruction_Id {
	id := ir.instruction_id(len(state.instructions))
	append(&state.instructions, instruction)
	return id
}

clone_args :: proc(values: []ir.Instruction_Id, allocator: mem.Allocator) -> []ir.Instruction_Id {
	result := make([]ir.Instruction_Id, len(values), allocator)
	copy(result, values)
	return result
}

sentinel :: proc(value_type: types.Type, selected := target.DEFAULT) -> i64 {
	if types.is_float(value_type, selected) {
		return i64(0x7fc0_0000) if types.bits(value_type, selected) == 32 else transmute(i64)u64(0x7ff8_0000_0000_0000)
	}
	switch types.bits(value_type, selected) {
	case 8:  return -86
	case 16: return -21846
	case 32: return -1431655766
	case:    return -6148914691236517206
	}
}

append_recovery_value :: proc(
	state: ^State,
	span: source.Span,
	value_type: types.Type,
	diagnostic := source.INVALID_DIAGNOSTIC,
) -> ir.Instruction_Id {
	append_instruction(state, ir.Instruction{
		op=.Trap,
		span=span,
		type=types.VOID,
		target=ir.INVALID_REF,
		a=ir.INVALID_INSTRUCTION,
		b=ir.INVALID_INSTRUCTION,
		diagnostic=diagnostic,
	})
	fallback := value_type
	if !types.is_valid(fallback) {
		fallback = types.I64
	}
	return append_instruction(state, ir.Instruction{
		op=.Const,
		span=span,
		type=fallback,
		integer=sentinel(fallback, state.hir_module.target),
		target=ir.INVALID_REF,
		a=ir.INVALID_INSTRUCTION,
		b=ir.INVALID_INSTRUCTION,
		diagnostic=source.INVALID_DIAGNOSTIC,
	})
}

Lower_Expr_Frame :: struct {
	expr:      hir.Expr_Id,
	stage:     u8,
	left:      ir.Instruction_Id,
	arg_index: int,
	args:      []ir.Instruction_Id,
}

lower_nested_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instruction_Id {
	outer := state.expr_stack
	state.expr_stack = nil
	state.expr_stack.allocator = state.allocator
	result := lower_expr(state, expr_id)
	delete(state.expr_stack)
	state.expr_stack = outer
	return result
}

lower_location :: proc(state: ^State, expr_id: hir.Expr_Id, for_write := false) -> ir.Instruction_Id {
	if expr_id == hir.INVALID_EXPR || int(expr_id) >= len(state.hir_module.exprs) {
		return ir.INVALID_INSTRUCTION
	}
	expr := state.hir_module.exprs[expr_id]
	#partial switch expr.kind {
	case .Local:
		local := hir.as_local(expr.target)
		if local != hir.INVALID_LOCAL && int(local) < len(state.local_slots) {
			return state.local_slots[local]
		}
	case .Global:
		global := hir.as_global(expr.target)
		if global != hir.INVALID_GLOBAL && int(global) < len(state.hir_module.globals) {
			return append_instruction(state, ir.Instruction{
				op=.Address_Global, span=expr.span, type=expr.type,
				target=ir.global_ref(ir.Global_Id(global)),
				a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
	case .Deref:
		return lower_nested_expr(state, expr.left)
	case .Index:
		container_type := state.hir_module.exprs[expr.left].type
		container := lower_nested_expr(state, expr.left)
		if types.is_array(container_type, &state.hir_module.types) {
			container = lower_location(state, expr.left, for_write)
		}
		index := lower_nested_expr(state, expr.right)
		return append_instruction(state, ir.Instruction{
			op=.Index_Address, span=expr.span, type=expr.type,
			integer=0 if for_write else 1,
			target=ir.INVALID_REF, a=container, b=index,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Field:
		base_type := state.hir_module.exprs[expr.left].type
		base := lower_nested_expr(state, expr.left)
		if !types.is_pointer(base_type, &state.hir_module.types) {
			base = lower_location(state, expr.left, for_write)
		}
		return append_instruction(state, ir.Instruction{
