c - How does GCC implement variable-length arrays?

Question

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

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.

Answer 1

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];

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

So it looks to be essentially alloca().