>>> all internal usage of lightuserdata (for hooks,
>>> profilers, built-in package, IR and so on) is changed to special values
>>> on Lua Stack.
>> 
>> Can you add at least _some_ test to verify memprof is fine?
> 
> Memprof avoids such extroversions. Do you mean the test for `jit.p`?
> 

I can see memeprof is out of the business, sure I meant any our new
functionality - such as stats on gc - that can be hit by this in some
way.
 
> The new commit message is the following:
> 
> ===================================================================
> Add support for full-range 64 bit lightuserdata.
> 
> (cherry picked from commit e9af1abec542e6f9851ff2368e7f196b6382a44c)
> 
> LuaJIT uses special NaN-tagging technique to store internal type on
> the Lua stack. In case of LJ_GC64 the first 13 bits are set in special
> NaN type (0xfff8...). The next 4 bits are used for an internal LuaJIT
> type of object on stack. The next 47 bits are used for storing this
> object's content. For userdata, it is its address. For arm64 a pointer
> can have more than 47 significant bits [1]. In this case the error BADLU
> error is raised.
> 
> For the support of full 64-bit range lightuserdata pointers two new
> fields in GCState are added:
> 
> `lightudseg` - vector of segments of lightuserdata. Each element keeps
> 32-bit value. 25 MSB equal to MSB of lightuserdata 64-bit address, the
> rest are filled with zeros. The lentgh of the vector is power of 2.
> 
> `lightudnum` - the length - 1 of aforementioned vector (up to 255).
> 
> When lightuserdata is pushed on the stack, if its segment is not stored
> in vector new value is appended to of this vector. The maximum amount of
> segments is 256. BADLU error is raised in case when user tried to add
> userdata with the new 257-th segment, so the whole VA-space isn't
> covered by this patch.
> 
> Also, in this patch all internal usage of lightuserdata (for hooks,
> profilers, built-in package, IR and so on) is changed to special values
> on Lua Stack.
> 
> Also, conversion of TValue to FFI C type with store is no longer
> compiled for lightuserdata.
> 
> [1]: https://www.kernel.org/doc/html/latest/arm64/memory.html <https://www.kernel.org/doc/html/latest/arm64/memory.html>
> 
> Sergey Kaplun:
> * added the description and the test for the problem
> 
> Resolves tarantool/tarantool#2712
> Needed for tarantool/tarantool#6154
> ===================================================================
> 
> Branch is force-pushed.

LGTM.

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