Comment 17 for bug 539632

http://gcc.gnu.org/ml/gcc/2007-12/msg00503.html
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33721

Yeah, it's been broken forever. So long that instead of fixing it by default that they added an argument to fix it if you want. I think that basically means the de-facto ABI is that the stack is aligned to 16 bytes on x86, despite what the ABI actually says.

If you want GCC to follow the ABI, you can use this argument or per-function attribute:
       -mstackrealign
           Realign the stack at entry. On the Intel x86, the -mstackrealign
           option will generate an alternate prologue and epilogue that realigns
           the runtime stack if necessary. This supports mixing legacy codes that
           keep a 4-byte aligned stack with modern codes that keep a 16-byte stack
           for SSE compatibility. See also the attribute
           "force_align_arg_pointer", applicable to individual functions.