c++ - 通过宏替换展开循环无效

Question

我正在尝试使用以下代码在 gcc 中执行循环展开。

#define SINGLE_INSTRUCTION(x,y) x + y
#define FIVE_INSTRUCTIONS(x,y) SINGLE_INSTRUCTION(x,y); SINGLE_INSTRUCTION(x,y); SINGLE_INSTRUCTION(x,y); SINGLE_INSTRUCTION(x,y); SINGLE_INSTRUCTION(x,y);
#define TWENTYFIVE_INSTRUCTIONS(x,y)  FIVE_INSTRUCTIONS(x,y); FIVE_INSTRUCTIONS(x,y); FIVE_INSTRUCTIONS(x,y); FIVE_INSTRUCTIONS(x,y); FIVE_INSTRUCTIONS(x,y);
#define ONEHUNDRED_INSTRUCTIONS(x,y);  TWENTYFIVE_INSTRUCTIONS(x,y); TWENTYFIVE_INSTRUCTIONS(x,y); TWENTYFIVE_INSTRUCTIONS(x,y); TWENTYFIVE_INSTRUCTIONS(x,y);

#define FIVEHUNDERED_INSTRUCTIONS(x,y); ONEHUNDRED_INSTRUCTIONS(x,y); ONEHUNDRED_INSTRUCTIONS(x,y); ONEHUNDRED_INSTRUCTIONS(x,y); ONEHUNDRED_INSTRUCTIONS(x,y); ONEHUNDRED_INSTRUCTIONS(x,y);

#define TWOTHOUSAND_INSTRUCTIONS(x,y); FIVEHUNDERED_INSTRUCTIONS(x,y); FIVEHUNDERED_INSTRUCTIONS(x,y); FIVEHUNDERED_INSTRUCTIONS(x,y); FIVEHUNDERED_INSTRUCTIONS(x,y);

#define TENTHOUSAND_INSTRUCTIONS(x,y) TWOTHOUSAND_INSTRUCTIONS(x,y); TWOTHOUSAND_INSTRUCTIONS(x,y); TWOTHOUSAND_INSTRUCTIONS(x, y); TWOTHOUSAND_INSTRUCTIONS(x,y); TWOTHOUSAND_INSTRUCTIONS(x,y);

#define FIFTYTHOUSAND_INSTRUCTIONS(x,y) TENTHOUSAND_INSTRUCTIONS(x,y); TENTHOUSAND_INSTRUCTIONS(x,y); TENTHOUSAND_INSTRUCTIONS(x, y); TENTHOUSAND_INSTRUCTIONS(x,y); TENTHOUSAND_INSTRUCTIONS(x,y);

#define TWOHUNDREDTHOUSAND_INSTRUCTIONS(x,y) FIFTYTHOUSAND_INSTRUCTIONS(x,y); FIFTYTHOUSAND_INSTRUCTIONS(x,y); FIFTYTHOUSAND_INSTRUCTIONS(x, y); FIFTYTHOUSAND_INSTRUCTIONS(x,y);

#define ONEMILLION_INSTRUCTIONS(x,y) TWOHUNDREDTHOUSAND_INSTRUCTIONS(x,y); TWOHUNDREDTHOUSAND_INSTRUCTIONS(x,y); TWOHUNDREDTHOUSAND_INSTRUCTIONS(x, y); TWOHUNDREDTHOUSAND_INSTRUCTIONS(x,y); TWOHUNDREDTHOUSAND_INSTRUCTIONS(x,y);

无效主(){

ONEMILLION_INSTRUCTIONS(50,100);
ONEMILLION_INSTRUCTIONS(200,300);
/*
    ONEMILLION_INSTRUCTIONS(200,300);
ONEMILLION_INSTRUCTIONS(50,100);
ONEMILLION_INSTRUCTIONS(50,100);
ONEMILLION_INSTRUCTIONS(200,300);
ONEMILLION_INSTRUCTIONS(50,100);
ONEMILLION_INSTRUCTIONS(200,300);
ONEMILLION_INSTRUCTIONS(50,100);
*/

}但每次我增加指令宏时，输出箱似乎总是<8KB。我预计大小约为 40MB。我不明白发生了什么。

Answer 1

您的指令不会产生任何可观察到的行为。 C 是根据抽象机定义的。只要程序的可观察行为符合规范规定，编译器就可以生成任何代码。

在 C 中，可观察到的行为是:调用库函数、将数据写入文件以及访问 volatile 变量。

作为一种解决方法，您可以将一条指令更改为 { volatile int z = x + y; 。