			op=.Field_Address, span=expr.span, type=expr.type, integer=expr.integer,
			target=ir.INVALID_REF, a=base, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	return ir.INVALID_INSTRUCTION
}

lower_compound_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instruction_Id {
	expr := state.hir_module.exprs[expr_id]
	#partial switch expr.kind {
	case .String:
		return append_instruction(state, ir.Instruction{
			op=.String, span=expr.span, type=expr.type, integer=expr.integer,
			target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Array, .Struct:
		args := make([]ir.Instruction_Id, len(expr.args), state.allocator)
		for arg, index in expr.args {
			args[index] = lower_nested_expr(state, arg)
		}
		return append_instruction(state, ir.Instruction{
			op=.Aggregate, span=expr.span, type=expr.type, args=args, integer=expr.integer,
			target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .None:
		return append_instruction(state, ir.Instruction{
			op=.None, span=expr.span, type=expr.type,
			target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Optional_Some:
		value := lower_nested_expr(state, expr.left)
		return append_instruction(state, ir.Instruction{
			op=.Optional_Some, span=expr.span, type=expr.type,
			target=ir.INVALID_REF, a=value, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Address:
		location := lower_location(state, expr.left, types.is_mutable(expr.type, &state.hir_module.types))
		if location != ir.INVALID_INSTRUCTION {
			return append_instruction(state, ir.Instruction{
				op=.Address_Of, span=expr.span, type=expr.type,
				target=ir.INVALID_REF, a=location, b=ir.INVALID_INSTRUCTION,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
		return append_recovery_value(state, expr.span, expr.type, expr.diagnostic)
	case .Deref, .Index, .Field:
		location := lower_location(state, expr_id)
		if location == ir.INVALID_INSTRUCTION {
			return append_recovery_value(state, expr.span, expr.type, expr.diagnostic)
		}
		return append_instruction(state, ir.Instruction{
			op=.Load, span=expr.span, type=expr.type,
			target=ir.INVALID_REF, a=location, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Slice:
		container := lower_nested_expr(state, expr.left)
		if types.is_array(state.hir_module.exprs[expr.left].type, &state.hir_module.types) {
			container = lower_location(state, expr.left)
		}
		args := make([]ir.Instruction_Id, len(expr.args), state.allocator)
		for arg, index in expr.args {
			args[index] = ir.INVALID_INSTRUCTION
			if arg != hir.INVALID_EXPR {
				args[index] = lower_nested_expr(state, arg)
			}
		}
		return append_instruction(state, ir.Instruction{
			op=.Slice, span=expr.span, type=expr.type, args=args,
			target=ir.INVALID_REF, a=container, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Unwrap:
		value := lower_nested_expr(state, expr.left)
		return append_instruction(state, ir.Instruction{
			op=.Unwrap, span=expr.span, type=expr.type,
			target=ir.INVALID_REF, a=value, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Length:
		container_type := state.hir_module.exprs[expr.left].type
		item, ok := types.node(&state.hir_module.types, container_type)
		if ok && item.kind == .Array {
			return append_instruction(state, ir.Instruction{
				op=.Const, span=expr.span, type=types.USIZE, integer=i64(item.count),
				target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
		_, array, array_ok := types.array_pointer(container_type, &state.hir_module.types)
		if array_ok {
			return append_instruction(state, ir.Instruction{
				op=.Const, span=expr.span, type=types.USIZE, integer=i64(array.count),
