Unaligned memory access

 Hi list,

I usually embed C structures and C++ classes directly into Lua userdata
blocks, instead of creating them in the heap and embedding the pointer in
the userdata. This avoids a pointer indirection, and having to remember to
delete the object when the userdata is collected. Not big deals I know, but
this is how I like to do it.

However, I have alignment issues in the userdata from time to time. It
happens when the userdata contains something that requires a 16 byte
alignment. Since this is not enforced by lua_newuserdata, the program will
crash with things like this:

   0x00007ff6fe911135 <+85>:    xorps  %xmm6,%xmm6
=> 0x00007ff6fe911138 <+88>:    movaps %xmm6,0x50(%rdi)

where %rdi is not 16-bytes aligned.

The workaround is easy enough: allocate the object in the heap with new,
and embed the pointer in the userdata. Compiler flags that will generate
movups for the case above are also an option.

This is not a request to change anything or add new APIs, it's just to let
the Lua maintainers know that this happens in the wild in code that uses
Lua. Maybe they will find it useful to fix it somehow.

Cheers,

Andre

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