如何在编译时使GCC评估函数？ [英] How to make GCC evaluate functions at compile time?

问题描述

我正在考虑以下问题：我想用一个查找表来编程一个微控制器（假设是一个AVR兆类型）。

第一次尝试是将表定位在单独的文件中，并使用任何其他脚本语言/程序/ ...创建它。在这种情况下，为C创建必要的源文件需要相当多的努力。

我现在的想法是使用预处理器和编译器来处理事情。我试图用一个正弦值表来实现这一点（就像一个例子）：

  #include  #include< math.h> 
 
 #define S1（i，n）（（uint8_t）sin（M_PI *（i）/ n * 255））
 #define S4（i，n）S1 ），S1（i + 1，n），S1（i + 2，n），S1（i + 3，n）
 
 uint8_t lut [] = {S4（0,4）}; 
 
 void main（）
 {
 uint8_t val，i; （i = 0; i <4; i ++）
 {
 val = lut [i]; 
 
 
 
 
 
 
 
 
 $ b如果我编译这段代码，我会收到有关 sin 函数。进一步在程序集中，在 .data 部分中没有任何内容。如果我只是删除第三行中的 sin ，我会得到程序集中的数据。显然，所有的信息都可以在编译时使用。
 
 
 
你能告诉我是否有办法实现我的意图：编译器计算出的离线值可能是多少？或者是使用外部脚本/程序/ ...来计算表项并将其添加到单独的文件中的最好方法，该文件将只是 #include  d？ 
解决方案
这里的一般问题是 sin 根据C语言的规则，事实上是非法的，因为它本身不是常量表达式本身，而且你正在初始化静态存储持续时间的数组，这就需要这样做。这也解释了为什么你的数组不在 .data 部分。
 
 
 
 C11（N1570）§6.6/ 2， 3常量表达式（强调我的） 
 
 
 
 
常量表达式可以在转换过程中被评估，而不是
运行时，因此可用于常量可能为
的任何地方。
 
 
 
常量表达式不得包含赋值，增量，
递减，函数调用或逗号运算符，除非它们是包含在未评估的子表达式中的
.115）
 
 
然而，由@ ShafikYaghmour的评论GCC将用它的内置对象替换 sin 函数调用（除非 -fno-builtin 选项存在），这可能会被视为常量表达式。根据 6.57 GCC提供的其他内置功能：
 
 
 GCC包含
标准C库中许多函数的内置版本。前缀为 __ builtin _ 的版本总是被
视为与C库函数具有相同的含义，即使
指定了 
 
 
 
I am thinking about the following problem: I want to program a microcontroller (let's say an AVR mega type) with a program that uses some sort of look-up tables.


The first attempt would be to locate the table in a separate file and create it using any other scripting language/program/.... In this case there is quite some effort in creating the necessary source files for C.

My thought was now to use the preprocessor and compiler to handle things. I tried to implement this with a table of sine values (just as an example):
#include <avr/io.h>
#include <math.h>

#define S1(i,n) ((uint8_t) sin(M_PI*(i)/n*255))
#define S4(i,n) S1(i,n), S1(i+1,n), S1(i+2,n), S1(i+3,n)

uint8_t lut[] = {S4(0,4)};

void main()
{
    uint8_t val, i;

    for(i=0; i<4; i++)
    {
        val = lut[i];
    }
}
If I compile this code I get warnings about the sin function. Further in the assembly there is nothing in the section .data. If I just remove the sin in the third line I get the data in the assembly. Clearly all information are available at compile time.

Can you tell me if there is a way to achieve what I intent: The compiler calculates as many values as offline possible? Or is the best way to go using an external script/program/... to calculate the table entries and add these to a separate file that will just be #included?
解决方案
The general problem here is that sin call makes this initialization de facto illegal, according to rules of C language, as it's not constant expression per se and you're initializing array of static storage duration, which requires that. This also explains why your array is not in .data section.

C11 (N1570) §6.6/2,3 Constant expressions (emphasis mine)

  
A constant expression can be evaluated during translation rather than
  runtime, and accordingly may be used in any place that a constant may
  be.
  
  
Constant expressions shall not contain assignment, increment,
  decrement, function-call, or comma operators, except when they are
  contained within a subexpression that is not evaluated.115)
However as by @ShafikYaghmour's comment GCC will replace sin function call with its built-in counterpart (unless -fno-builtin option is present), that is likely to be treated as constant expression. According to 6.57 Other Built-in Functions Provided by GCC:

  
GCC includes built-in versions of many of the functions in the
  standard C library. The versions prefixed with __builtin_ are always
  treated as having the same meaning as the C library function even if
  you specify the -fno-builtin option.


                        
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