				target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
		value := lower_nested_expr(state, expr.left)
		return append_instruction(state, ir.Instruction{
			op=.Length, span=expr.span, type=types.USIZE,
			target=ir.INVALID_REF, a=value, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Slice_Ptr:
		container_type := state.hir_module.exprs[expr.left].type
		value := lower_nested_expr(state, expr.left)
		if types.is_array(container_type, &state.hir_module.types) {
			value = lower_location(state, expr.left)
		}
		return append_instruction(state, ir.Instruction{
			op=.Slice_Ptr, span=expr.span, type=expr.type,
			target=ir.INVALID_REF, a=value, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Orelse:
		value := lower_nested_expr(state, expr.left)
		begin := append_instruction(state, ir.Instruction{
			op=.Orelse_Begin, span=expr.span, type=expr.type,
			target=ir.INVALID_REF, a=value, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
		fallback := lower_nested_expr(state, expr.right)
		return append_instruction(state, ir.Instruction{
			op=.Orelse, span=expr.span, type=expr.type,
			target=ir.INVALID_REF, a=begin, b=fallback,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Not:
		value := lower_nested_expr(state, expr.left)
		return append_instruction(state, ir.Instruction{
			op=.Not, span=expr.span, type=types.BOOL,
			target=ir.INVALID_REF, a=value, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .Eq, .Ne, .Lt, .Le, .Gt, .Ge:
		left := lower_nested_expr(state, expr.left)
		right := lower_nested_expr(state, expr.right)
		predicate := ir.Compare_Predicate.Eq
		#partial switch expr.kind {
		case .Eq: predicate = .Eq
		case .Ne: predicate = .Ne
		case .Lt: predicate = .Lt
		case .Le: predicate = .Le
		case .Gt: predicate = .Gt
		case .Ge: predicate = .Ge
		}
		return append_instruction(state, ir.Instruction{
			op=.Compare, span=expr.span, type=types.BOOL, integer=i64(predicate),
			target=ir.INVALID_REF, a=left, b=right,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	case .And, .Or:
		// Short-circuit via a bool slot: store the left operand, branch on it, and
		// only evaluate/store the right operand when needed. Avoids phi nodes.
		slot := append_instruction(state, ir.Instruction{
			op=.Alloca, span=expr.span, type=types.BOOL,
			target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
		left := lower_nested_expr(state, expr.left)
		append_instruction(state, ir.Instruction{
			op=.Store, span=expr.span, type=types.BOOL,
			target=ir.INVALID_REF, a=slot, b=left, diagnostic=source.INVALID_DIAGNOSTIC,
		})
		rhs_lbl := fresh_label(state)
		done_lbl := fresh_label(state)
		true_target := rhs_lbl if expr.kind == .And else done_lbl
		false_target := done_lbl if expr.kind == .And else rhs_lbl
		append_instruction(state, ir.Instruction{
			op=.Cond_Br, span=expr.span, type=types.VOID,
			a=left, integer=true_target, target=ir.Ref(u32(false_target)),
			b=ir.INVALID_INSTRUCTION, diagnostic=source.INVALID_DIAGNOSTIC,
		})
		append_instruction(state, ir.Instruction{
			op=.Label, span=expr.span, type=types.VOID, integer=rhs_lbl,
			target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
		right := lower_nested_expr(state, expr.right)
		append_instruction(state, ir.Instruction{
			op=.Store, span=expr.span, type=types.BOOL,
			target=ir.INVALID_REF, a=slot, b=right, diagnostic=source.INVALID_DIAGNOSTIC,
		})
		append_instruction(state, ir.Instruction{
			op=.Br, span=expr.span, type=types.VOID, integer=done_lbl,
			target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
