C预处理程序可以执行算术吗？如果可以，怎么做？ [英] Can the C preprocessor perform arithmetic and if so, how?

问题描述

我目前正在为微控制器编写代码；由于ATMega128没有硬件乘法器或除法器，因此这些操作必须在软件中完成，并且占用大量的周期。但是，出于代码的可移植性和易用性，我宁愿不要将预先计算的值硬编码到我的代码中。因此，例如，我有许多任务取决于系统时钟频率。目前，我的运行频率为16MHz，但是我应该选择降低它，例如为了减少电池应用的功耗，我想更改一行代码，而不是多行代码。

I'm currently writing code for a microcontroller; since the ATMega128 does not have a hardware multiplier or divider, these operations must be done in software and they take up a decent amount of cycles. However, for code portability and ease of use, I'd prefer not to hard-code precomputed values into my code So for instance, I have a number of tasks which are dependent on the system clock frequency. Currently I' running at 16MHz, but should I choose to lower that, say to reduce power consumption for battery applications, I'd like to change one line of code rather than many.

这样说，C预处理程序可以计算算术表达式，然后将结果粘贴到我的代码中，而不是将原始表达式粘贴到代码中吗？如果是这样，我将如何去做？他们的编译器选项是我需要考虑的吗？

So with that said, can the C preprocessor compute arithmetic expressions and then "paste" the result into my code rather than "pasting" the original expression into the code? If so, how would I go about doing this? Are their compiler options and whatnot that I need to consider?

注意：我要计算的值是常量，所以我看不出为什么这不是常数

NOTE: The values I want to compute are constant values, so I see no reason why this would not be a feature.

推荐答案

这是一个问题：

• 第一季度。 C预处理程序可以执行算术吗？

这是另一个：

• 第二季度。 C预处理程序可以计算算术表达式，然后将结果
  粘贴到我的代码中，而不是将原始表达式粘贴到代码中吗？

对第一季度的答案是肯定的。问题2的答案是否定的。这两个事实都可以用以下文件来说明
：

The answer to Q1 is Yes. The answer to Q2 is No. Both facts can be illustrated with the following file:

foo.c

#define EXPR ((1 + 2) * 3)
#if EXPR == 9
int nine = EXPR;
#else
int not_nine = EXPR;
#endif

如果我们将其传递给C预处理器，则用 cpp foo.c 或等价的
gcc -E foo.c ，我们看到的输出如下：

If we pass this to the C preprocessor, either by cpp foo.c or equivalently gcc -E foo.c, we see output like:

# 1 "foo.c"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 30 "/usr/include/stdc-predef.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/predefs.h" 1 3 4
# 31 "/usr/include/stdc-predef.h" 2 3 4
# 1 "<command-line>" 2
# 1 "foo.c"


int nine = ((1 + 2) * 3);

预处理器保留定义 int的行的事实和
删除了定义 not_nine 的行，这表明我们已经正确执行了
求值＃的算法。如果EXPR == 9 。

The fact that the preprocessor retains the line defining int nine and has dropped the line defining not_nine shows us that it has correctly performed the arithmetic required to evaluate #if EXPR == 9.

定义的预处理文本为 int的事实int九=（（1 + 2）* 3）;
向我们显示 #define 指令使预处理器替换
<$ c定义为（（1 + 2）* 3）的$ c> EXPR ，以及具有算术值的 not
的定义为 9 。

The fact that the preprocessed text of the definition is int nine = ((1 + 2) * 3); shows us that the #define directive causes the preprocessor to replace EXPR with its definition ((1 + 2) * 3), and not with the arithmetic value of its definition, 9.

C预处理程序除<$ c $以外是否还有其他指令c> #define 具有第二个
效果？

Does the C preprocessor have any directive besides #define which has the second effect? No.

但这当然并不意味着 int 9 的定义必须包含
运行时计算，因为编译器几乎可以肯定会计算
算术表达式（（1 + 2）* 3）在编译时将其替换为常量 9 的
。

But this does not of course imply that the definition of int nine must entail a runtime calculation, because the compiler will almost certainly evaluate the arithmetic expression ((1 + 2) * 3) at compiletime and replace it with the constant 9.

我们可以看到编译器通过检查
编译后的目标文件来翻译源文件。大多数工具链都会提供类似GNU binutils
objdump 之类的帮助。如果我使用gcc编译 foo.c ：

We can see how the compiler has translated the source file by examining the compiled object file. Most toolchains will provide something like GNU binutils objdump to assist with this. If I compile foo.c with gcc:

gcc -c -o foo.o foo.c

然后调用：

objdump -s foo.o

到看到 foo.o 的全部内容，我得到：

to see the full contents of foo.o, I get:

foo.o:     file format elf64-x86-64

Contents of section .data:
 0000 09000000                             ....            
Contents of section .comment:
 0000 00474343 3a202855 62756e74 752f4c69  .GCC: (Ubuntu/Li
 0010 6e61726f 20342e38 2e312d31 30756275  naro 4.8.1-10ubu
 0020 6e747539 2920342e 382e3100           ntu9) 4.8.1.

还有希望的 9 在 .data 部分中进行硬编码。

And there is the hoped-for 9 hard-coded in the .data section.

注意：预处理器的算术功能仅限整数算术

Note that the preprocessor's arithmetic capabilities are restricted to integer arithmetic

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