I had the same problem with my jit, which also generated stack frames not aligned to 16-byte. My test program crashed on an SSE instruction in the Rust standard library (I dont' recall if this bug only occured in release mode, may have been already compiled code). I was pretty proud when I fixed this. Although I have to admit that after finding out that the accessed address was actually valid, I was already supposing that alignment was a problem. Fixing it was then straightforward since it was my own toy compiler.