diff --git a/src/middle/lir.ts b/src/middle/lir.ts
new file mode 100644
index 0000000..5e72e5f
--- /dev/null
+++ b/src/middle/lir.ts
@@ -0,0 +1,236 @@
+export class Mod {
+    public fns: Fn[] = [];
+
+    constructor() {}
+
+    createFn(): Fn {
+        return new Fn(this);
+    }
+}
+
+export class Fn {
+    public bbs: BasicBlock[] = [];
+
+    constructor(
+        public mod: Mod,
+    ) {}
+
+    createBasicBlock(): BasicBlock {
+        return new BasicBlock(this);
+    }
+
+    pushBasicBlock(bb: BasicBlock) {
+        this.bbs.push(bb);
+    }
+}
+
+export class BasicBlock {
+    public insts: Inst[] = [];
+
+    constructor(
+        public fn: Fn,
+    ) {}
+}
+
+export class Inst {
+    constructor(
+        public bb: BasicBlock,
+        public ty: Ty,
+        public kind: InstKind,
+    ) {}
+}
+
+export type InstKind =
+    | { tag: "Int"; value: number }
+    | { tag: "Bool"; value: boolean }
+    | { tag: "Fn"; fn: Fn }
+    | { tag: "Param"; idx: number }
+    | { tag: "GetElemPtr"; base: Inst; offset: number }
+    | { tag: "Call"; callee: Inst; args: Inst[] }
+    | { tag: "Alloca" }
+    | { tag: "Load"; source: Inst }
+    | { tag: "Store"; target: Inst; source: Inst }
+    | { tag: "ReturnVoid" }
+    | { tag: "Return"; source: Inst }
+    | { tag: "Jump"; target: BasicBlock }
+    | { tag: "Branch"; cond: Inst; truthy: BasicBlock; falsy: BasicBlock }
+    | { tag: "Select"; cond: Inst; truthy: Inst; falsy: Inst }
+    | { tag: "Trunc"; source: Inst }
+    | { tag: "ZeroExt"; source: Inst }
+    | { tag: "SignExt"; source: Inst }
+    | { tag: UnaryOp; source: Inst; signed: boolean }
+    | { tag: BinaryOp; left: Inst; right: Inst; signed: boolean };
+
+export type UnaryOp = "Not" | "Neg";
+
+export type BinaryOp =
+    | "Eq"
+    | "Ne"
+    | "Lt"
+    | "Gt"
+    | "Lte"
+    | "Gte"
+    | "Or"
+    | "Xor"
+    | "And"
+    | "Shl"
+    | "Shr"
+    | "Add"
+    | "Sub"
+    | "Mul"
+    | "Div"
+    | "Rem";
+
+export class Ty {
+    constructor(
+        public kind: TyKind,
+    ) {}
+
+    hash(): string {
+        return JSON.stringify(this.kind);
+    }
+}
+
+export type TyKind =
+    | { tag: "I8" | "I16" | "I32" | "I64" }
+    | { tag: "Ptr" }
+    | { tag: "Array"; ty: Ty; length: number };
+
+export class Context {
+    private tys = new Map<string, Ty>();
+    private instsHashIds = new Map<Inst, number>();
+    private fnHashIds = new Map<Fn, number>();
+    private bbHashIds = new Map<BasicBlock, number>();
+
+    private insts = new Map<string, Inst>();
+
+    createTy<
+        Tag extends TyKind["tag"],
+    >(
+        tag: Tag,
+        kind: Omit<TyKind & { tag: Tag }, "tag">,
+    ): Ty {
+        const ty = new Ty({ ...kind, tag } as TyKind);
+        const key = ty.hash();
+        const found = this.tys.get(key);
+        if (found) {
+            return found;
+        }
+        this.tys.set(key, ty);
+        return ty;
+    }
+
+    createInst(
+        bb: BasicBlock,
+        ty: Ty,
+        kind: InstKind,
+    ): Inst {
+        const newId = this.instsHashIds.size;
+        const inst = new Inst(bb, ty, kind);
+        const key = this.hashInst(inst);
+        const found = this.insts.get(key);
+        if (found) {
+            return found;
+        }
+        this.insts.set(key, inst);
