From 754a24fafcea637cab8bc01bb2702069109a0358 Mon Sep 17 00:00:00 2001
From: Rui Ueyama <ruiu@cs.stanford.edu>
Date: Sun, 25 Aug 2019 11:48:44 +0900
Subject: [PATCH] Add va_start to support variadic functions

---
 chibicc.h       |  1 +
 codegen.c       | 30 ++++++++++++++++++++++++++++++
 parse.c         |  2 ++
 test/function.c | 23 +++++++++++++++++++++++
 4 files changed, 56 insertions(+)

diff --git a/chibicc.h b/chibicc.h
index 0addaaa..39055cc 100644
--- a/chibicc.h
+++ b/chibicc.h
@@ -92,6 +92,7 @@ struct Obj {
   Obj *params;
   Node *body;
   Obj *locals;
+  Obj *va_area;
   int stack_size;
 };
 
diff --git a/codegen.c b/codegen.c
index 1371fa8..f6860f5 100644
--- a/codegen.c
+++ b/codegen.c
@@ -567,6 +567,36 @@ static void emit_text(Obj *prog) {
     println("  mov %%rsp, %%rbp");
     println("  sub $%d, %%rsp", fn->stack_size);
 
+    // Save arg registers if function is variadic
+    if (fn->va_area) {
+      int gp = 0;
+      for (Obj *var = fn->params; var; var = var->next)
+        gp++;
+      int off = fn->va_area->offset;
+
+      // va_elem
+      println("  movl $%d, %d(%%rbp)", gp * 8, off);
+      println("  movl $0, %d(%%rbp)", off + 4);
+      println("  movq %%rbp, %d(%%rbp)", off + 16);
+      println("  addq $%d, %d(%%rbp)", off + 24, off + 16);
+
+      // __reg_save_area__
+      println("  movq %%rdi, %d(%%rbp)", off + 24);
+      println("  movq %%rsi, %d(%%rbp)", off + 32);
+      println("  movq %%rdx, %d(%%rbp)", off + 40);
+      println("  movq %%rcx, %d(%%rbp)", off + 48);
+      println("  movq %%r8, %d(%%rbp)", off + 56);
+      println("  movq %%r9, %d(%%rbp)", off + 64);
+      println("  movsd %%xmm0, %d(%%rbp)", off + 72);
+      println("  movsd %%xmm1, %d(%%rbp)", off + 80);
+      println("  movsd %%xmm2, %d(%%rbp)", off + 88);
+      println("  movsd %%xmm3, %d(%%rbp)", off + 96);
+      println("  movsd %%xmm4, %d(%%rbp)", off + 104);
+      println("  movsd %%xmm5, %d(%%rbp)", off + 112);
+      println("  movsd %%xmm6, %d(%%rbp)", off + 120);
+      println("  movsd %%xmm7, %d(%%rbp)", off + 128);
+    }
+
     // Save passed-by-register arguments to the stack
     int i = 0;
     for (Obj *var = fn->params; var; var = var->next)
diff --git a/parse.c b/parse.c
index 510cf5a..26711fc 100644
--- a/parse.c
+++ b/parse.c
@@ -2212,6 +2212,8 @@ static Token *function(Token *tok, Type *basety, VarAttr *attr) {
   enter_scope();
   create_param_lvars(ty->params);
   fn->params = locals;
+  if (ty->is_variadic)
+    fn->va_area = new_lvar("__va_area__", array_of(ty_char, 136));
 
   tok = skip(tok, "{");
   fn->body = compound_stmt(&tok, tok);
diff --git a/test/function.c b/test/function.c
index 0c0d36d..12cb8fb 100644
--- a/test/function.c
+++ b/test/function.c
@@ -72,6 +72,25 @@ short short_fn();
 
 int add_all(int n, ...);
 
+typedef struct {
+  int gp_offset;
+  int fp_offset;
+  void *overflow_arg_area;
+  void *reg_save_area;
+} __va_elem;
+
+typedef __va_elem va_list[1];
+
+int add_all(int n, ...);
+int sprintf(char *buf, char *fmt, ...);
+int vsprintf(char *buf, char *fmt, va_list ap);
+
+char *fmt(char *buf, char *fmt, ...) {
+  va_list ap;
+  *ap = *(__va_elem *)__va_area__;
+  vsprintf(buf, fmt, ap);
+}
+
 int main() {
   ASSERT(3, ret3());
   ASSERT(8, add2(3, 5));
@@ -121,8 +140,12 @@ int main() {
   ASSERT(6, add_all(3,1,2,3));
   ASSERT(5, add_all(4,1,2,3,-1));
 
+  { char buf[100]; fmt(buf, "%d %d %s", 1, 2, "foo"); printf("%s\n", buf); }
+
   ASSERT(0, ({ char buf[100]; sprintf(buf, "%d %d %s", 1, 2, "foo"); strcmp("1 2 foo", buf); }));
 
+  ASSERT(0, ({ char buf[100]; fmt(buf, "%d %d %s", 1, 2, "foo"); strcmp("1 2 foo", buf); }));
+
   printf("OK\n");
   return 0;
 }
-- 
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