为什么GDB“跳回”当用c源代码进行调试时 [英] Why is GDB "jumping back" when debugging with c source codes

问题描述

现在我发现gdb有时会在各行之间跳来跳去，比如考虑c源代码如下所示：

  char * XXX; 
 int a; 
 ... 
 
 if（...）
 {
 
} 
  

当我到达 if if 子句时，我输入 n 它会跳回到 int a 部分。这是为什么？

如果我再次执行该命令，它会在中输入括号，如果。如果可能的话，我想避免那部分，直接输入if（当然，如果条件匹配）

解决方案

当我到达if子句时，我键入n，它将跳回到int的一个部分。为什么会这样？

由于您的代码是通过优化编译的，编译器可以（并且经常）重新安排你的程序以这样的方式：属于不同源代码行的指令是交错的（代码运动优化尝试（除其他之外）将加载指令移到很长时间才需要它们的结果;这有助于隐藏存储器延迟）。 >

如果您使用的是 gcc-4.8 或更高版本，请使用 -Og 。否则，请参阅这个回答。

I'm debugging the goldfish android kernel (version 3.4), with kernel sources.

Now I found that gdb sometimes jump back and forth between lines, e.g consider c source code like the following:

char *XXX;
int a;
...

if (...)
{

}

When I reached the if clause, I type in n and it will jump back to the int a part. Why is that?

If I execute that command again, it would enter the brackets in the if.

If possible, I want to avoid that part, and enter the if directly (of course, if condition matches)

解决方案

When I reached the if clause, I type in n and it will jump back to the int a part. Why is that?

Because your code is compiled with optimization on, and the compiler can (and often does) re-arrange instructions of your program in such a way that instructions "belonging" to different source lines are interleaved (code motion optimizations attempt (among other things) to move load instructions to long before their results are needed; this helps to hide memory latency).

If you are using gcc-4.8 or later, build your sources with -Og. Else, see this answer.

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