		append_instruction(state, ir.Instruction{
			op=.Label, span=expr.span, type=types.VOID, integer=done_lbl,
			target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
		return append_instruction(state, ir.Instruction{
			op=.Load, span=expr.span, type=types.BOOL,
			target=ir.INVALID_REF, a=slot, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
	}
	return append_recovery_value(state, expr.span, expr.type, expr.diagnostic)
}

lower_expr :: proc(state: ^State, expr_id: hir.Expr_Id) -> ir.Instruction_Id {
	stack := state.expr_stack
	clear_dynamic_array(&stack)
	defer {
		for frame in stack {
			delete(frame.args, state.allocator)
		}
		clear_dynamic_array(&stack)
		state.expr_stack = stack
	}
	append(&stack, Lower_Expr_Frame{expr=expr_id})
	last := ir.INVALID_INSTRUCTION
	for len(stack) > 0 {
		frame_index := len(stack)-1
		frame := stack[frame_index]
		if frame.expr == hir.INVALID_EXPR || int(frame.expr) >= len(state.hir_module.exprs) {
			last = append_recovery_value(state, source.Span{}, types.I64)
			_ = pop(&stack)
			continue
		}
		expr := state.hir_module.exprs[frame.expr]
		if frame.stage == 0 {
			#partial switch expr.kind {
			case .Invalid:
				last = append_recovery_value(state, expr.span, expr.type, expr.diagnostic)
				_ = pop(&stack)
			case .Integer, .Float, .Bool:
				last = append_instruction(state, ir.Instruction{
					op=.Const, span=expr.span, type=expr.type, integer=expr.integer,
					target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION, diagnostic=source.INVALID_DIAGNOSTIC,
				})
				_ = pop(&stack)
			case .String, .Array, .Struct, .None, .Optional_Some, .Address, .Deref,
			     .Index, .Slice, .Field, .Length, .Slice_Ptr, .Unwrap, .Orelse,
			     .Not, .Eq, .Ne, .Lt, .Le, .Gt, .Ge, .And, .Or:
				last = lower_compound_expr(state, frame.expr)
				_ = pop(&stack)
			case .Local:
				last = ir.INVALID_INSTRUCTION
				local := hir.as_local(expr.target)
				if local != hir.INVALID_LOCAL && int(local) < len(state.local_slots) && state.local_slots[local] != ir.INVALID_INSTRUCTION {
					last = append_instruction(state, ir.Instruction{
						op=.Load, span=expr.span, type=expr.type, target=ir.INVALID_REF,
						a=state.local_slots[local], b=ir.INVALID_INSTRUCTION, diagnostic=source.INVALID_DIAGNOSTIC,
					})
				} else if local != hir.INVALID_LOCAL && int(local) < len(state.local_values) &&
				          state.local_values[local] != ir.INVALID_INSTRUCTION {
					last = state.local_values[local]
				}
				if last == ir.INVALID_INSTRUCTION {
					last = append_recovery_value(state, expr.span, expr.type, expr.diagnostic)
				}
				_ = pop(&stack)
			case .Global:
				global := hir.as_global(expr.target)
				if global == hir.INVALID_GLOBAL || int(global) >= len(state.hir_module.globals) {
					last = append_recovery_value(state, expr.span, expr.type, expr.diagnostic)
				} else {
					last = append_instruction(state, ir.Instruction{
						op=.Load_Global, span=expr.span, type=expr.type, target=ir.global_ref(ir.Global_Id(global)),
						a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION, diagnostic=source.INVALID_DIAGNOSTIC,
					})
				}
				_ = pop(&stack)
			case .Function:
				function := hir.as_function(expr.target)
				if function == hir.INVALID_FUNCTION || int(function) >= len(state.hir_module.functions) {
					last = append_recovery_value(state, expr.span, expr.type, expr.diagnostic)
				} else {
					last = append_instruction(state, ir.Instruction{
						op=.Function_Address, span=expr.span, type=expr.type,
						target=ir.function_ref(ir.Function_Id(function)),
