如何实现GCC变长数组？ [英] How does GCC implement variable-length arrays?

问题描述

如何实现GCC可变长度的数组（VLA）？是这样的阵列基本指针，以如返回的alloca动态分配的存储空间？

How does GCC implement Variable-length arrays (VLAs)? Are such arrays essentially pointers to the dynamically allocated storage such as returned by alloca?

的其它替代我能想到的是，这种阵列被分配作为在一个功能最后一个变量，以便在编译时已知的变量的偏移量。然而，第二VLA的偏移量，然后再次不被在编译时已知的。

The other alternative I could think of, is that such an array is allocated as last variable in a function, so that the offset of the variables are known during compile-time. However, the offset of a second VLA would then again not be known during compile-time.

推荐答案

下面是分配code（86 - 在64 code是相似的）从的为VLA支持一些海合会文档：

Here's the allocation code (x86 - the x64 code is similar) for the following example line taken from some GCC docs for VLA support:

char str[strlen (s1) + strlen (s2) + 1];

其中计算的strlen（S1）+ strlen的（S2）+ 1 是 EAX （GCC MinGW的4.8.1 - 不优化）：

where the calculation for strlen (s1) + strlen (s2) + 1 is in eax (GCC MinGW 4.8.1 - no optimizations):

mov edx, eax
sub edx, 1
mov DWORD PTR [ebp-12], edx
mov edx, 16
sub edx, 1
add eax, edx
mov ecx, 16
mov edx, 0
div ecx
imul    eax, eax, 16
call    ___chkstk_ms
sub esp, eax
lea eax, [esp+8]
add eax, 0
mov DWORD PTR [ebp-16], eax

所以它看起来是基本的alloca（）。

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