英文:
Does gcc cache the address of a global array at a static index by default?
问题
让我们假设我有全局数组
char global_array[3] = {33, 17, 6}; 并且我访问 global_array[1]。当gcc解释 global_array[1] 时，它会在运行时将1添加到 global_array 的地址，还是将 global_array[1] 更改为静态地址，假设使用 -O0 编译选项？如果不是这样，是否有一个编译器标志，可以让gcc将数组的静态索引转换为静态指针？
英文:
Let's say I have the global array
char global_array[3] = {33, 17, 6}; and I access global_array[1]. When gcc interprets global_array[1], will it add 1 to the address of global_array at runtime, or change global_array[1] to a static address, assuming -O0? If not, is there a compiler flag to for gcc to make the static index of an array a static pointer?
答案1
得分: 0
当gcc解释global_array[1]时，假设使用-O0，它会在运行时将1添加到global_array的地址，还是将global_array[1]更改为静态地址？
关于gcc在不同优化级别下的具体行为没有特定的保证。以x86_64的gcc 12.2为例，考虑以下代码：
int func (void)
{
char global_array[3] = {33, 17, 6};
return global_array[1];
}
在-O0级别下，这将导致奇怪的汇编代码：
func:
push rbp
mov rbp, rsp
mov WORD PTR [rbp-3], 4385
mov BYTE PTR [rbp-1], 6
movzx eax, BYTE PTR [rbp-2]
movsx eax, al
pop rbp
ret
它将数组存储在堆栈上，但将神奇的数字4385作为一个字加载，然后在末尾加载6。那么4385是什么？它是0x1121的十六进制表示法。字节的值为0x11 = 17十进制和0x21 = 33，由于它是x86小端序，与数组的顺序相比，WORD指令的字节顺序相反。
即使我使用了-O0，这个字写入已经是某种形式的微优化。所以再次强调，不能提供保证。关于你的问题，访问部分rbp-2确实类似于在运行时将1添加到地址。在这种情况下，它是从堆栈帧指针减去一个偏移量，但本质上是相同的操作（并且可能与将1添加到地址一样快/慢）。
当启用优化时，通常的做法是用绝对值替换整个操作。
如果不是这样，是否有编译器标志可以使数组的静态索引成为静态指针？
通过更改编译器选项可能不是实现这一目标的方法。如果你希望有一个指向固定地址的指针，通常的步骤是：
- 在特定目标的链接器脚本中创建一个自定义段位于固定地址。
- 在文件范围内声明你的数组，并使用gcc的变量属性将其分配到你的自定义段内。
- 现在
&global_array[1]始终是相同的地址。
英文:
> When gcc interprets global_array[1], will it add 1 to the address of global_array at runtime, or change global_array[1] to a static address, assuming -O0?
There's no particular guarantees of what gcc does and does not depending on level of optimization. Take your example under gcc 12.2 for x86_64:
int func (void)
{
char global_array[3] = {33, 17, 6};
return global_array[1];
}
This results in peculiar assembly code under -O0:
func:
push rbp
mov rbp, rsp
mov WORD PTR [rbp-3], 4385
mov BYTE PTR [rbp-1], 6
movzx eax, BYTE PTR [rbp-2]
movsx eax, al
pop rbp
ret
It is storing the array of the stack but it loads the magic number 4385 as a word, then loads a 6 at the end. Now wth is 4385? It is 0x1121 hex. Where the bytes have values 0x11 = 17dec and 0x21 = 33, and since it is a x86 little endian, the byte order for the WORD instruction is reversed vs the order of the array.
This word write is already some manner of mini-optimization even though I used -O0. So again - no guarantees. Regarding your question, the access part rbp-2 is indeed similar to adding 1 the to the address in runtime. In this case it is subtracting an offset from the stack frame pointer, but that's essentially the same thing (and likely as fast/slow as adding 1 to an address).
The normal thing when optimizations are enabled would otherwise be to replace the whole thing with the absolute value.
> If not, is there a compiler flag to for gcc to make the static index of an array a static pointer?
Messing around with compiler options is probably not how. If you wish to have a pointer to a fixed address, then this is the usual steps:
- Create a custom segment at a fixed address in the linker script for the specific target.
- Declare your array at file scope and use gcc variable attributes to allocate it inside your custom segment.
- Now
&global_array[1]is always the same address.
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