						a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
						diagnostic=source.INVALID_DIAGNOSTIC,
					})
				}
				_ = pop(&stack)
			case .Widen, .C_Vararg_Promote, .Weaken_Pointer, .Weaken_Slice, .Decay_Array_Pointer:
				stack[frame_index].stage = 1
				append(&stack, Lower_Expr_Frame{expr=expr.left})
			case .Negate:
				stack[frame_index].stage = 5
				append(&stack, Lower_Expr_Frame{expr=expr.left})
			case .Add, .Pointer_Add:
				stack[frame_index].stage = 2
				append(&stack, Lower_Expr_Frame{expr=expr.left})
			case .Call:
				function := hir.as_function(expr.target)
				if function == hir.INVALID_FUNCTION {
					if expr.left == hir.INVALID_EXPR {
						last = append_recovery_value(state, expr.span, expr.type, expr.diagnostic)
						_ = pop(&stack)
						continue
					}
					stack[frame_index].stage = 6
					append(&stack, Lower_Expr_Frame{expr=expr.left})
					continue
				}
				if int(function) >= len(state.hir_module.functions) {
					last = append_recovery_value(state, expr.span, expr.type, expr.diagnostic)
					_ = pop(&stack)
					continue
				}
				stack[frame_index].args = make([]ir.Instruction_Id, len(expr.args), state.allocator)
				stack[frame_index].stage = 4
				if len(expr.args) > 0 {
					append(&stack, Lower_Expr_Frame{expr=expr.args[0]})
				}
			}
			continue
		}
		if frame.stage == 6 {
			stack[frame_index].left = last
			stack[frame_index].args = make([]ir.Instruction_Id, len(expr.args), state.allocator)
			stack[frame_index].stage = 4
			if len(expr.args) > 0 {
				append(&stack, Lower_Expr_Frame{expr=expr.args[0]})
			}
			continue
		}
		if frame.stage == 5 {
			last = append_instruction(state, ir.Instruction{
				op=.Neg_Checked, span=expr.span, type=expr.type, target=ir.INVALID_REF,
				a=last, b=ir.INVALID_INSTRUCTION, diagnostic=source.INVALID_DIAGNOSTIC,
			})
			_ = pop(&stack)
			continue
		}
		if frame.stage == 1 {
			op := ir.Opcode.Widen
			#partial switch expr.kind {
			case .Weaken_Pointer:      op = .Weaken_Pointer
			case .Weaken_Slice:        op = .Weaken_Slice
			case .Decay_Array_Pointer: op = .Decay_Array_Pointer
			case .C_Vararg_Promote:    op = .C_Vararg_Promote
			case:                      op = .Widen
			}
			last = append_instruction(state, ir.Instruction{
				op=op,
				span=expr.span, type=expr.type, target=ir.INVALID_REF,
				a=last, b=ir.INVALID_INSTRUCTION, diagnostic=source.INVALID_DIAGNOSTIC,
			})
			_ = pop(&stack)
			continue
		}
		if frame.stage == 2 {
			stack[frame_index].left = last
			stack[frame_index].stage = 3
			append(&stack, Lower_Expr_Frame{expr=expr.right})
			continue
		}
		if frame.stage == 3 {
			last = append_instruction(state, ir.Instruction{
				op=.Pointer_Add if expr.kind == .Pointer_Add else .Add_Checked,
				span=expr.span, type=expr.type, target=ir.INVALID_REF,
				a=frame.left, b=last, diagnostic=source.INVALID_DIAGNOSTIC,
			})
			_ = pop(&stack)
			continue
		}
		if frame.stage == 4 {
			if frame.arg_index < len(expr.args) {
				stack[frame_index].args[frame.arg_index] = last
				stack[frame_index].arg_index += 1
				if frame.arg_index+1 < len(expr.args) {
					append(&stack, Lower_Expr_Frame{expr=expr.args[frame.arg_index+1]})
					continue
				}
			}
			function := hir.as_function(expr.target)
			callee := frame.left
			if function != hir.INVALID_FUNCTION {
				callee = ir.INVALID_INSTRUCTION
			}
			last = append_instruction(state, ir.Instruction{
				op=.Call, span=expr.span, type=expr.type, target=ir.function_ref(ir.Function_Id(function)),
				a=callee, b=ir.INVALID_INSTRUCTION, args=stack[frame_index].args, diagnostic=source.INVALID_DIAGNOSTIC,
