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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（`$rdx` 原值）；而一旦关闭这一选项，即使不写签名，编译器也能通过内建的函数匹配机制获得完整的 `printf` 签名信息(`int printf(const char*, ...)`)，便能生成正确的汇编调用设置 `$rdx`，结果也自然没有问题了。
+
+C，很奇妙吧。 xDDD
+
+> Compiler Explorer 复现链接: https://godbolt.org/z/eh9WK9eqG
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diff --git a/content/scheduled-idling/index.md b/content/scheduled-idling/index.md
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++++
+title = "计划性空转"
+date = 2025-03-23
+authors = ["135e2 (Mole Shang)"]
+[taxonomies]
+tags = ["随想"]
++++
+
+> Surely idling is about doing what you feel? No, because if you do not schedule your idling, you will be easy prey for the attention merchants out there, the Silicon Valley platforms and scams, the advertising people, the distractors, who spend vast sums employing behavioural scientists to get you to hang around on their sites, clicking and spending. In scheduling is freedom.
+>
+> -- An Idler's Manual, Tom Hodgkinson
+
+可直觉上还是觉着「有计划地空转」是件挺不可理喻的事。
+
+<!-- more -->