How does GCC implement variable-length arrays?

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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().

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