			})
			stack[frame_index].args = nil
			_ = pop(&stack)
		}
	}
	return last
}

lower_statements :: proc(state: ^State, statements: []hir.Stmt_Id) {
	hir_module := state.hir_module
	for statement_id in statements {
		statement := hir_module.statements[statement_id]
		switch statement.kind {
		case .Declaration:
			value := lower_expr(state, statement.expr)
			if statement.local == hir.INVALID_LOCAL || int(statement.local) >= len(state.func_locals) {
				append_instruction(state, ir.Instruction{
					op=.Trap, span=statement.span, type=types.VOID,
					target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION, diagnostic=statement.diagnostic,
				})
				continue
			}
			local := state.func_locals[statement.local]
			slot := append_instruction(state, ir.Instruction{
				op=.Alloca,
				span=statement.span,
				type=local.type,
				target=ir.local_ref(ir.Local_Id(statement.local)),
				a=ir.INVALID_INSTRUCTION,
				b=ir.INVALID_INSTRUCTION,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
			state.local_slots[statement.local] = slot
			append_instruction(state, ir.Instruction{
				op=.Store,
				span=statement.span,
				type=local.type,
				target=ir.INVALID_REF,
				a=slot,
				b=value,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		case .Assignment:
			value := lower_expr(state, statement.expr)
			slot := ir.INVALID_INSTRUCTION
			value_type := types.INVALID
			if statement.target != hir.INVALID_EXPR {
				slot = lower_location(state, statement.target, true)
				if int(statement.target) < len(hir_module.exprs) {
					value_type = hir_module.exprs[statement.target].type
				}
			} else if statement.local != hir.INVALID_LOCAL && int(statement.local) < len(state.local_slots) {
				slot = state.local_slots[statement.local]
				value_type = state.func_locals[statement.local].type
			}
			if slot == ir.INVALID_INSTRUCTION || !types.is_valid(value_type) {
				append_instruction(state, ir.Instruction{
					op=.Trap, span=statement.span, type=types.VOID,
					target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION, diagnostic=statement.diagnostic,
				})
				continue
			}
			append_instruction(state, ir.Instruction{
				op=.Store,
				span=statement.span,
				type=value_type,
				target=ir.INVALID_REF,
				a=slot,
				b=value,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		case .Return:
			if statement.expr == hir.INVALID_EXPR {
				append_instruction(state, ir.Instruction{
					op=.Return_Void,
					span=statement.span,
					type=types.VOID,
					target=ir.INVALID_REF,
					a=ir.INVALID_INSTRUCTION,
					b=ir.INVALID_INSTRUCTION,
					diagnostic=source.INVALID_DIAGNOSTIC,
				})
			} else {
				value := lower_expr(state, statement.expr)
				append_instruction(state, ir.Instruction{
					op=.Return,
					span=statement.span,
					type=state.func_result,
					target=ir.INVALID_REF,
					a=value,
					b=ir.INVALID_INSTRUCTION,
					diagnostic=source.INVALID_DIAGNOSTIC,
				})
			}
		case .Expression, .Sink:
			_ = lower_expr(state, statement.expr)
		case .Trap:
			append_instruction(state, ir.Instruction{
				op=.Trap,
				span=statement.span,
				type=types.VOID,
				target=ir.INVALID_REF,
				a=ir.INVALID_INSTRUCTION,
				b=ir.INVALID_INSTRUCTION,
				diagnostic=statement.diagnostic,
			})
		case .If:
			// A conditional unwrap (`if opt |v| { ... }`) carries the binding local id in
			// `statement.local`; `expr` is then the optional, not a bool condition. Test it
			// for presence, and inside the then-block bind the unwrapped value to the local.