+        this.instsHashIds.set(inst, newId);
+        return inst;
+    }
+
+    hashInst(inst: Inst): string {
+        const id = this.instsHashIds.get(inst);
+        if (id) {
+            return `%${id}`;
+        }
+
+        const i = (inst: Inst) => this.hashInst(inst);
+        const fn = (fn: Fn): string => {
+            let id = this.fnHashIds.get(fn);
+            if (!id) {
+                id = this.fnHashIds.size;
+                this.fnHashIds.set(fn, id);
+            }
+            return `%${id}`;
+        };
+        const bb = (bb: BasicBlock): string => {
+            let id = this.bbHashIds.get(bb);
+            if (!id) {
+                id = this.bbHashIds.size;
+                this.bbHashIds.set(bb, id);
+            }
+            return `%${id}`;
+        };
+
+        const k = inst.kind;
+        switch (k.tag) {
+            case "Int":
+            case "Bool":
+                return `${k.tag}-${k.value}`;
+            case "Fn":
+                return `${k.tag}-${fn(k.fn)}`;
+            case "Param":
+                return `${k.tag}-${k.idx}`;
+            case "GetElemPtr":
+                return `${k.tag}-${i(k.base)}-${k.offset}`;
+            case "Call":
+                return `${k.tag}-${i(k.callee)}-${k.args.map(i).join("-")}`;
+            case "Alloca":
+                return `${k.tag}`;
+            case "Load":
+                return `${k.tag}-${i(k.source)}`;
+            case "Store":
+                return `${k.tag}-${i(k.target)}-${i(k.source)}`;
+            case "ReturnVoid":
+                return `${k.tag}`;
+            case "Return":
+                return `${k.tag}-${i(k.source)}`;
+            case "Jump":
+                return `${k.tag}-${bb(k.target)}`;
+            case "Branch":
+                return `${k.tag}-${i(k.cond)}-${bb(k.truthy)}-${bb(k.falsy)}`;
+            case "Select":
+                return `${k.tag}-${i(k.cond)}-${i(k.truthy)}-${i(k.falsy)}`;
+            case "Trunc":
+                return `${k.tag}-${i(k.source)}`;
+            case "ZeroExt":
+                return `${k.tag}-${i(k.source)}`;
+            case "SignExt":
+                return `${k.tag}-${i(k.source)}`;
+            case "Not":
+            case "Neg":
+                return `${k.tag}-${i(k.source)}-${k.signed}`;
+            case "Eq":
+            case "Ne":
+            case "Lt":
+            case "Gt":
+            case "Lte":
+            case "Gte":
+            case "Or":
+            case "Xor":
+            case "And":
+            case "Shl":
+            case "Shr":
+            case "Add":
+            case "Sub":
+            case "Mul":
+            case "Div":
+            case "Rem":
+                return `${k.tag}-${i(k.left)}-${i(k.right)}-${k.signed}`;
+        }
+    }
+}
+
+export class Builder {
+    constructor(
+        private bb: BasicBlock,
+        private cx: Context,
+    ) {}
+
+    createInt(ty: Ty, value: number) {
+        this.push(ty, { tag: "Int", value });
+    }
+
+    private push(ty: Ty, kind: InstKind) {
+        this.bb.insts.push(this.cx.createInst(this.bb, ty, kind));
+    }
+}
diff --git a/src/middle/mod.ts b/src/middle/mod.ts
new file mode 100644
index 0000000..c420f97
--- /dev/null
+++ b/src/middle/mod.ts
@@ -0,0 +1 @@
+export * as lir from "./lir.ts";