diff --git a/chibicc.h b/chibicc.h
index 83f81fb..ad4a6d7 100644
--- a/chibicc.h
+++ b/chibicc.h
@@ -153,6 +153,7 @@ struct Obj {
   Node *body;
   Obj *locals;
   Obj *va_area;
+  Obj *alloca_bottom;
   int stack_size;
 
   // Static inline function
diff --git a/codegen.c b/codegen.c
index 69cc423..907c1aa 100644
--- a/codegen.c
+++ b/codegen.c
@@ -560,6 +560,34 @@ static void copy_struct_mem(void) {
   }
 }
 
+static void builtin_alloca(void) {
+  // Align size to 16 bytes.
+  println("  add $15, %%rdi");
+  println("  and $0xfffffff0, %%edi");
+
+  // Shift the temporary area by %rdi.
+  println("  mov %d(%%rbp), %%rcx", current_fn->alloca_bottom->offset);
+  println("  sub %%rsp, %%rcx");
+  println("  mov %%rsp, %%rax");
+  println("  sub %%rdi, %%rsp");
+  println("  mov %%rsp, %%rdx");
+  println("1:");
+  println("  cmp $0, %%rcx");
+  println("  je 2f");
+  println("  mov (%%rax), %%r8b");
+  println("  mov %%r8b, (%%rdx)");
+  println("  inc %%rdx");
+  println("  inc %%rax");
+  println("  dec %%rcx");
+  println("  jmp 1b");
+  println("2:");
+
+  // Move alloca_bottom pointer.
+  println("  mov %d(%%rbp), %%rax", current_fn->alloca_bottom->offset);
+  println("  sub %%rdi, %%rax");
+  println("  mov %%rax, %d(%%rbp)", current_fn->alloca_bottom->offset);
+}
+
 // Generate code for a given node.
 static void gen_expr(Node *node) {
   println("  .loc %d %d", node->tok->file->file_no, node->tok->line_no);
@@ -732,6 +760,13 @@ static void gen_expr(Node *node) {
     return;
   }
   case ND_FUNCALL: {
+    if (node->lhs->kind == ND_VAR && !strcmp(node->lhs->var->name, "alloca")) {
+      gen_expr(node->args);
+      println("  mov %%rax, %%rdi");
+      builtin_alloca();
+      return;
+    }
+
     int stack_args = push_args(node);
     gen_expr(node->lhs);
 
@@ -1241,6 +1276,7 @@ static void emit_text(Obj *prog) {
     println("  push %%rbp");
     println("  mov %%rsp, %%rbp");
     println("  sub $%d, %%rsp", fn->stack_size);
+    println("  mov %%rsp, %d(%%rbp)", fn->alloca_bottom->offset);
 
     // Save arg registers if function is variadic
     if (fn->va_area) {
diff --git a/parse.c b/parse.c
index b3c57f8..1b39afb 100644
--- a/parse.c
+++ b/parse.c
@@ -2912,8 +2912,10 @@ static Token *function(Token *tok, Type *basety, VarAttr *attr) {
     new_lvar("", pointer_to(rty));
 
   fn->params = locals;
+
   if (ty->is_variadic)
     fn->va_area = new_lvar("__va_area__", array_of(ty_char, 136));
+  fn->alloca_bottom = new_lvar("__alloca_size__", pointer_to(ty_char));
 
   tok = skip(tok, "{");
 
@@ -2999,8 +3001,16 @@ static void scan_globals(void) {
   globals = head.next;
 }
 
+static void declare_builtin_functions(void) {
+  Type *ty = func_type(pointer_to(ty_void));
+  ty->params = copy_type(ty_int);
+  Obj *builtin = new_gvar("alloca", ty);
+  builtin->is_definition = false;
+}
+
 // program = (typedef | function-definition | global-variable)*
 Obj *parse(Token *tok) {
+  declare_builtin_functions();
   globals = NULL;
 
   while (tok->kind != TK_EOF) {
diff --git a/test/alloca.c b/test/alloca.c
new file mode 100644
index 0000000..ed6ffbc
--- /dev/null
+++ b/test/alloca.c
@@ -0,0 +1,28 @@
+#include "test.h"
+
+void *fn(int x, void *p, int y) { return p; }
+
+int main() {
+  int i = 0;
+
+  char *p1 = alloca(16);
+  char *p2 = alloca(16);
+  char *p3 = 1 + (char *)alloca(3) + 1;
+  p3 -= 2;
+  char *p4 = fn(1, alloca(16), 3);
+
+  ASSERT(16, p1 - p2);
+  ASSERT(16, p2 - p3);
+  ASSERT(16, p3 - p4);
+
+  memcpy(p1, "0123456789abcdef", 16);
+  memcpy(p2, "ghijklmnopqrstuv", 16);
+  memcpy(p3, "wxy", 3);
+
+  ASSERT(0, memcmp(p1, "0123456789abcdef", 16));
+  ASSERT(0, memcmp(p2, "ghijklmnopqrstuv", 16));
+  ASSERT(0, memcmp(p3, "wxy", 3));
+
+  printf("OK\n");
+  return 0;
+}
diff --git a/test/test.h b/test/test.h
index f6fa2db..1e640ae 100644
--- a/test/test.h
+++ b/test/test.h
@@ -10,3 +10,4 @@ int memcmp(char *p, char *q, long n);
 void exit(int n);
 int vsprintf();
 long strlen(char *s);
+void *memcpy(void *dest, void *src, long n);