			is_unwrap := statement.local != hir.INVALID_LOCAL
			opt := ir.INVALID_INSTRUCTION
			cond: ir.Instruction_Id
			if is_unwrap {
				opt = lower_expr(state, statement.expr)
				cond = append_instruction(state, ir.Instruction{
					op=.Optional_Is_Some, span=statement.span, type=types.BOOL,
					target=ir.INVALID_REF, a=opt, b=ir.INVALID_INSTRUCTION,
					diagnostic=source.INVALID_DIAGNOSTIC,
				})
			} else {
				cond = lower_expr(state, statement.expr)
			}
			has_else := statement.else_body != nil
			then_lbl := fresh_label(state)
			else_lbl := fresh_label(state) if has_else else then_lbl
			merge_lbl := fresh_label(state)
			false_target := else_lbl if has_else else merge_lbl
			append_instruction(state, ir.Instruction{
				op=.Cond_Br, span=statement.span, type=types.VOID,
				a=cond, integer=then_lbl, target=ir.Ref(u32(false_target)),
				b=ir.INVALID_INSTRUCTION, diagnostic=source.INVALID_DIAGNOSTIC,
			})
			append_instruction(state, ir.Instruction{
				op=.Label, span=statement.span, type=types.VOID, integer=then_lbl,
				target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
			if is_unwrap && int(statement.local) < len(state.func_locals) {
				local := state.func_locals[statement.local]
				slot := append_instruction(state, ir.Instruction{
					op=.Alloca, span=statement.span, type=local.type,
					target=ir.local_ref(ir.Local_Id(statement.local)),
					a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
					diagnostic=source.INVALID_DIAGNOSTIC,
				})
				state.local_slots[statement.local] = slot
				inner := append_instruction(state, ir.Instruction{
					op=.Optional_Value, span=statement.span, type=local.type,
					target=ir.INVALID_REF, a=opt, b=ir.INVALID_INSTRUCTION,
					diagnostic=source.INVALID_DIAGNOSTIC,
				})
				append_instruction(state, ir.Instruction{
					op=.Store, span=statement.span, type=local.type,
					target=ir.INVALID_REF, a=slot, b=inner,
					diagnostic=source.INVALID_DIAGNOSTIC,
				})
			}
			lower_statements(state, statement.then_body)
			append_instruction(state, ir.Instruction{
				op=.Br, span=statement.span, type=types.VOID, integer=merge_lbl,
				target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
			if has_else {
				append_instruction(state, ir.Instruction{
					op=.Label, span=statement.span, type=types.VOID, integer=else_lbl,
					target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
					diagnostic=source.INVALID_DIAGNOSTIC,
				})
				lower_statements(state, statement.else_body)
				append_instruction(state, ir.Instruction{
					op=.Br, span=statement.span, type=types.VOID, integer=merge_lbl,
					target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
					diagnostic=source.INVALID_DIAGNOSTIC,
				})
			}
			append_instruction(state, ir.Instruction{
				op=.Label, span=statement.span, type=types.VOID, integer=merge_lbl,
				target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
		}
	}
}

lower_body :: proc(hir_module: ^hir.Module, function: hir.Function, allocator: mem.Allocator) -> []ir.Instruction {
	state := State{
		hir_module=hir_module,
		allocator=allocator,
		func_locals=function.locals,
		func_result=function.result,
		local_values=make([]ir.Instruction_Id, len(function.locals), allocator),
		local_slots=make([]ir.Instruction_Id, len(function.locals), allocator),
	}
	state.instructions.allocator = allocator
	state.expr_stack.allocator = allocator
	defer {
		delete(state.local_values, allocator)
		delete(state.local_slots, allocator)
		delete(state.expr_stack)
	}
	for _, index in state.local_values {
		state.local_values[index] = ir.INVALID_INSTRUCTION
		state.local_slots[index] = ir.INVALID_INSTRUCTION
	}
	for local_id in function.params {
		param := append_instruction(&state, ir.Instruction{
			op=.Param,
			type=function.locals[local_id].type,
			target=ir.local_ref(ir.Local_Id(local_id)),
			a=ir.INVALID_INSTRUCTION,
			b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
