diff --git a/src/codegen/x86_64/isel.ts b/src/codegen/x86_64/isel.ts
index 6c305ab..7f57613 100644
--- a/src/codegen/x86_64/isel.ts
+++ b/src/codegen/x86_64/isel.ts
@@ -1,4 +1,134 @@
 import * as mir from "../../mir.ts";
+import { Block, Fn, Inst, Reg, Regs } from "./module.ts";
 
-export function selectFnInstructions(fn: mir.Fn) {
+export function selectFnInstructions(fn: mir.Fn): Fn {
+    return new InstructionSelector(fn).generateFn();
+}
+
+class InstructionSelector {
+    private localsSize = 0;
+    private locals = new Map<mir.Inst, number>();
+    private exitBlock!: Block;
+
+    constructor(
+        private fn: mir.Fn,
+    ) {}
+
+    generateFn(): Fn {
+        this.generateFrameLayout();
+
+        const entry = new Block();
+        const exit = new Block();
+
+        entry.push(new Inst({ tag: "PushR", src: Reg.reg(Regs.bp) }));
+        entry.push(
+            new Inst({
+                tag: "MovRR",
+                dst: Reg.reg(Regs.bp),
+                src: Reg.reg(Regs.sp),
+            }),
+        );
+        entry.push(
+            new Inst({
+                tag: "SubRI",
+                dst: Reg.reg(Regs.sp),
+                imm: this.localsSize,
+            }),
+        );
+
+        this.exitBlock = exit;
+
+        const blocks: Block[] = [];
+        for (const bb of this.fn.bbs) {
+            blocks.push(this.generateBasicBlock(bb));
+        }
+
+        entry.push(new Inst({ tag: "Jmp", bb: blocks[0] ?? exit }));
+
+        exit.push(
+            new Inst({
+                tag: "MovRR",
+                dst: Reg.reg(Regs.sp),
+                src: Reg.reg(Regs.bp),
+            }),
+        );
+        entry.push(new Inst({ tag: "PopR", dst: Reg.reg(Regs.bp) }));
+        entry.push(new Inst({ tag: "Ret" }));
+
+        return new Fn([entry, ...blocks, exit]);
+    }
+
+    private generateFrameLayout() {
+        let offset = 0;
+
+        const align = (alignment: number) => {
+            if (offset % alignment != 0) {
+                offset += alignment - offset % alignment;
+            }
+        };
+
+        const pushLocal = (inst: mir.Inst, size: number, alignment: number) => {
+            align(alignment);
+            this.locals.set(inst, offset);
+            offset += size;
+        };
+
+        this.fn.visit({
+            visitInst(inst) {
+                if (inst.kind.tag === "Alloca") {
+                    const ty = inst.ty.as("PtrMut").kind.ty;
+                    switch (ty.kind.tag) {
+                        case "Int": {
+                            switch (ty.kind.intTy) {
+                                case "i32":
+                                    pushLocal(inst, 4, 4);
+                                    break;
+                                case "i64":
+                                    pushLocal(inst, 8, 8);
+                                    break;
+                                default:
+                                    throw new Error(
+                                        `not implemented (${ty.kind.intTy})`,
+                                    );
+                            }
+                            break;
+                        }
+                        default:
+                            throw new Error(
+                                `not implemented (${ty.kind.tag})`,
+                            );
+                    }
+                }
+            },
+        });
+
+        align(16);
+
+        this.localsSize = offset;
+    }
+
+    private generateBasicBlock(bb: mir.BasicBlock): Block {
+        const block = new Block();
+        for (const inst of bb.insts) {
+            const k = inst.kind;
+            switch (k.tag) {
+                case "Alloca":
+                    break;
+                case "Int":
+                    block.push(
+                        new Inst({
+                            tag: "MovRI",
+                            dst: Reg.reg(Regs.bp),
+                            imm: k.value,
+                        }),
+                    );
+                    break;
+                default:
+                    // throw new Error(
+                    //     `not implemented (${k.tag})`,
+                    // );
+            }
+        }
+        return block;
+    }
 }
diff --git a/src/codegen/x86_64/module.ts b/src/codegen/x86_64/module.ts
index 78b9947..1b8787f 100644
--- a/src/codegen/x86_64/module.ts
+++ b/src/codegen/x86_64/module.ts
@@ -3,7 +3,9 @@ export class Module {
 }
 
 export class Fn {
-    private blocks: Block[] = [];
+    constructor(
+        private blocks: Block[] = [],
+    ) {}
 }
 
 export class Block {
@@ -15,6 +17,9 @@ export class Block {
 }
 
 export class Inst {
+    constructor(
+        public kind: InstKind,
+    ) {}
 }
 
 export type InstKind =
@@ -26,7 +31,7 @@ export type InstKind =
     | { tag: "MovMRR"; dst: Reg; offset: number; src: Reg }
     | { tag: "MovRMR"; dst: Reg; src: Reg; offset: number }
     | { tag: "AddR"; dst_op1: Reg; op2: Reg }
-    | { tag: "Jmp"; bb: number }
+    | { tag: "Jmp"; bb: Block }
     | { tag: "Ret" };
 
 export class Reg {
diff --git a/src/front/check.ts b/src/front/check.ts
index 97db688..7c8baf4 100644
--- a/src/front/check.ts
+++ b/src/front/check.ts
@@ -107,7 +107,7 @@ class TypeChecker {
                             );
                             this.reporter.info(
                                 this.fn.kind.retTy?.loc ?? this.fn.loc,
-                                `return type '${retTy}' defined here`,
+                                `return type '${retTy.pretty()}' defined here`,
                             );
                             this.reporter.abort();
                         }
diff --git a/src/main.ts b/src/main.ts
index c438bcb..d8239c0 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -1,4 +1,5 @@
 import * as ast from "./ast.ts";
+import { selectFnInstructions } from "./codegen/x86_64/isel.ts";
 import { Reporter } from "./diagnostics.ts";
 import * as front from "./front/mod.ts";
 import * as middle from "./middle.ts";
@@ -49,6 +50,9 @@ if (Deno.args.includes("--print-mir")) {
     }
 }
 
+// const result = selectFnInstructions(mainMiddleFn);
+// console.log(JSON.stringify(result, null, 4));
+
 if (!reporter.ok()) {
     reporter.printAll();
     Deno.exit(1);