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++++
+title = "MinGW, x86_64 调用规范与代码高尔夫"
+date = 2025-03-28
+authors = ["135e2 (Mole Shang)"]
+[taxonomies]
+tags = ["技术"]
++++
+
+最近和友人研究短码竞赛，用 MinGW 跑测试的时候出现了诡异的现象，在此留作记录。
+
+<!-- more -->
+
+由于常用的短码 OJ 均使用 [TCC](https://www.bellard.org/tcc/) 编译，自然为了模拟其行为，并节省字符数，我们选择了 C90 标准（不强制要求声明函数签名）并开启了 `-fno-builtin` 编译选项（以忽略烦人的编译器警告）。
+
+跑了几段测试后，黑魔法出现：
+
+![❓](./erroneous_output.png)
+
+一开始我们都觉得是 MinGW 犯了什么毛病，于是拉进 GDB 里一通猛调：
+
+根据 [Windows x64 calling convention](https://learn.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-170#example-of-argument-passing-2---all-floats)，`printf`函数的第二个浮点参数应当存在 `$xmm1` 中，调试一下却发现已经正确写入。
+
+![xmm1 register](./xmm1.png)
+
+用 GDB 跳进 `printf` 实现里大概翻了一下，也没有注意到什么奇怪的地方。将各种寄存器倒腾一番之后，依然百思不得其解。
+
+友人突然想起来看看编译参数；删掉 `-fno-builtin` 之后，尽管多了不少警告，MinGW 突然产出了正确的结果：
+
+![❗](./correct_output.png)
+
+那么问题来了：开不开 builtin 和 `printf` 的输出结果有什么关系？
+
+~~又到了喜闻乐见的编译原理检测时间~~
+
+GCC 文档如此解释 `no-builtin` 选项：
+```
+-fno-builtin
+-fno-builtin-function
+
+   Don't recognize built-in functions that do not begin with
+   __builtin_ as prefix.  GCC normally generates special code to
+   handle certain built-in functions more efficiently; for
+   instance, calls to "alloca" may become single instructions
+   which adjust the stack directly, and calls to "memcpy" may
+   become inline copy loops.  The resulting code is often both
+   smaller and faster, but since the function calls no longer
+   appear as such, you cannot set a breakpoint on those calls, nor
+   can you change the behavior of the functions by linking with a
+   different library.  In addition, when a function is recognized
+   as a built-in function, GCC may use information about that
+   function to warn about problems with calls to that function, or
+   to generate more efficient code, even if the resulting code
+   still contains calls to that function.  For example, warnings
+   are given with -Wformat for bad calls to "printf" when "printf"
+   is built in and "strlen" is known not to modify global memory.
+```
+
+再次仔细阅读 [Windows x64 calling convention](https://learn.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-170#varargs)，注意到 `Varargs` 一节有这么一行小字：
+
+> For floating-point values only, both the integer register and the floating-point register must contain the value, in case the callee expects the value in the integer registers.
+
+如此对比两者产生的汇编，理由就很容易想清楚了：
+
+![asm diff](./asm_diff.png)
+
+显然我们在没有函数签名定义的情况下，又开启了 `no-builtin` 选项，编译器因而无从获取 `printf` 的签名信息，只能从调用推断其签名为 `void printf(const char*, double)`，无法遵循正确的 `varargs` 调用规范设置通用寄存器 `$rdx`，导致 Windows 运行时标准库中的 `printf` 作为 callee 获取第二个参数时永远是全0（`$rdx` 原值）；而一旦关闭这一选项，即使不写签名，编译器也能通过内建的函数匹配机制获得完整的 `printf` 签名信息(`int printf(const char*, ...)`)，便能生成正确的汇编调用设置 `$rax`，结果也自然没有问题了。
+
+C，很奇妙吧。 xDDD
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