		state.local_values[local_id] = param
	}
	for local_id in function.params {
		slot := append_instruction(&state, ir.Instruction{
			op=.Alloca, type=function.locals[local_id].type,
			target=ir.local_ref(ir.Local_Id(local_id)),
			a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION,
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
		append_instruction(&state, ir.Instruction{
			op=.Store, type=function.locals[local_id].type,
			target=ir.INVALID_REF, a=slot, b=state.local_values[local_id],
			diagnostic=source.INVALID_DIAGNOSTIC,
		})
		state.local_slots[local_id] = slot
	}

	lower_statements(&state, function.body)
	if len(state.instructions) == 0 ||
	   (state.instructions[len(state.instructions)-1].op != .Return &&
	    state.instructions[len(state.instructions)-1].op != .Return_Void) {
		if types.is_void(function.result) {
			append_instruction(&state, ir.Instruction{op=.Return_Void, type=types.VOID, target=ir.INVALID_REF, a=ir.INVALID_INSTRUCTION, b=ir.INVALID_INSTRUCTION, diagnostic=source.INVALID_DIAGNOSTIC})
		} else {
			value := append_instruction(&state, ir.Instruction{
				op=.Const,
				type=function.result,
				integer=sentinel(function.result, hir_module.target),
				target=ir.INVALID_REF,
				a=ir.INVALID_INSTRUCTION,
				b=ir.INVALID_INSTRUCTION,
				diagnostic=source.INVALID_DIAGNOSTIC,
			})
			append_instruction(&state, ir.Instruction{op=.Return, type=function.result, target=ir.INVALID_REF, a=value, b=ir.INVALID_INSTRUCTION, diagnostic=source.INVALID_DIAGNOSTIC})
		}
	}
	return state.instructions[:]
}

lower_global_initializer :: proc(hir_module: ^hir.Module, global: hir.Global, allocator: mem.Allocator) -> []ir.Instruction {
	state := State{hir_module=hir_module, allocator=allocator}
	state.instructions.allocator = allocator
	state.expr_stack.allocator = allocator
	defer delete(state.expr_stack)
	value := lower_expr(&state, global.expr)
	append_instruction(&state, ir.Instruction{
		op=.Return,
		type=global.type,
		target=ir.INVALID_REF,
		a=value,
		b=ir.INVALID_INSTRUCTION,
		diagnostic=source.INVALID_DIAGNOSTIC,
	})
	return state.instructions[:]
}

lower :: proc(hir_module: ^hir.Module, allocator := context.allocator) -> ir.Module {
	module := ir.init_module(hir_module.target, allocator)
	types.destroy_store(&module.types)
	module.types = types.clone_store(&hir_module.types, allocator)
	for value in hir_module.strings {
		append(&module.strings, fmt.aprintf("%s", value, allocator=allocator))
	}
	for global in hir_module.globals {
		_ = ir.global_id(len(module.globals))
		append(&module.globals, ir.Global{
			name=global.name,
			link_name=fmt.aprintf("%s", global.link_name, allocator=allocator),
			type=global.type,
			is_static=global.is_static,
			external=global.external,
			writable=global.writable,
			static_value=global.static_value,
			initializer=nil if global.is_static || global.external else lower_global_initializer(hir_module, global, allocator),
			problematic=global.problematic,
			diagnostic=global.diagnostic,
		})
	}
	for function in hir_module.functions {
		param_types := make([]types.Type, len(function.params), allocator)
		for local_id, index in function.params {
			param_types[index] = function.locals[local_id].type
		}
		_ = ir.function_id(len(module.functions))
		append(&module.functions, ir.Function{
			link_name=fmt.aprintf("%s", function.link_name, allocator=allocator),
			calling_convention=.C if function.calling_convention == .C else .Brolang,
			implementation=.Declaration if function.implementation == .Declaration else .Definition,
			linkage=.External if function.linkage == .External else .Internal,
			is_main=function.is_main,
			variadic=function.variadic,
			param_types=param_types,
			result=function.result,
			instructions=nil if function.implementation == .Declaration else lower_body(hir_module, function, allocator),
			problematic=function.problematic,
		})
	}
	return module
}