It gets worse than that. On x86-32, you are only guaranteed 8-byte stack alignment, and if you try to use __attribute__((aligned(16)) on a stack variable, gcc just silently fails to enforce the alignment. Or it did 15 years ago when x86-32 was relevant. A lot of times the code works anyway by luck. Until it doesn't. No idea if this was ever fixed.
Wasn't that a Windows limitation? I recall that trouble from ffmpeg having to align the stack for functions that might be called from microsoft compiled code. I recall gcc assuming alignment but no guarantee that you were actually called like that.
